Tire size question

[wulley1]

If you put a 225-40-16 on instead of a 225-50-16, would it screw up your speedometer enough to notice? The overal diameter of the tire and wheel would be a little bit smaller than if a 50 series was installed. I'm sure the speedo would read a little bit faster than it should. Any one have any ideas or have tried this before?

[97alpineM3]

I dont think you will be able to notice... but there are some crazy math guys on here.. that will probably write you out an explanation in mathmatical terms.... Your wheel will probably only travel an extra inch more with a thicker tire... than with the 40.... so if you times that extra inch by how ever many to equal a mile.. then you will probably get a decimal answer to how much it actually changes your speedometer....

[El Presidente]

yes, there would be a difference. You may not notice, but the cop with the radar gun might. its best to know you're real speed in case you get pulled over.

[e36_m3_Turbo]

Originally posted by El Presidente@Jun 28 2004, 09:20 PM
yes, there would be a difference. You may not notice, but the cop with the radar gun might. its best to know you're real speed in case you get pulled over.

yes i second that, lik my car is 5 mph off, haha ghetto huh, and i cant change it. :

[mixalot]

Use this link for the exact difference.

gauge speed 60
true speed 55.7 mph


http://www.miata.net/garage/tirecalc.html

[wulley1]

great website! thanks for the help.

[CiscoKid]

my friends m3 has differant speeds on the OBC and the spedo? wtf?

[bitcore]

haha

[Q_BMW]

ya
my OBC shows a few miles lower too...wonder if the OBC is doing the calculations and giving us the real (cop) speed?
Q

[wulley1]

maybe, I know mine is off by about 5 or 6 mph when I'm going 70. Pretty typical for all cars to be 5% faster than they really are.

[97alpineM3]

Just a guess... but I think that the obc and the speedometer read there speed from two different locations.... right or wrong?

[Kevin325i]

I concure. The speedo is off of the differential and should be the little bendy bar type of speedo so as it gets old it can be off a little more. The OBC is probably reading it from the same thing, but it's electronic and doesn't have the wear-factor.

Kevin