Questions about how to get better mileage.

[scr4]

I don’t see why you guys are so critical of Rick’s result. Overcoming air resistance at 75 mph requires 150% more power (i.e. 2.5 times more) than at 55 mph. OK, you also get there 36% faster, but even then you’ve used up 85% more energy for the whole trip.

[Rick]

@AskNott

Rick, although going slower usually is more fuel-effecient, your spare tire example is flawed, if the spare was on a driven wheel. The space saver spare is smaller in diameter than your original tire, so your speedometer and odometer were off. You were going slower than 55 mph, and you drove less distance than the odometer said you drove.

BZZT not.
The spare was on a rear wheel. The speedo calculation on a Volvo is done by ABS control module. The ABS averages the speed of the two front tires, and then broadcasts this info on the computer net. So no, my speedo was not off.
Secondly, the OD of the space saver spare is within millimeters of what the factory tire is, the space saving is in the width, and weight.

@kanicbird

Some donuts have the same outer diameter as the OEM tires, this is usually on cars that need them to prevent drivetrain dammage. This would increase MPG, again due to less rolling resistance. If they are smaller they basically change the gear ratio and may end up decreasing it.

Rick in a word, nobody buys a car because of the sole reason of mpg. Using 4 donuts would cause harsh handling, fast tire ware, poor ground clearence and a host of other disadvantages, but will increase mpg and also increase acceleration in most cars that can’t spin their wheels when floored.

I didn’t say that gas mileage was the sole reason people bought cars, go back and read what I wrote. Here I will save the trip upthread

If two cars are equal, the one with better gas mileage will sell better.

But hey, I will go with gas mileage as being the single most important item on some car buyer’s list. Want proof? Try this
If you believe that gas mileage does not sell cars that explain why Toyota Dealers have waiting lists for Prius and Ford Expeditions are sitting on the lot?
Hint, it’s not because the Prius is bigger, or faster, or carries more. Gee what does a Prius do better than an Expedition? Could it be gas mileage?
As far as my contention that slowing down saves gas, scr4 got it right, there is one hell of a lot less air to push out of the way at 55 then at 75.
And since we are in GQ
cite

Avoid driving at high speeds. Gasoline powered cars operate with maximum efficiency in the highest gear at the lowest speed in that highest gear without engine lugging. This effect is largely due to aerodynamic drag. In highway driving over 80-90 km/h (50-55 mph) (depending on the car’s aerodynamics; low for small cars and high for SUV, trucks), the aerodynamic drag will rise sharply and so rises the fuel consumption.

another cite

But the bottom line is, if you double your speed, this equation says that you will increase the power required by much more than double. A hypothetical medium sized SUV that requires 20 horsepower at 50 mph might require 100 horsepower at 100 mph.

and a final cite

While each vehicle reaches its optimal fuel economy at a different speed (or range of speeds), gas mileage usually decreases rapidly at speeds above 60 mph.

As a rule of thumb, you can assume that each 5 mph you drive over 60 mph is like paying an additional $0.21 per gallon for gas.

If you are going to continue to insist that my observed real world gas mileage improvement is due to my having a spare tire mounted and was not a result of a lower speed, all I can say is when come back bring cite.

[Declan]

@metj

I don’t understand this one - why not?

Just a wag , but I think it has to do with conservation of energy. Starting from a standing stop , it might take an ounce of gasoline to get the car up to highway speed ,but teaspoons to keep moving at highway speed. Reduce the pressure on the gas pedal and you start to slow down, then re-apply a greater amount of pressure to regain your previous speed and the car only uses say a ticture of gasoline.

Using the brake would cause the car to slow to a point where your back to using an ounce of gasoline to regain your speed, do this enough times and your wasting gasoline.

Declan

[kanicbird]

Rick Your cites don’t prove that anyone would buy a new car w/ 4 donuts.

You said it yourself, if all other things are equal someone would tend to buy car w/ better mpg, but using 4 donuts on one car and 4 standard tires makes things unequal, so your statement invalidates itself.

By your own words you have not performed a valid test, the only cite I need is this thread.

Questions about how to get better mileage.

Also there are low rolling resistant tires which are better then donuts but don’t seem to really sell on new cars as OEM.

[Rick]

I never said that anyone would buy a car with 4 donuts, what I said was [making fun of the stupid statement that a 6mpg improvement in gas mileage was totally due to one count it one donut spare tire]

If you could jump the gas mileage from 22 to 28 by changing one tire, then why wouldn’t the car maker put four of those tires on the car and then have an SUV that got 46MPG? I guarentee that if you could build a SUV the size of an XC 90 Volvo that got 46 MPG, you would not be able to keep any of them on the lot.

If you believe that you could get Prius type gas mileage out of an XC 90 by simply changing the tires, and yet the car makers don’t for whatever reason, then your tin foil must be slipping. Dude get a grip. There are not four tires on this planet anywhere that will allow you to get 46 mpg in an XC90. If there was then at least some (read every single one) carmakers would be lining up to put them as OE on their cars and trucks. This is for the simple reason that that great a change in gas mileage would be an unbeatable sales advantage. Anybody that did not put these wonder tires on, would have a hell of a time getting anybody to buy their sleds.
Oh and cute trick of citing this thread :wally
FTR let’s look at some of the comments in this thread.

@Muad'Dib (OP)

I know that going 55mph will use less gas then going 75mph

Plus comments by Tuckerfan, Chaparral v8, Mr. Slant, and scr4 that slower = better gas mileage. Then on the it’s all due to rolling resistance we have you. Your “cite” isn’t looking too strong here.
Now I agree that maybe, just maybe my spare tire has a lower rolling resistance. So in that 6 mpg increase maybe 0.1% is due to a different tire, maybe it was 0.2%, but no way it hell 100% of the observed gas mileage increase is due to lower rolling resistance. Hell let’s go crazy and say that the rolling resistance contributed 10 times more to the improved gas mileage than I think it did. That means that the improvement due to the one tire was 1% of the measured improvement in fuel mileage (6mpg). for a whopping increase due to that one tire of 0.06mpg. Whoop-de-fucking-do.