Not if you do it right. If the vehicle is a Chrysler, most independent mechanics can reset tire size in the vehicle's computer to keep the transmission shift opoints and speedometer correct and on a GM, the dealer can do this for you as well.
If the vehicle is something else, just make sure the overall diameter of the tire is the same. Unfortunately, the sizes are usually a bit complicated, as they use a combination of US Standard (fractional) and Metric (SI) units in a single size. While a 31x10.5x15 tire has an overall diameter of pi*31in, a P205/75r15 has an overall size of (((205mm*0.75*2)+15in)*pi).
Depending on your tire size type, just plug in the numbers and you'lkl see what the overall diameter is. If can do 6th grade algebra, you can compute the most appropriate size tire for the new rim size to keep overall size nearly the same. Ask you kids for help if needed.
The speedometer is calibrated based on the overall tire diameter on the vehicle from the factory. If you put larger rims on with sidewalls as high as the original tires, the speedometer will not be accurate as the overall tire diameter has increased.
I've been told that you can "net out" the difference of bigger rims with thinner tires (i.e. the diameter of the tire/wheel stays the same).
If you change the diameter your speedo will NOT be accurate. Tire shops may have some idea as to how much it will be off. The best thing would be to have a mechanic recalculate your speedo.
if you get bigger tires (like i have) i have 35x12.5 tires on 15 inch rims.. but in your case if you had a 15 inch rim on 20 inch tires and bumped it up to a 20 inch tire on 18 rims, your speedo shouldn't change, and if you are having it done, they would recal. anyway..just ask the tire place.answer
if combined wheel and tire diameter is bigger than factory specs , your speedo will be inaccurate. most tire shops have no idea or the equipment to recalibrate a good mechanic can recalibrate if your vehicle has a computer
dang, i think all he wanted to know was--- you will be going faster that what the speeddometer says
I don't know about the rest , but when i changed mine to a bigger wheel yes the speedometer did change thus i got a ticket for going TO FAST.....
taller tires always slows the speedometer. so you are actually traveling a little bit faster than the speedo indicates to you..
rim size has nothing to do with this.
tire height is where it is.
WRONG! IT IS THE OVERALL DIAMETER WHICH IS IMPORTANT. SPEEDOMETER CALIBRATION CAN BE ACCEPTABLY OK IF A LARGER RIM IS FITTED WITH A LOWER SIDEWALL HEIGHT TIRE, BUT YOU NEED TO CALCULATE BOTH TOTAL DIAMETERS. SO, RIM SIZE IS IMPORTANT TOO.
if you want to change speedometer,
you can get a odometer correction tool to change.
If you are buying larger rims, have the dealer figure the tire that will still match the tire rotation speed as the OEM tire.
I would like to add that many of the same exact vehicles have different size tires and rims from the factory.