I have conflicting reports: If I change my rims/tires on my A4 from 16" to
18" does it affect the speedo/odometer readings? My tire guy says the dealer needs to make an adjustment, but the dealer says it will have no effect.Seems logical that a larger diameter wheel/tire would make the apparent mileage lower (fewer rotations to travel a given distance), but I can't find an authoritative source. Any help?
what is important is the outside diameter of the tire. Choose a lower-profile tire when you go to a larger diameter wheel...
When getting a larger rim, you need to get a lower profile tire, so that the overall rim/tire combo diameter remains the same. For example, if what you have right now is 205/55/16, then when you get your 18" rims, you'll get
225/40/18 tires in order to stay as close as possible to the OEM diameter. This way, the difference in diameter will be so small, that you will not affect your speedo/odo in any significant way.You can play around wih this calculator to see what I mean:
Pete
lower-profile
I agree I have changed my wheels on a Audi A4 from 15" to 17" RS4 copies with no problems. If you go for 18" expect to loose some comfort as the profile of the tyre needs to be very low
Yoda
Remember also that, if your total wheel/tire diameter changes, not only will your speedo be off, but you will effectively be changing the overall gearing. If your overall wheel/tire diameter increases, you will probably have worse acceleration (with the stock tranny, anyway).
-- Mike Smith
In theory, if the overall rolling circumference increases, acceleration will suffer, but top speed increase. Unlikely to be noticeable. More significant is the decrease in ride comfort and less resistance to aquaplaning with the very lower profiles.
-- Doug Ramage
Eh? Ride comfort, sure, but I'd argue that less resistance to aquaplaning is not truly attributable to aspect ratio.
On a given vehicle (so the performance differences are strictly due to tires) resistance to aquaplaning is primarily a function of tread *width* and pattern.
That width is a factor in calculating aspect ratio does not somehow convey cause to aspect ratio...
/daytripper '00 s4 6spd
Well, hell's fire, boys, if we're goin' to go gettin' all
*theoretical* here, let's go all the way! The other thing that the wide aspect tire (usually, unless you're getting the same width as the original tires)
"Postus Interruptus" ;-) notwithstanding, I believe you reiterated my point: the size of the contact patch, not the aspect ratio, is what's in play wrt hydroplaning...
/daytripper '00 s4 6spd w/low aspect S03's, quite good in the wet...
That's *exactly* what it was. As soon as I realized I was repeating what you'd said, I hit the "Cancel message" button. Only it said, "Post message". Unfortunately, AFAIK, there's no 'overs' in Usenet ...
-- C.R. Krieger (no more embarrassed than usual)
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