Lower Speed Limits, Save Fuel? Michigan Thinks So! Will Others Follow?

[Sniffs]

I think if people would drive at the "scrooge point" for their vehicles, and have a limiter SET there...but of course, Americans would automatically hack the car computer to remove it.

the "scrooge point" is, apparently, where the horsepower and torque curves cross. what I've seen, however, it that it's ALWAYS at the red-line...or even past it I've seen a couple curves that NEVER cross

 

[Famine]

Horsepower and torque curves cross at exactly 5,252rpm in every car ever. In my car, that'd be a hair under 100mph in top gear.

 

[Sureboss]

It certainly does vary with the size of car and engine - remember the TG Prius/M3 test from this season's opener? At the same average speed, the M3 was the more economical car...

If anything the UK National Speed Limit needs raising to 80mph - perhaps even 85mph.

Everyone drives at 80 anyway, don't they? Barring M1 and M25 obviously.

 

[nick09]

My dad uses the cruise control when he takes a drive on the highway in the Jetta '01.

Honestly, why do you drive that fast if you want to conserve gas?

 

[Famine]

Why use cruise control if you want to conserve gas?

 

[wfooshee]

My car gets 32mpg at 100 mph, and 43 at 70mph. If I had another gear (for fuel conservation) im sure at 100mph I could get 37mpg.

Maybe not. The force required to push air out of the way goes up as about the square of the speed difference, not linearly. Having a gear to reduce revs doesn't account for the fact that you still need nearly 3 times the energy to hold 100 mph compared to 70. Even with reduced revs, you'll have to hold a higher throttle setting to feed enough fuel to shove that air aside.

When you get to those speeds, dropping a few hundred RPM doesn't necessarily do what you think it will do. Many "high performance" street cars reach aerodynamically limited top speed in the next-to-highest gear simply because they can't move the air in the taller gear. But at normal highway speeds they get better mileage in the taller gear. Dunno about 100+ mph.

Has to do with the 5250 RPM point Famine mentioned above. Very loosely speaking (and there was a HUGE thread on this a couple years ago) you need horsepower to shove air, and horsepower is a function of revs. Drop the revs too far and you don't have the power to go that fast any more.

How much less fuel do you use though? I mean if you are going 55mph it will take longer to get somewhere meaning the engine will have to run for a longer period. I don't know the maths but I would be curious to see how it works out.

Time is irrelevant. The engine doesn't burn "extra" fuel by running longer. If it gets 25 miles per gallon, then you used 4 gallons to get 100 miles, no matter how long it took. If you slow down and get 33 miles per gallon, then it only took 3 gallons, even though it might take an hour or more longer.

It might be less confusing if we measured fuel as consumed, rather than engine efficiency.

In other words, instead of saying, "I get 35 miles per gallon in town on my motorcycle and 42 on the highway," I could say, "My motorcycle uses 2.86 gallons per hundred miles in town and 2.38 on the highway." Americans use MPG because it's a bigger number that's better, which is how we think. It's also more impressive to say "42 up from 35" instead of saying "half a gallon less over a hundred miles." The former sounds like a bigger difference.

Another example of how consumption (fuel per distance) is more realistic than efficiency (distance per fuel) is the following, which I'm stealing from a Car and Driver column several years ago, I think by Patrick Bedard.

Which saves more fuel, improving an SUV from 14 to 15 miles per gallon, or a compact sedan from 41 to 51 miles per gallon? The answer will surprise you if you bother to work it out.

OK, I'll save you the trouble of doing all the reciprocals and unit conversions. Both changes improve consumption by just a tick under half a gallon per hundred miles, actually .47 or so. EXACTLY THE SAME IMPROVEMENT IN ACTUAL GAS SAVED!

Obviously the compact car is more efficient (distance per fuel) in the first place, but to save the same quantity of gas (fuel per distance) it has to become way more efficient.




Now, as for the original thread topic, lowering speed limits to save fuel. That's what Nixon did in the '70's fuel crisis, mandated a 55 mph limit on the nation's highways, even though speed limits are state's rights, not federal. He did it by blackmailing the states with federal highway funds. State of {insert your state's name here}, if you don't demonstrate enforcement of 55, you don't get highway money.

Then when the fuel crisis was over, the safety nazis took over, saying, "Hey, look! Highway deaths are down! We have to leave the speed limit slower." It took decades to overcome that, and ya know what? The carnage never came. Highway deaths are still dropping measured as passenger miles per death. Maybe drunk driving enforcement, better cars, tires, and brakes, things like that.

So Michigan's governor apparently hopes we don't know our recent history. Yeah, if I want to save gas, I'll go slower. But look, be-otch, my time has value, too, and if I'm billing at 125 bucks an hour to get to a customer site, they want me there now, not an hour from now. It ain't your place to tell me where that value point is.

 

[Touring Mars]

But look, be-otch, my time has value, too

Brilliant post 👍

 

[Gil]

I wonder how long till someone plays the lower speed limits are safer card.

Lower speed limits are NOT safer. Unless EVERYONE is going within about 10-15 MPH of each other.
Less variation between speeds is safer.
When the speed limit was 55, and some were obeying that, and quite a few were going 75 and 80, you get a 25MPH difference in speeds between cars. That will not only contribute to unsafe conditions, it also causes "accordian effect" slowing down the whole shebang, and creating traffic jams.

As for driving slower saving gas, it's true. You can see as much as a 15% gain in fuel economy.

However, one of my sons drives a Mazda3, rated at 32 MPG highway. He routinely pulls down 37 MPG, and the car doesn't even have 10,000 miles on it yet. I expect that figure to go up.

He drives at a reasonable speed (65 MPH), does his best to hit all lights when they are green, and to use his brakes as sparingly as possible.

65 MPH is accepted as the highest speed you can travel before you are exponentially fighting wind resistance compared with travelling 55 MPH.

If you use all "hypermiler's" tricks you could get 2 to 3 times the rated MPG for your car. You would also be miserable from driving too damn slow, Not to mention a hazard on most freeways.

BTW, hypermilers do not condone "drafting" to pick up a few extra MPG.

 

[Tornado]

Lower speed limits are NOT safer.

I think that was his point.

 

[Gil]

See, Joe and I have a unique relationship.
I know that he was saying that eventually politicians will bring back that old saw that got America driving 55 (and Sammy so pissed about it).
I was just whipping out the NHTSA study facts.
He says stuff and I bring the proof.
Or the other way around.👍

 

[Joey D]

That's a big 10-4 Gil!

On my trip this weekend the average speed was easily 95 on the highway with runs up to 110mph. We blew past Michigan State Police at 90 and they never even flinched. I agree that as long as everyone is going about the same speed it's much safer then one or two people going way over or way under the limit. This just furthers my belief Michiganders will not take kindly to a drop in speed limit. If you want to save fuel, do 55mph on back highways.

 

[V1P3R]

95!? wow... you do that here and that will be a GUARANTEED speeding ticket. plus some other stuff

People here go 80 - 85 but that's pushin it.

 

[Joey D]

The only way you get a ticket here is if you are doing 15 over the flow of traffic, weaving dangerously, or driving recklessly. We all know and most of us obey it.

 

[wfooshee]

Then there is a selective enforcement problem. They don't need to drop the limit, they need to enforce the limit in place a little bit more strictly.

If that's really the average speed up there, do trucks keep up with it, or is there a severe (15-20 mph) difference between cars and trucks? I'm gonna speculate that they would have a bit of trouble justifying much over 75 or 80, as the fuel cost for them goes up just too much past that.

So much for everyone being close to the same speed. . . . .

 

[Joey D]

Then there is a selective enforcement problem. They don't need to drop the limit, they need to enforce the limit in place a little bit more strictly.

If that's really the average speed up there, do trucks keep up with it, or is there a severe (15-20 mph) difference between cars and trucks? I'm gonna speculate that they would have a bit of trouble justifying much over 75 or 80, as the fuel cost for them goes up just too much past that.

So much for everyone being close to the same speed. . . . .

No they don't need to enforce the limit more strictly. And yes semi trucks easily go 80-85mph and they stay in the right lane (if they are caught in the left it's a ticket, no questions asked). Most of our freeways where any great deal of people would be are 3-5 lanes in one direction, the only time it's 2 lanes in each direction is in the boondocks.

People who aren't from Michigan don't really know what to make of us up here and why we hate out of state drivers with a passion. Our traffic laws are enforced but only when they matter. If you do any more then 5 over in a construction zone you will be fined (double actually) and will be written for the speed you were doing. Do anything over the speed limit in a school zone and you will more of less have your ass handed to you. Actually in most city driving you can't stray to far from the speed limit or you will be fined because it is dangerous.

The government shouldn't force us to conserve fuel, if we chose to drive the way we do and if our economy drops greatly at higher speeds then so be it. I still averaged 36mpg this weekend with about 60% 90-100mph highway driving, sure I would have gotten 40mpg with 55mph driving but the trip would have take almost twice as long and where is the fun in that? Not to mention I would have caused dangerous driving conditions on I-69 by doing so.

Honestly the limit here should be 90 with a minimum of 70 and all vehicles should have to keep right unless passing. You would have less slowing down, less wasted time, and it would be completely safe. If you want to conserve fuel, drive back roads where the limit is still 55...but we go 70 anyways.

 

[niky]

Maybe not. The force required to push air out of the way goes up as about the square of the speed difference, not linearly. Having a gear to reduce revs doesn't account for the fact that you still need nearly 3 times the energy to hold 100 mph compared to 70. Even with reduced revs, you'll have to hold a higher throttle setting to feed enough fuel to shove that air aside.

When you get to those speeds, dropping a few hundred RPM doesn't necessarily do what you think it will do. Many "high performance" street cars reach aerodynamically limited top speed in the next-to-highest gear simply because they can't move the air in the taller gear. But at normal highway speeds they get better mileage in the taller gear. Dunno about 100+ mph.

Has to do with the 5250 RPM point Famine mentioned above. Very loosely speaking (and there was a HUGE thread on this a couple years ago) you need horsepower to shove air, and horsepower is a function of revs. Drop the revs too far and you don't have the power to go that fast any more.



Time is irrelevant. The engine doesn't burn "extra" fuel by running longer. If it gets 25 miles per gallon, then you used 4 gallons to get 100 miles, no matter how long it took. If you slow down and get 33 miles per gallon, then it only took 3 gallons, even though it might take an hour or more longer.

It might be less confusing if we measured fuel as consumed, rather than engine efficiency.

In other words, instead of saying, "I get 35 miles per gallon in town on my motorcycle and 42 on the highway," I could say, "My motorcycle uses 2.86 gallons per hundred miles in town and 2.38 on the highway." Americans use MPG because it's a bigger number that's better, which is how we think. It's also more impressive to say "42 up from 35" instead of saying "half a gallon less over a hundred miles." The former sounds like a bigger difference.

Another example of how consumption (fuel per distance) is more realistic than efficiency (distance per fuel) is the following, which I'm stealing from a Car and Driver column several years ago, I think by Patrick Bedard.

Which saves more fuel, improving an SUV from 14 to 15 miles per gallon, or a compact sedan from 41 to 51 miles per gallon? The answer will surprise you if you bother to work it out.

OK, I'll save you the trouble of doing all the reciprocals and unit conversions. Both changes improve consumption by just a tick under half a gallon per hundred miles, actually .47 or so. EXACTLY THE SAME IMPROVEMENT IN ACTUAL GAS SAVED!

Obviously the compact car is more efficient (distance per fuel) in the first place, but to save the same quantity of gas (fuel per distance) it has to become way more efficient.




Now, as for the original thread topic, lowering speed limits to save fuel. That's what Nixon did in the '70's fuel crisis, mandated a 55 mph limit on the nation's highways, even though speed limits are state's rights, not federal. He did it by blackmailing the states with federal highway funds. State of {insert your state's name here}, if you don't demonstrate enforcement of 55, you don't get highway money.

Then when the fuel crisis was over, the safety nazis took over, saying, "Hey, look! Highway deaths are down! We have to leave the speed limit slower." It took decades to overcome that, and ya know what? The carnage never came. Highway deaths are still dropping measured as passenger miles per death. Maybe drunk driving enforcement, better cars, tires, and brakes, things like that.

So Michigan's governor apparently hopes we don't know our recent history. Yeah, if I want to save gas, I'll go slower. But look, be-otch, my time has value, too, and if I'm billing at 125 bucks an hour to get to a customer site, they want me there now, not an hour from now. It ain't your place to tell me where that value point is.

invisible +rep. Well said.

 

[ilovethescudo]

According to the news, pain at the pump has been saving lives, via fewer fatal accidents. People have been driving less.

 

[Joey D]

According to the news, pain at the pump has been saving lives, via fewer fatal accidents. People have been driving less.

That would make sense, the less you drive the less chance you have of an accident.

 

[Dave A]

Ofcourse on the original subject, you could always lower the speed limit to 0mph. You would prevent a lot of road accidents and save a hell of a lot of fuel both at the same time.

 

[Joey D]

Ofcourse on the original subject, you could always lower the speed limit to 0mph. You would prevent a lot of road accidents and save a hell of a lot of fuel both at the same time.

You probably would see an increase of people dying from starvation though because American's can't walk anywhere...even if there were a lack of cars.

But as I've said lower speed limits don't save lives, Michigan shows that if everyone is doing 100mph you are just as save than if everyone is doing 55mph.

 

[ExigeEvan]

Ofcourse on the original subject, you could always lower the speed limit to 0mph. You would prevent a lot of road accidents and save a hell of a lot of fuel both at the same time.

Nah Dai, that's wrong. Sitting there idling the engine for an hour would give you 0 MPG, how's that more efficient?

 

[Dave A]

, great angle.

Joey, it was tongue in cheek. I'm very much against lowering speed limits on pretenses like this, they've made proposals for lower limits on saftey grounds here in the UK last year. The proposals dissapeared though, at least for now.

 

[Joey D]

Oh I realised it was tongue in cheek, hence the starvation comment.

 

[Gil]

I was reading yesterday's paper today.
It seems the sales of Motor Scooters are going thru the roof in Kansas City.
People seem to be buying them for running errands in town.
Getting 60 - 100 MPG....

 

[Joey D]

We have motor scooters around here and it drives me nuts because they feel the need to do 20mph on a 50mph road where I can't pass and they won't move over. I'm pretty sure they are not road legal as they do not have a license plate...I could be wrong though.

 

[Tornado]

I think it is funny that many people around here are starting to trade in their SUVs and trucks for full size cars that don't get any better mileage.

 

[YSSMAN]

We have motor scooters around here and it drives me nuts because they feel the need to do 20mph on a 50mph road where I can't pass and they won't move over. I'm pretty sure they are not road legal as they do not have a license plate...I could be wrong though.

They're legal. My brother's scooter is only a 50cc unit, classified as a Moped, totally road legal. The "top speed" is supposed to be 35 MPH with most of them, but every one we've come across has had the limiter removed and they'll happily cruise at 55 MPH all day. Get one with a bigger engine, you'll be doing nearly 80 MPH, and will be highway legal.

====

That being said... The "other Michigander" is here...

In all honesty, I don't think anyone in Michigan is dumb enough to fall for it again. We had the same 55 MPH rule that went out of effect in what, the late '80s or the early '90s? It never did much, and to be honest, I always recall having everyone in my family break the law and consequently law enforcement never went by the book.

I've been able to manage nearly 28 MPG on average with the Celica by keeping the car under 70 MPH and the RPMs as close to 3,000 as possible at all times. My guess is that a dip to 55 MPH may increase the fuel efficiency that I'm getting, but that's where we reach a problem.

What if we take a trip from Grand Rapids to Chicago?

180 Miles / 55 MPH = 3.3 Hours (Proposed Speed Limit)

180 Miles / 70 MPH = 2.5 Hours (Current Speed Limit)

180 Miles / 85 MPH = 2.1 Hours (Speed Usually Observed)

The time difference between limits is outrageous. I would also be under the assumption that the differences in the rate of fuel being burned would be offset by the extended time at which the fuel would be spent... However, I cannot be completely certain. Fuzzy math dictates that the differences in consumption would be minimal at best, but personally speaking, I know I'd want to spend less time in the car if at all possible. Particularly when I know that most vehicles tend to get better fuel economy in the 65-70 MPH range when out on the highway. The best average I ever had with the Jetta came after a trip to Chicago, averaging about 85 there and back, I turned in over 30 MPG as opposed to when I'd regularly average about 27 with a City/Highway combination.

 

[ROAD_DOGG33J]

I would also be under the assumption that the differences in the rate of fuel being burned would be offset by the extended time at which the fuel would be spent... However, I cannot be completely certain. Fuzzy math dictates that the differences in consumption would be minimal at best, but personally speaking, I know I'd want to spend less time in the car if at all possible.

As wfooshee explained, time is irrelevant when you consider MPG. By finding the speed that the car gets the most miles per gallon, you get the most efficient use of your gas. That's the speed that will allow you to cover the distance you are traveling with the least amount of gallons used. I would guess that's also the point after which the rate of consumption would overcome the benefit of spending less time traveling.

Check post 36 for wfooshee's explanation

 

[niky]

Yup... again, MPG =/= GPH. You're trying to approach fuel use as gallons per hour, when you should be looking at it as total gallons for the entire trip.

Trust me, driving at 45-55 will increase your fuel economy by a huge factor. 28 mpg? 32-36 is probably possible... and 40 mpg if conditions are favorable. Remember, Japanese car, Japanese gearing... and it'll be most economical at speeds your typical Japanese motorist would be driving out in the countryside, which is about 30-40 mph.

 

[Sniffs]

I don't get it.

I've kept track oF every vehicle's mileage I've ever had. you think I've broken 25? only once. and in an AMERICAN car...a Chevy to boot!! even a first year Camry wouldn't go over 25 per (because some moron blue-gooed the engine instead of putting actual gaskets in)

of course, this isn't a FLAT area, either. the only high-speed bits my way are the Interstate.