A Question for a Math Genius

[semperfiguy]

If a car's speedometer and odometer are calibrated for 14 inch tires and 15 inch tires are substituted, will the speedometer register a higher or lower speed than the car is actually going? Also, will the odometer register a lower or greater distance than the car has actually traveled. This cold weather has got me in a temporary brain freeze! Your replies are very much appreciated.

[vidmaster]

Of course

[Raoul Duke]

My understanding is that the revolution of a tyre is equivalent to the circumference and the odometer calculates the distance based on the number of revolutions of your tyre.
I have heard before that smaller diameter tyres will impact odometer readings more than larger tyres.
The calculation should be on the Internet somewhere.

[Adhoc]

A lot depends on the profile of the tyres. A very rough calculation, all things being equal, your speed will be about 5% more than the speedo reading.
Try this site: http://www.tyresizecalculator.com/

[Giggle]

If you change to larger tires than the car is calibrated for, your actual speed will be higher than the indicated speed and the odometer reading will be lower based on farther distance per revolution.

14 inch tires x pi gives you a circumference of 43.98
15 inch tires x pi gives you a circumference of 47.12

[stattointhailand]

Doesn't matter if your using 14 or 15 inch tyres it's still B200 when BIB stop ya

[Barney]

MMM Genius math questions.

[Nigglyb]

MMM Genius math questions.

4

14 inch tires x pi gives you a circumference of 43.98
15 inch tires x pi gives you a circumference of 47.12

I like pi

[wazza]

Doesn't matter if your using 14 or 15 inch tyres it's still B200 when BIB stop ya

Emeritus Professor Stattointhailand

[tutone]

SPEEDOMETER ACCURACY
No speedometer can be 100 percent accurate. In fact, most manufacturers build speedometers so they fall within a fairly narrow tolerance range, no more than 1 percent to 5 percent too slow or too fast. As long as a car is maintained at factory specs, its speedometer should continue to register vehicle speed within this range. But, if a car is modified, its speedometer may need to be recalibrated.

Changing tire size is one of the most common things car owners do that can affect speedometer accuracy. That’s because larger tires cover more ground in one complete revolution. Consider the example below.

Your car comes with factory-installed tires that are 21.8 inches in diameter. That means the circumference of each tire is 68.5 inches. Now let’s say you want to replace the stock tires with new tires that are 24.6 inches in diameter. Each new tire has a circumference of 77.3 inches, which means it travels almost 10 inches farther with each complete revolution. This has a tremendous affect on your speedometer, which will now indicate a speed that is too slow by almost 13 percent. When your speedometer reads 60 miles per hour, your car will actually be traveling 67.7 miles per hour!

Depends on tire diameter. Wheel and tire diameter are two different things.
https://www.etrailer.com/faq-determinin ... meter.aspx

[papafarang]

to know if any difference you need total tyre height if you went to 16in tyres you would buy 175/60-16 and so on all these tyres 14/15/16in @ 75/67/60 give you 829 rev per mile. when talking tyres and you say 14in or 15in your talking rim size not tyre size.

[JMT42]

giggle has it wright

[papafarang]

not a question for geniuses , just a question for tyre fitters if you had 14 in tyres it means the centre of the tyre is 14in, 15in tyres mean 15in centres . the real question is if I change 18in high tyres to 19in high tyres

[glalt]

An accurate way to check your actual speed is with a GPS. My old Nissan speedometer read 105 KPH when the GPS said 100. when I replaced the tires, I went just one size bigger. I was pleased when that made the speedometer exactly match the GPS. Changing the rim size larger would made a bigger difference. The speedometer would read slower than the actual speed. Not a big deal as long as you know how fast you are actually going.

If you need an estimate, you can measure the circumference with a tape measure. It also depends on the tire. Low profile tires on a larger rim could possibly be smaller than regular tires.

[papafarang]

14inch tyre 23.22 inch high
15 inch tyre 23.11 inch high
difference of 2.79mm not enough to be noticeable in speed or rpm

[Giggle]

This is a question about tires, not rims. The diameter of those tires is about 23 inches, not 13, 14, or 15. The measurement of the rim is immaterial and has no impact on speed or odometer.

The etymology of "tire" is that the word is a short form of "attire", and that a wheel with a tire is a dressed wheel. Tyre is a Brit aberration.