Does rim or tire size change your speed?

Answer 1

"Your speedometer can misread if you have larger tires. I run 35s on my '02 F-150, and when my speedometer says I'm going 80, I'm really going about 73 or 74 miles per hour. My truck also has 20" rims if that matters to you."

Actually, putting larger than original tires will cause the speedometer to be incorrect in the opposite direction. Say, your truck came with 285/70/20 and you put larger 315/70/20 tires on it, your speedometer might be reading 75, yet you would be going approximately 82. Car computers read either axle or wheel revolutions, thus producing the mph you see. Therefore, when your car thinks your tires have completed one full revolution and traveled X distance, your tires have actually only completed 80% of a revolution, yet your car has traveled the same X distance, so when your tires complete the revolution they have actually traveled X + Y distance, where Y is the difference in size between the new and old tires.

Think of it this way, if you have longer legs than someone, and you both take one full stride, you will have traveled farther then that person because of your longer legs.

Another example: if you mark one place on both tires, and proceed to roll them on the ground a full revolution until you see that mark, you will find that the larger tire has traveled farther than the smaller tire.

Larger tires can also affect your odometer, with the same concept, that larger tire will travel more distance than the computer is calculating.

Interestingly, no consumer cars are manufactured with speedometers and odometers calibrated to be perfectly accurate. Cops allow +/- 3 mph at 75mph because your speedometer may be reading different than the speed your going. Only police cars are perfectly calibrated to reflect speed. Manufactures admit purposefully calibrating car consumer speedometers to show faster than they are going so they can not be sued for someone getting a ticket.

If you have any questions, drive your car onto a highway with mile markers, drive 60 mph, since it takes 60 seconds to travel a mile, you should see a .1 mile marker every 10 seconds, if you don't, read your local laws regulating the allowance for variation and make sure it is in the allowed specifications, if not, visit a trusted mechanic and have them adjust it, because trying to explain to a cop that it's not your fault probably won't fly. And remember, bigger is always better when it comes to tires on trucks!