Tire Size & Miles Per Hour

Using larger tires on a vehicle changes the miles per hour.

Speedometers are set to calculate mileage based on the stock tires that come with the vehicle. If stock tires are replaced with larger models, this will change the miles per hour (mph) and the speedometer reading will no longer be accurate.

Calculating Actual Speed

To calculate actual speed with larger tires, use the following equation: new tire diameter times speedometer-indicated speed divided by old tire diameter equals actual speed in miles per hour. As an example, 36 inches times 55 mph divided by 28 inches equals 70.7 mph.

Calculating Miles Per Hour from the Speedometer Reading

If you want to know what the speedometer-indicated speed in mph would be when you are traveling at a given speed with larger tires, use this equation: old tire diameter times actual speed divided by new tire diameter equals mph indicated on speedometer. For instance, 28 inches times 60 mph divided by 36 equals 46.6 mph actual speedometer reading.

Calculating Miles Per Hour Using Gear Ratio and RPM

Tire size, gear ratio, mph and revolutions per minute (rpm) all are interrelated. If one changes, all the others do too. To calculate mph using these values, the equation is rpm times tire diameter divided by the rear-end gear ratio times 336 to give mph. If a car with a 4.11:1 gear ratio that is turning at 4500 rpm has a 28-inch tire, its mph would be 91. With 32-inch tires, the mph would be 104.

References

• Offroaders: Tires, Gears, MPH and RPM
• "How to Build a Hot Rod Model A Ford"; Dennis Parks; 2001