Drivetrain & Track
Speedometer Error & RPM at Speed
See how a combined tire size and final drive swap changes true speed, speedometer error, cruise RPM at 30–80 MPH, and speed per gear at redline — before vs. after.
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How to Use This Tool
- Select your original and new tire sizes.
- Enter the original and new final drive ratio, then load a transmission preset or type your gear ratios.
- Choose where your speedometer reads from: the transmission output (driveshaft sensor) or the wheel-speed / ABS sensors.
- Check an indicated speed or an RPM in gear, then read the cruise RPM and redline speed tables before vs. after the swap.
The Formula & Physics Behind the Math
Tire diameter = rim diameter + 2 × (width × aspect ratio ÷ 100 ÷ 25.4). If the speedometer counts wheel-speed sensor pulses, only the tire matters: actual ÷ indicated = new diameter ÷ old diameter. If it counts transmission output or driveshaft revolutions, the final drive matters too: actual ÷ indicated = (new diameter ÷ old diameter) × (old final drive ÷ new final drive). Speedometer error = (ratio − 1) × 100%, where positive means the car is going faster than the gauge shows. Engine RPM at a true speed = speed × gear ratio × final drive × 63,360 ÷ (tire diameter × π × 60), and speed at redline in each gear is the inverse.
Frequently Asked Questions
Do shorter gears make my speedometer read wrong?
On cars whose speedometer is driven from the transmission output or driveshaft — common on older rear-drive cars and trucks — yes. A numerically higher final drive spins the driveshaft faster at the same road speed, so the gauge reads high. Going from 3.73 to 4.10 gears makes it read about 10% fast unless the tires grow or the speedometer is recalibrated.
Can bigger tires cancel out a gear swap?
Partly or fully. Taller tires make the gauge read low while shorter gears make it read high, so the two can offset. This calculator combines both so you can pick a tire and gear pair that keeps error near zero, or see how much recalibration you need.
How is this different from the tire size and gear ratio calculators?
The Tire Size Calculator compares two tire sizes on their own, and the Gear Ratio Calculator charts speed per gear for one setup. This tool combines a tire change and a final drive change and shows before vs. after speedometer error and cruise RPM together.
What speedometer error is acceptable?
Many enthusiasts aim to stay within about ±3% so the gauge, odometer, and cruise control stay close. Beyond that, plan a recalibration with a tuner, a handheld programmer, or a speedometer drive gear change on older cars.