Will adding bigger rims to a car make the speed gage read faster or slower?

Question

If you have 14" rims and add 18" rims to the car, will the speed gage show you are going fater than you actually are or slower?

Also, is it true it can hurt the tranny on some vehicles?

Answer 1

it depends on the tire size for the rims...normally it would read slower because the wheels would be spinning slower with bigger rims

Answer 2

All depends if the replacement tires are higher than the stock 14".  If they are higher & your speedo reads 30 your ground speed may be closer to 34.

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Answer 3

The size of the rim does not matter. The overall diameter of the wheel and tire is what changes the speedometer reading. Measure the diameter of your stock wheel and tire. then measure the diameter of the combo you want to install. If it is larger the speedometer will read slow. If it is smaller the speedometer will read fast.

Stick to within an inch of the stock diameter and you will have no problems with the trans or anything else.