This isn't the 70's anymore. Oil conditions are similar, but the dynamics of how most vehicles use gas is different.

Recently drove a long trip and did it at 55. I got almost nothing for my trouble, but a loss of time. The car I have does 42-45MPG on the freeway. Most of the newer rentals I drive get 35 or so.

They get these miles at 55, 60, 65, even 70.

Really, the older cars and large cars like SUV's, Trucks and such see a significant difference at 55.

IMHO, limiting everybody wouldn't pay off all that well like it would in the 70's when fuel consumption curves vs. speed were different.

I get the feeling that most people in a position to advance this regulation drive guzzlers, thinking that experience is typical when it just isn't.

How about we regulate the Trucks and SUV's to capture that savings instead?

Is the reason that modern vehicles do not run more efficiently at 55 vs. 70 MPH predominantly due to changes in the aerodynamics or is it due to modern cars having a large number of engine sensors and engine control computers?

Surely a big reason for improved fuel efficiency at higher speeds today vs 30 years ago is the predominance of overdrive transmissions, which were not so common in the 1970s when the 55mph speed limit was imposed. Overdrive wasn't a standard feature on cars then the way it is today, and you get much better mileage with overdrive on the freeways than without it.

I've heard conflicting things about the optimal speed for average fuel efficiency in today's cars, but in general I've heard it is closer to 55mph than to 70mph. Of course, you're talking an average, since it's not the same for every vehicle.

Andrew

I think that average is skewed. I've driven a fair number of different cars in the last coupla years.

The under told story is the difference between optimal at 55 and 70. For the larger cars, it's significant. For older cars it's very significant, and I think the overdrive is a big factor too.

For most mid to small cars, made in the late 80's or newer, that difference is NOT significant.

Alfredo, it's been my experience that the larger number of sensors and control devices is about clean air and performance, not overall consumption. They could be oriented that way, but I've not seen it much, if at all.

Newer cars are heavier, and have more stringent emissions controls. That seems to balance out any MPG gains.

Case in point, a new GM Cobalt.

At 55, and if you are EXTREMELY careful, you can get 37MPG average. At 65 it's maybe 34!! At 70, it drops to 32 - 28, depending.

I sampled this car on a long drive up to Seattle and back recently.

55 wouldn't get us anything in this car, that 60 - 65 wouldn't.

Best gain is really displaying the MPG for the driver to conserve, and they've done that.

The biggest factor underrated by the public in mileage performance is aerodynamics. The less wind resistance the vehicle presents results in pushing that optimum mileage performance point higher.
Given that at a certain engine speed in top gear there is an optimum mileage point, the actual speed of the vehicle is going to be higher when there is less resistance. I have noticed that mileage traveling up the gorge against those east winds can vary my mileage by 5 mpg. My best mileage sans wind is at about half way to redline (3300 rpm) in 5th gear, but the velocity you will get at that point will vary due to headwind/crosswind. Contrary to the claim in another similar thread, you don't necessarily get better mileage by slowing down (the act of taking your foot off the gas obviously uses less fuel, but reapplying acceleration to reach a speed slightly less than optimum won't make a significant difference over the life of the tank of gas).

Perhaps 60 MPH would be an ideal speed limit. Man, once you hit this speed you're up against a physics wall where more and more energy is required to overcome it than to simply move a car. Unless one intends to become airborne, that would be the ideal speed for vehicles using a fuel on the downside of availability.

Large freight companies are restricting even their teamster drivers to 58 MPH. Clearly fuel savings are worth more than labor costs here.

I don't think the cars should display just MPG, because most people can't relate to that. The average person doesn't do math in his/her head. He/she surely knows that 35MPG is better than 25MPG, but just ask the average person to figure out how much a 10-mile trip costs in gas? 8 in 10 people wouldn't have a clue how to figure that out.

So why not display the cost of a trip? It's much more complicated to automate than a simple MPG display - but it's not that hard in the era of wireless technology for your car's computer to record what you just paid to fill up your tank. And it can record how much fuel you used to drive from home to the store. So if you paid $4.10/gallon and you used 1.2 gallons to drive to the store and back, then your trip cost $4.92 in gas. THAT is what people need to see, because they can relate to.

Connect it up with GPS, and you can email people a monthly summary of their trips including store names, like your cell phone bill:

 
7/26 Home to Costco Tigard, OR............................$4.92 
7/26 Costco Tigard to Fry's Electronics, Wilsonvile, OR...$4.18 
7/26 Fry's Electronics to Home............................$9.18 
 

Speed limits don't work. I know, liberal that I am I should support the idea. However, we don't have endless resources to put all our cops on the road the way we did in the 70's. I think we really need to focus on designing cars that use minimal fuel.

Can't say I disagree there.

If it has to happen, then maybe we can do the conservation law violation bit again, keeping the damage off of peoples insurance policies.

Maybe leave limits where they stand, but impose a conservation cap for those cars that warrant it. Could break down into a few coarse classes, leaving most people with solid choices.

SUV? 55 Mid to long Truck? 55 Carrying anything besides people? 55 Old car? 55 (maybe use 1986 or something like that) That just can't be all that hard. Publish the limits, and let people sort them out, and drive accordingly.

If a given car is ambigious, well? That's a gamble just like speeding is right?

Everybody else then just does what they think is best for fuel conservation.

I'm all for that, I just want the hassle to actually matter, not be a feel good kind of thing. I'd like to see some kind of conversion be possible for a lot of the newer, better cars we have too. Shame to trash them all. Then again, maybe it's a focus for jobs. Don't know.

Andy, what's the impact of the cars weight on the aerodynamic element? The shape of the body, seems to me to exhibit the same force, but am I wrong on that somehow?

Andrew, that's an extremely good idea

Call the TomTom guys. They will love it.

The government shouldn't be trying to solve the energy crisis by bringing America to a crawl. It won't change much, but it will sure piss a lot of people off. I still dislike Nixon to this day for the 55 mph limit. I called it the Republican National Speed Limit.

I have no problem trying to educate people what it costs to make an unnecessary trip at $4.00 per gallon, but then let them decide if they want to spend the money.

That's the Conservative in me, let people make their own decisions, not the government making the decisions for them.

I would have to agree with Deane. Speed limits should factor in safety only. If it's safe to drive 80 but I get better gas mileage at 60, then get out of my way. Time is money.

Its never safe to drive at 80 unless we start upping the skills needed to obtain a driver's license.

We ought to compare the "time is money" thinking in terms with the Concorde. The Concorde saved a huge amount of time, but wasn't worth the fuel used. People quit flying on it.

$4/gallon gas has many people rethinking the term.

"SUV? 55 Mid to long Truck? 55 Carrying anything besides people? 55 Old car? 55"

Here's a simpler solution:

A "speeding permit" and a higher "grade" of fuel that you can buy at the pump to allow you to exceed a 60 MPH speed limit up to 70. This dyed gas -- same octane as regular gas -- can be purchased for lets say $2.50 more than regular unleaded -- hence, if you want to be fuelish, "time is money", you'll just have to pay for it.

The money can be earmarked to each state's DOT with a percentage going to enforcement. So if you are caught exceeding 60 with a "speeding" permit and NO dyed gas, you're hit with a whoppingly high fine -- higher than us regular guys.

This is relatively easy to enforce because the DOT already deal with non-taxed dyed diesel for off road use (farmers). If you're caught with dyed diesel in your semi, you're dead meat.

I don't know if that's simpler. Maybe just different.

Still, that hoses people like me, who drive 60-70 and don't waste gas.

That's the rub with me on this. If we are gonna regulate stuff, that hassle needs to actually do something. And it needs to be worth it.

Not sure that's the case with the 55 deal.

Okay, how about this idea: a .75/gal federal surcharge on unleaded gas and non-PUC diesel and no speed limit reduction anywhere unless the surcharge is repealed?

Those that can afford it can do so, and others can, well, slow down, to get their money back.

This way 1) the planet breathes a bit better, 2) we cut our dependence on OPEC and mideastern nuttiness, and 3) the RICH pay more than the poor to accomplish nearly the same thing? Hmm?

Your ideas are nuts. Clearly it's plenty safe to drive 80, as we have hundreds of thousands of Oregonians doing it every day with very little traffic deaths as a result. And if those driving faster use more fuel, too bad. If I'm paying the bill, it's really none of your concern anyhow.

Nobody here has yet raised this question: how much would the fuel savings from lowering speed limits be compared to total fuel usage? One could say that fuel is wasted either when one is driving a car at a speed where the fuel economy takes a nosedive (as in the example that Missing_KSKD gave above, where a GM Cobalt's miles per gallon drop from 34 to ~30 when one goes from 65 to 70 MPH) or when one is stuck in traffic with the engine idling. For that reason, I think that speed limits should be about safety. To promote fuel economy, I think that it would be helpful if somebody were to compile and publish data about the fuel economy vs. speed curves for various cars. I think that most people would "get it" if they saw a plot that takes a nosedive above a certain speed.

vitalogy sez: Your ideas are nuts. Clearly it's plenty safe to drive 80

Hmm, personal insults to defend your position. Clearly you haven't spent much time behind the wheel as I have. Besides, everybody here knows of people that shouldn't have a drivers license in the first place, let alone drive 80. So if we're gonna bump the limit up to 80, we have to make sure EVERY DRIVER can handle 80. Unless we want to have tiered licenses.

Missing - am I reading you wrong, or are you claiming that the 10-20% reduction in fuel economy between 55-70 isn't significant?

I think what the argument boils down to is the question of just how valuable is your time? The answer varies from person to person. For example, I can stand driving 55 on my daily commute, as that only adds a couple of minutes to the trip, I don't really notice it, and it gives me a chance to relax. On a multi-hour road trip, on the other hand, I drive as fast as I think I can get away with, because I can't bear the tedium of driving slowly for hours! In my car (a diesel VW with an instantaneous MPG gauge) that can mean the difference between 40 and 50 MPG, which is roughly the same 20% that missing was talking about - and I think that is a very significant difference.

All else being equal (i.e. if you're not downshifting into a less efficient gear/RPM range), slowing down will always save you fuel. It's a simple law of physics - the resistance from aerodynamic drag, *regardless* of how low your drag coefficient is, will always increase as the CUBE of your speed. It's an exponential curve that quickly approaches vertical as you accellerate.

As for lowering the speed limit, it's a pointless idea. Experience has shown that people will drive as fast as they want to, as long as they think they can get away with it. If the government wants to encourage people to save fuel, they should raise the fuel tax to give an economic incentive.

Skep - I'm with you 100%. If we want to save fuel *and* improve safety *AND* improve public transit, lets get some of the idiots with no driving skills off the road!

I am currently living in a 75 MPH state. Here is the problem: About 20% of the people have slowed down to around 65-70 MPH to save gas, the other 80% are still going 75-80 MPH. Do I need say how dangerous this has become?

I generally try to go 55 on the highways anyways, regardless. Been doing that for about four years now. I have only been pulled over for it once--that was spring of last year in Montana while en route to North Dakota. (I never thought it possible you could get nabbed in MT for going *under* the speed limit, but it turns out you can.)

Paulwalker, I try to avoid the stretch of I5 between the Hazel Dell interchange and Longview (the "dead zone") if at all possible, for that very reason. The speed limit along there is 70, trucks 60. Hell, I even sometimes feel terrified driving that stretch even when I am going the speed limit! Lotta semi trucks and RVs travel that stretch, and my insurance is already high enough without being clobbered by one of those big rigs.

Nope, I am not going to take my chances!! My last job involved doing courier runs out to Woodland on a three-times-weekly basis and ten months of that was way more than enough for me. That, of course, was when gas was still in the sub-$2.00 range.

Incidentally I have an older truck (87 Ford Bronco.) 55 and under on the highway, I usually can get around 35MPG. Unfortunately travelling to/from Pendleton or G-dale via 84 can take forever.........

Motozak2, that doesn't surprise me about being pulled over for going too slow in Montana. Afterall, that was a State that had NO speed limit for a time in the 90's!

BTW, the highest speed limit in the US currently is in parts of Texas where it is 80.

For those of you who have posted "I pay for the gas so let me go as fast as I can", I'm sorry, but you don't just pay for the gas.

Once you exceed a ceratin speed your RPM's increase to burn more fuel per mile. In addition as gas prices have gone up the same lot of you then want to pay less in Income and property taxes to offset your higher expenses.

Conservation with a lower speed and less useage of gasoline by driving much less will cause the gas prices to drop.

The past week the prices didn't drop because of Bush talking about drilling offshore, but the economy is going in the toilet and people are driving substancially less, just in May, 2008 driving was reduced by more than 1 Billion miles in the US.

In the last months of Bush Corporate America is hyper greedy knowing that the GOP will protect them. However some polls in the country are starting to show that the Democrats might win 7 to 9 Senate Seats and the oil companies are getting scared and lowering the price of gas to make the Presidential Race stays closer and reduce the GOP loses in the Senate.

Screw the idiot that proposed this.

He obviously doesn't drive. Probably flys around in a taxpayer funded jet.

The SOLE reason to decrease the speed limit is to INCREASE revenue for the states. Speeders = tickets = revenue.

Why do you think Oregon has a 65 mph speed limit when every stare around it has a 70 mph limit? Hint; think revenue.

If the speed limit is reduced to 55 mph, it will add nearly 3 hours to a 10 hour trip at 70 mph.

Leave the speed limit the hell alone.

If you want to voluntarily slow down to save yourself gas and money, more power to you. Just say in the right lane and let those of us that want to get where we are going pass you or at least get out of the way when someone is coming up behind you at a rate of speed faster than yours.

Speed limit +9, or if someone passes me when I'm doing +9, then I'm going to keep up with them!

When I'm on a long trip I mostly don't care what my mileage is.

I'm driving down to Livermore, CA again next week. When I did that drive 3 weeks ago, it was 646 miles from my house to the hotel. It took me 10.5 hours.

This time I am going to pull a trailer down there so automatically I'm limited to 55 mph (64 actually) so I figure it's going to take me 14 hours or so. I'm still going to do it in 1 shot, but I'm going to be cussing the traffic managers the whole way!

If it means driving 3+ extra hours forget it.

Somebody needs to whack the senator who proposed 55 mph upside the head with a boat paddle.

Tadc, what I am saying is that the savings is NOT 10 to 20 percent in a lot of cars, and yes that 10 to 20 percent isn't worth saving financially to a whole lot of people.

You are right about simple laws of physics. However, the power / fuel consumption curve is not a linear one. Producing a fraction of the engine horsepower is very efficient. Getting a larger fraction becomes less efficient, and nearing the full potential is downright wasteful.

I recently rented some U-haul vehicles. The very largest one consumed a LOT of gas. The thing got maybe 9 MPG peak. Very costly to drive. On a 40 mile trip, driving 50 to 55 saved about 15 dollars! Worth it, given the overall cost of the rental was about 50/50 fuel / vehicle use charges. Total rental fees were about $100, so that's a nice gain.

I buy that, it's safe and I'll do it every single time I'm driving a big truck. No brainer.

The Cobalt sits in the middle of this. Overall, the car is kind of crappy in that it's damn near impossible to drive it while consuming fuel at a rate more than 35MPG. However, driving it at 55 vs 65 saves a few dollars over the life of the tank. It's just not that much, given the overall time and safety burden. Really, my only complaint here is that this car underperforms given it's technology level. I know, given some consieration, that thing, essentially unchanged, but with better parameters chosen, would get that same 42MPG my old corolla does.


Paulwalker brings up a good point! Traffic is best when consistent. For a lot of people time is money, and I'm gonna just put it out there: 55 is a hard speed in most cars. They WANT to go about 65 --as if they are built for it, and they probably are. PITA to spend so much mental energy regulating speed. Better to focus that energy on the traffic scene and have a better / safer drive.

There is a good case for the newer car to be safer, and I think I would agree. That's a personal choice of mine though. I'll just compensate by driving smarter, and call it good.

Finally, the corolla is a complete waste of time at 55. The difference in consumption is not significant in that I average 400 miles per tank doing the 55 deal, and I still average 400 miles per tank doing 65 - 70, which is my norm.

I can notice the difference between the corolla and the newer cobalt. Over the tank, it all equates to 5-10 dollars between the two. Not enough money to force people to choose a new car, because the return on that new investment would span many years, but also just lame ass wasteful. The cars could easily be more equal, and that's a fuel waste we just built in to what should be a gas saving car. Bummer.

So my point is, driving the 55 does not get me anything that matters! And I think this is true for a very large set of drivers.

Physics does demand that more fuel be consumed at the higher speed. However, what a lot of people don't realize is the non-linear power curve of your engine may well come into play, skewing what should be a simple thing.

For a lot of cars I've recently driven, it's the same deal. I need to get something, if the speed regulation is justified, and I don't get anything!

I measure my driving and take notes on gas purchased. Know what really saves the dollars and fuel?

It's not 55 on the freeway, but all the stupid little trips I didn't need to take, and as Alfredo mentioned, idling. Cars idle away about a gallon per hour! Add that up every day, and now maybe those electric conversions make some sense!

Just think if you could be charging a battery while driving with gas, hit traffic, kill the engine and bump the car along, until using the gas makes sense again! I know I could cut my fuel bill significantly doing that.

Sadly, cutting it significantly still does not warrant the expensive hybrids we have right now. Dollars per mile is still way cheaper in my current car, so that's what I'm driving as dollars per mile over the long time I drive is what matters to me.

I would pay for a conversion that did this. If it got me 500 miles for that 10 gallon tank, that's about $500 per year savings at current prices for me. Could double that if prices continue to rise.

For me, dollars per mile is currently about $0.12! Cheap ass. Really hard to get cheaper actually. So, all those dollars I don't pay to transport can go to other things. All good.

That's also why I mentioned SUV's. In the bigger cars, the difference in fuel consumption ends up being significant. It's gonna be pretty easy to drive the 55, pay at the pump, do some math at the end of the month and see what that 55 got you.

Bet it's a nice dinner. That's something.

The savings I get in the corolla doing 55, is about a nice fountain pop and pepperoni stick! Not worth it.

Too many of our cars will perform well enough over 55 as to not impact fuel consumption, so I'm not in favor of the regulation for that reason.

As for our overall consumption of fuel, I'm gonna be a bit radical and propose that if we use significantly less, then China and others will just use more.

Better to spend our dollars right now, making better cars and building out alternatives as quickly as we can. BTW, doing those things takes oil in large quantities.

So, another way to look at this regulation then is what would we be getting nationally? If we just consume less, and don't put that oil savings into our technology investments to reduce dependance, then we get NOTHING, because China and others will consume more, and leverage that to their advantage, not ours.

On the other hand, say we really cut down on transportation fuel consumption, and ramp up consumption for build outs and investment, we are getting something for that effort and that something is significant relative to what our competing nations are getting.

We are being very, very poor competitors right now, where improving our selves and building real wealth is concerned. We just don't make stuff, and that failure to innovate and build off our labor is hurting our dollar.

That lower dollar, BTW, seriously impacts our perception of how bad the fuel costs are.

Put people to work building wealth, see the dollar improve, watch the perception of how hard it is to buy a gallon of gas drop. It's that simple as the value of that gallon of gas has not changed much. What we buy it with has!

Finally, there is the psychological impact of a 55 speed limit. I HATE this kind of crap. It's obvious we need to build out those things that get us off of oil. I don't need additional pressure in the form of regulation that does not actually deliver anything toward that goal, which I think this regulation is.

So yeah, just consuming less fuel, no matter what, is a feel good thing that does us no good. It's either got to equate to real dollars that can be measured against time, and our ability to apply the fuel to other purposes (opportunity gains), or it's bunk.

Maybe I'm a bit conservative on this, but I gotta see the dollars matter, or it's just not worth regulating.

For the record . . . Even I think 55 is too slow. I think 60 is the sweet spot for today's vehicles.