Milage claims

Here in my town there is quite a battle brewing about the lowering of the speed limits around town. One of the claims going around is that driveing 35 mph verses 40 mph (going slower)is causeing a increase of gas consumption by 15 to 20% I for one find this claim totaly outrages. I am willing to bet that there is no difference in milage by driveing 5mph slower. Any comments?

Gas mileage will be better at the slower speeds, but I don’t think you are looking at anything close to 15 to 20% lower. If you are talking about reductions from 70 or 75 mph down to 60 perhaps you’d get those kinds of numbers.

Wind resistance is the biggest factor in mpg especially as speeds increase. Wind resistance would be a bit less at 35 vs 40 mph but not very much. If the town wants to really save fuel they should invest in traffic signal coordination to reduce sitting and idling at traffic lights. Of course this would be a significant expenditure. New signs are cheap and there will be increased revenues from the greater number of traffic tickets and greater fines to be collected. Slower speeds might contribute to safety and that would be a better rational than fuel savings.

This is a local political issue. Id ask the supports of this initiate for their scientific proof behind the fuel savings they are predicting with the lower speeds.

A 15%-20% increase in mileage for a 5 MPH decrease in speed is a very unrealistic estimate IMHO.

“A 15%-20% increase in mileage for a 5 MPH decrease in speed is a very unrealistic estimate IMHO.”

Maybe at a higher speed, but 40 MPH is 14% faster than 35 MPH. Wind resistance is linearly related to the square of velocity, so the drag at 40 MPH is actually 30% higher than at 35 MPH. So, there should be a measurable change in fuel consumption if the car remains in the same gear at 30 and 40 MPH. The 15% to 20% figure quoted by the OP would not be at all surprising.

Political Correctness for increased revenue is my guess.

Fat chance. It is possible thqt there may be a change, but they are being very The actural results will vary with the car driver and trafic conditions. It is even possible for the reduction to increase fuel consuption.

If this is important to you, get the discussion back on safety - especially for pedestrians and bike riders. I would make a point of challenging only if it becomes an issue swaying decision makers. Not everyone on the City Council can be expected to understand this is just plain nuts.

Sorry, I may have miss lead everyone. Those that are against the reduction in the speed limit are claiming that the slower speed 35 mph is increaseing their gas consumtion. In other words they claim a reduction in their milage by 15 to 20%
(If you get 20 mpg at 40 mph, the slower speed, 35 mph would be 16 mpg) I just aint convinced. Sorry for any confusion.

The stop lights and stop signs along with slowing/accelerating will kill any fuel savings. Could it be that this is intended as a traffic ticket increase?

At these speeds, it’s tire rolling resistance, not air resistance that dominates and tire rolling resistance tends to be linear as far as I know.
The real gas mileage killer in urban driving is the stop signs and lights. Getting rid of 4-way stop signs, replacing stop signs with yield signs, and syncronizing the lights to the traffic will really save gas.
The main effect of the lower speed limit in stop and go traffic is that it only takes about 76% as much energy to accelerate a car to 35 mph as it does to accelerate it to 40 mph. The only thing worse than 40 mph stop and go traffic is 50, 55, and 60 mph stop and go traffic.

The reason you get better gas mileage on the highway is because you are not constantly stopping and re-accelerating, not because you are going faster. You will be amazed at the gas mileage you can get at a steady 35 mph on the highway.

The slower speeds will not increase gas mileage much, and certainly will not decrease gas mileage at all. A decrase in gas mileage defy’s logic.

This is not entirely true. As you keep getting slower, at some speed (probably less than 35), your mileage will drop. The reason is that as the amount of drive power is reduced, the percentage of parasitic power increases. This is the power used to drive the AC, water pump, fan, and so on. For example, the car will get 0 MPG when going 0 MPH. It will do better at 1 MPH, but not as good as it would at 35 MPH. What is the speed that will give the peek MPG? It won’t be 1 MPH, but it might be less than 40 MPH. It will depend somewhat on the car in question.

Fully agree.

a car uses more gas, when its going 35 MPH at 3000 RPM, in 2nd, then a car going 40, in 3rd, at 1500 RPM.

Agree; the difference in gas mileage will be hard to measure.

Again. Any difference isn’t worth spit and you are being distracted from the issue.
What speeds are safe for drivers, pedestrians and bike riders in your town? The City or County engineer needs to step up and make a statement. He or she knows exactly what the various speed limits should be based on traffic counts, signaling, lioghting, road designs, etc. This is NOT a discussion about how city parks should be used. Why are citizens sticking their nose into something like this? Grow up and get real.

Where is it written in stone that a car must be in 2nd gear at 35 mph? Your premise is a strawman.
If I’m no longer accelerating, my car is in 4th or 5th at 35 mph.

That’s ALWAYS what it is.

I agree with this. We have two cars with readouts that show instant fuel mileage. 40 is better than 25 but I have not checked 35 vs 40. I suspect that the difference will be small, if any.

This is the situation in the Delaware, Ohio area of state route 23. The speed limit for most of the area is 50 to 65 mph. However, with the numerous stop lights in the area, it is very difficult to maintain, let alone achieve, these speeds in several places. Heading south from Marion(or Findlay), once you reach route 229(first stop light going south), it’s over. As you get closer to Columbus, the traffic lights increase.