Speedometer Difference

Wasup fellas. I've been wondering. Ever since I replaced my stock 15 in wheels with my 18's, I know the speedometer has been off. Not noticably, but science dictates that adding wheel diameter will throw off the ECU which still thinks it is rotating 15 inches worth of rim and rubber.

So my question is this: is the speedometer over or underestimated?

ie, when the speedo says 60, am I really doing 58 or 62???

Sorry if I gave you a headache from reading this. I know I got a headache writing it.:frown:

what series tired do you have?

actually i just saw your sig. you got 225/40/18.

this is right from www.whalentire.com the place i got my rims and tires

SELECTED TIRE SIZE:225/40R18

DIAMETER: 25.1" SECTION WIDTH: 8.9" SIDEWALL HEIGHT: 3.5"
MINIMUM RIM WIDTH: 7.5" MAXIMUM RIM WIDTH: 9.0"
REVOLUTIONS PER MILE: 804

IF YOU TRAVELED 60 MPH WITH THIS TIRE INSTALLED ON YOUR VEHICLE
YOUR SPEEDOMETER WOULD READ 59.7 MPH


so your speedometer is almost right, it is .3 mph off.

Thanx for the info. I knew the speedo couldn't be too off. I thought maybe 1mph.