my stock 17 inch rims with 215/45/17
my aftermarket 18x7.5 225/40/18

will my speedometer be affected? Even if its like 3 mph I would love to know this is for a court case tommorow!

 

Doing a quick calc you should have increased your diameter by approx, .5 inches. So it will cause your speedo to read slightly slow.

 

Emphasis on "slightly".

 

Yep.. should be off by roughly 1.6%

 

meaning close to nil.... and probably wouldn't help for any court case....

which.. btw.... how did that go?

 

yeah.. I dont think that roughly 1mph at 65mph will help much

Curious as well how your court case went

 

cop: "i clocked you doing 100 mph..."
person: "well i do have aftermarket rims... my speedo must be off... i swore i was much lower than that..."
cop: "ok, what size are they?"
person: "(blah blah blah vs. stock)"
cop: "so then, when i clocked you at 100, your speedo would've shown around 98-99 mph...."
person:

 

I don't think that the court would like to hear that your speedometer is off. I know that it's against the law to have a speedometer in kilometers and if you try to use that as an excuse to get out of tickets cops don't take it well.

Either way, the difference is only slight so it's probably not going to help you much anyways.