Found another way to save on gas.

[MetroMPG]

it was a couple of degrees above freezing today and i was going on a 200 km highway round trip. warm enough that the suspension was somewhat more supple; it wasn't going to completely punish me for pumping up the tires.

so i tried it. i went from about 38/36 up to 50/46. (the tires are rated for either 44 or 46, i forget which exactly).

one of the first things i noticed was the car rose up a few milimeters - almost imperceptibly - as i inflated the tires (noticeable because the top of the wheel arch was at eye level).

which reminded me of this thread.

so after the trip i went and measured the difference in effective wheel/tire circumference that 12psi makes to my tires.

- i released 12 psi while watching a mark on the front wheel arch with a ruler placed against the car

- bleeding off 12 psi dropped the wheel arch by 2 mm

- that equates to an effective 4 mm wheel/tire diameter difference (well probably not exactly, since the bottom of the tire has more "squish" in it to remove before the outside diameter would change to that measurement)

- a 4 mm diameter increase from 155/80/13 wheels & tires works out to a 0.69% increase in circumference

- i saw 53.1 mpg (US) indicated on the scangauge today. so, corrected by 0.69% it would be 53.5 mpg (53.4673) (top speed was 85 km/h)

what this tells me is (1) tire pressure does affect the speedo to a small degree (2) i bet the effect is different for different tire shapes (aspect ratios); (3) i really should calibrate my odometer. i keep on saying that, but i haven't done it yet...

final notes:

- some of the rough roads & potholes were brutal at 50 psi. i discovered today that i have a loose power wire in my cruise control installation that i didn't know about. it shut itself off about 5 different times because of some really hard bumps!

- the change in tread depth as my tires wear down (effective 10mm diameter difference) will have a much larger effect on speedo accuracy than a 12psi inflation difference (4mm dia. delta)

- i really should calibrate my speedo/odometer...

[JanGeo]

I said this before - the diameter / radius from ground to center of the tire will increase when you add more air but that will not change the circumference of the tire. All you are doing is reducing the flat spot on the tire contact patch. The tire will still have to travel the same amount if it was under inflated or over inflated to make one revolution. The steel cords do not stretch with more air pressure.

Compensating for speedo error in my ScanGauge results in an odometer difference of about 11 miles in 400 miles traveled or about the 3% that I adjusted the ScanGauge speed by.

[MetroMPG]

edit: see further down this thread for evidence that pressure DOES affect circumference

[krousdb]

Originally Posted by MetroMPG

on the tire topic - i'm sure it's been said before, but the super-high-pressure folks are probably prematurely wearing down the centre of their treads.

For the same reason that pressure does not affect tire diameter, it also does not affect tire wear. The steel belts prevent this from happening. In 2 years, 60 PSI (44 max) in the OEM Integritys on the Prius, 45k miles, perfect even wear before I changed them. Most Prius drivers struggle to get 30k with the OEM's.

[MetroMPG]

is the uneven wear warning a throwback to older tire technology? bias ply design?

[Compaq888]

I don't know what the hell you guys are talking about but the more my tire pressure goes up the better my gas mileage. I now get for the street what I got last year for mixed driving. I get for mixed driving what I got for freeway only last year.

I have only raised my tire pressure 8psi in the front and 7psi in the back over OEM car recommendations. I'm still below the max rating of 44psi of the tire.

[MetroMPG]

compaq, nobody's disputing the idea that higher air pressure helps fuel economy (through lower rolling resistance).

we were just looking at the question of whether more air pressure changes the effective circumference of the tire. it doesn't.

[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.

[kickflipjr]

Yes but the higher you go the more risky it gets. Your could probally put 70psi in them (But it would be VERY VERY dangerous).

[MetroMPG]

if they're rated to 44, they'll take 44.

whether you'll like the ride/handling effects of 44 psi is up to you

some people are exceeding their tires' ratings, but with the understanding that we all experiment at our own risk. mine are rated at 44.