Does rim size chanage mph

Answer 1

The accuracy of your speedometer reading depends on if the overall circumference of your wheel and tire has changed or not. For instance, if you increased your rim size by an inch but mounted lower profile tires, the overall size could be about the same as stock and the speedometer reading would be as accurate as it ever was.

However, when one mounts as large a rim as possible, so the vehicle looks a little like it's standing on stilts, the speedometer will be off. For every revolution of the wheel you will go farther...which means you are also going faster than w/stock setup. So your speedometer might say you're doing 65mph but you might be doing 75 or more.

How to figure out how inaccurate your speedometer might be (so when a black&white is following you, you can adjust your speed down to a "no ticket for you" zone.)

A few methods are shown below...my favorite I have saved to last:

1a. Before changing your rims and tires, measure the circumference of your original wheel and tire. BEST way to do this is not to put a string around it, because the EFFECTIVE circumference after accounting for flex in the tire is what counts. Make a chalk mark on the side of the tire in the middle of where it is touching the ground...and on the street too. Then slowly have someone drive your car forward one revolution of the wheel, until the tire mark is again at the center bottom. Measure the distance from the original mark on the street to your new mark...that is how far you travel with 1 revolution of the original setup.

1b. Mount your new rims and tires, repeat the marking and measuring. The ratio of the new measurement divided by the old measurement will tell you how far off your listed speed is. Hypothetically if the new measurement was 120 inches and the old measurement was 100 inches, you are driving at a speed 20% higher than your speedometer shows. (speedometer says 100mph, you're doing 120. speedometer says 50 mph, you're doing 60.)

2. Borrow someone's GPS that has a speed reading on it. Find a long straight stretch of road. Drive a steady speed, and observe the GPS and your speedometer. I used this method when I changed differential gear ratios on my 4x4...I had a rider with me who wrote a list of speedometer speeds next to gps readings. Later I checked various readings (like at speedometer 30mph, 45mph, 65mph) and made sure the ratios to the GPS readings were all the same. Later I put small marks on my speedometer's face showing TRUE 25mph, 45mph, 55mph, 65mph (common speed limits in my area.)

3. My favorite and easiest to read later on method was used on a Russian motorcycle that had it's speedometer in kilometers rather than miles: I bought a cheap bicycle computer (I think it was called Mitymite or something similar), that had a speedometer function. I adjusted the bike computer to match the circumference of my motorcycle tires...and then just used THAT rather than the bike's speedometer to see my speed. You can get them where you don't have an actual wire from the sending unit by your tire to the computer, which would make installation easier. This could be difficult to mount on a car however, as it involves putting a sensor close to your wheel, and also attaching a small unit to the tire. You might be able to mount the sensor to your disk brake caliper, and the sendor epoxied to your rim in close proximity. Easy to do on a motorcycle since it's wheels are setup similar to a bike's.

All in all, get yourself a GPS with a speedometer function and just use that to see your real speed! No messing around with adapting bike computers, no measuring of wheels and rims before and after (since you probably already have the large rims mounted, my guess.)

Good luck,

Paul H.