Found another way to save on gas.

[krousdb]

Originally Posted by Compaq888

oh well. Let me ask you this.. I have Kumho 716's all season. If I were to raise pressure to 44psi would they take it??? right now i'm at 40psi.

I have 50 in my Kumho 795's. The max pressure rating is 35. I thought you said earlier that yours had a 44 PSI max. If so, they can take 44 it no problem.

[Compaq888]

ok, I will bump them up to 44 then. I was at 35 and I got no improvement in gas mileage. I went to 37 and got like 2-3mpg improvement, then I went to 40 and got a 3-4mpg improvement. Hopefully at 44psi I get a 4-5mpg from stock. I'm talking about street mpg wise. I went from 20mpg to 22.5mpg when I had 37psi in the tires.

How many mpgs did you guys gain from inflating the tires more???

[SVOboy]

How many mpgs did you guys gain from inflating the tires more???

I added 15 and got no perceptible difference in my suburban driving, which is consistent with the numbers posted around this site relating to drag on the wheels. Most improvement is on the highway but I drive no highway so I figure I'll never notice the difference though it prolly exists.

[Compaq888]

I drive mostly on the street and I notice a difference. My fuel gauge moves a lot slower and I refuel now after 400 miles. Before I had to refuel at around 350miles or I would run out of fuel.

[SVOboy]

I'd call that a placebo affect. The reduction in drag is minimal especially between 35 and 37.

[Compaq888]

Call it the whatever effect, lol. I'm at 50miles now and my gauge is nowhere close the 3/4 mark. Before at 50 miles it would hit the 3/4 mark. My gauge right now is between the 3/4 mark and full. I seriously doubt this is the placebo effect otherwise I got to start suing every gas station owner for ripping me off the last few months. 20+ fuel receipts don't lie.

[JanGeo]

I usually get about 90 miles before the needle gets down to the full mark, at 3/4 I have about 150-175 miles, half about 210-250 miles.

[Compaq888]

Originally Posted by JanGeo

I usually get about 90 miles before the needle gets down to the full mark, at 3/4 I have about 150-175 miles, half about 210-250 miles.

One time my needle hit the full mark at 92miles but it was all highway and very flat. The test I did for matt killed my mpg quick because I went up some steep mountains. It's really hard to keep my car in overdrive because it's heavy so going up the mountain was very hard and it went to 3rd gear sometimes.

[MetroMPG]

hate to revive this old debate, but i believe i have found some solid support for the issue of PSI and tire diameter/circumference.

more air pressure DOES slightly change the diameter of a tire. this difference is the basis of operation of some tire pressure monitoring systems, which detect pressure discrepancies based on input from the ABS wheel sensors (looking for different rotational speeds of the tires).

Taken from http://www.aa1car.com/library/tire_monitors.htm:

The other method of monitoring tire pressure does not require any additional hardware inside or outside the wheel. It is the "indirect" method. This approach makes use of the existing wheel speed sensors in the antilock brake (ABS) system. By modifying the ABS operating software, the ABS system learns the average speed at which each wheel rotates when traveling straight at a constant velocity, and detects low tires by comparing relative wheel speeds. The auto makers like this approach because it is a simple, cost-effective way to monitor tire pressure. But it has some limitations.

One is that it can't detect a low tire if two tires on the same axle are low, if two tires on the same side are low, or if all four tires are low by an equal amount. But it can detect one or more low tire if the rest are at or near the recommended inflation pressure.

Here's how the indirect ABS-based TPMS works. If a tire loses air, the diameter of the tire shrinks slightly causing it to turn at a slightly faster rate than the others. But on most applications, a tire has to loose anywhere from 8 to 14 pounds of air pressure before there's enough of a difference in diameter to be detected by the wheel speed sensors. It depends on the type of tires, tire diameter and aspect ratio, and the sensitivity of the ABS system.

Low profile tires with short stiff sidewalls change diameter less than tires with taller aspect ratios and more compliant sidewalls. Consequently, a loss of 10 psi in a low profile tire may only change the diameter of the tire less than 1 mm (0.040 inches). Most ABS systems cannot detect changes smaller than 1 mm. For this reason, indirect ABS-based tire pressure monitoring systems are not as sensitive as direct TPMS systems that have a pressure sensor inside each wheel.

ABS-based systems must also relearn the rotational "signature" of each tire when tires are replaced, repaired or rotated. This requires the driver to push a reset button on the dash or to follow a menu on the driver information display. It's also essential that the tires all be properly inflated before the recalibration procedure takes place -- otherwise the module won't recalibrate correctly and may not detect a low tire.

[krousdb]

Originally Posted by MetroMPG

hate to revive this old debate, but i believe i have found some solid support for the issue of PSI and tire diameter/circumference.

more air pressure DOES slightly change the diameter of a tire.

Hmmm. I'm not convinced that you can make that conclusion from the source you quoted. I would have agreed with you if you had said:
"more air pressure added to an underinflated tire DOES slightly change the diameter of a tire." I agree because the steel belts and sidewalls may not be fully tensioned when underinflated. But i see nothing in the quote that says that adding air pressure to a properly inflated tire changes the diameter. I know that i am splitting hairs here but I like to do that.