How Does Tire Size Affect Speed?
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Transmission Speed
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Despite popular beliefs, speed is not measured by the number of tire rotations a car makes while moving. Speedometer readings are based on the speed of the gears in a car's transmission. When a car is built, it is stocked with wheels that rotate in accordance with the speed of the transmission, resulting in accurate speedometer readings.
Changes in Tire Size
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When factory-calibrated tires are replaced with larger tires and rims, a car's transmission will continue to run at the same speed. Larger wheels will make fewer rotations than smaller tires. This means, as the transmission continues to clock a specific speed, larger tires will actually cause the car to move more slowly than the speedometer reads. Depending on the increase in tire size, a car will usually travel about 5 to 15 percent slower. The opposite effect is realized when smaller tires are installed.
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Calculating Acutal Speed
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It is important to know exactly how fast a car is moving to ensure speeding laws are not broken. To ensure this is always known when traveling, the following formula allows drivers who ride on larger or smaller ties know how fast they are actually moving at all times:
New tire size (diameter) x speedometer reading / stock tire size = actual speed
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- Photo Credit Winston Motor Sports
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Comments
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You said that with larger wheels the actual speed will be slower than the speed displayed by the speedometer, and u gave a formula that proves the inverse! In fact if you substitute in the formula the new tire size with larger size, the actual speed will be higher than the speedometer reading :)
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u
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calculate using a gps.It gets reading of position with 11 sats.,with larger tires the actual speed is lower than what is being displayed on the speedometer.Ehow is correct in what they are saying! PHYSICS--simpler thinking-- picture a ten speed bike and the way the gears work.The smaller the gear the faster the pedals rotate with the bike going the same speed!
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Lisa Stewart Mayer
Jan 27, 2011
eHow - you are incorrect. -
Tomislav TeaBee Bjelonic
Oct 27, 2010
IMHO, I think it's oposite: Speedometer reads the radian speed (revolutions per second). If you have larger tire, it's circumference is larger, thus when wheel makes a spin, it travels larger distance than a smaller tire. Example: Earth makes only 1 rotation per day, but building on a surface travels 24,859.82 miles (40,008 km). So, if you had tires a size of an Earth, you would travel 1035 mph :)
