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...
- 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...