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Build Planner / Speedometer

Speedometer calculator

How far off your speedometer reads after bigger tires or new gears, what you're really doing at every reading, and the number to program to fix it. Or skip the math and check it against your phone's GPS.

Tires
New gears too

Only if your speedometer reads from the transmission or transfer case. Many trucks from the mid-2000s on read wheel speed from the ABS sensors, and a regear doesn't change those.

Check against GPS

Hold a steady speed on flat road with a GPS app open. This overrides the tire math above.

mph
mph

Add the tire the truck is set for and the one you're running.

How the numbers work

Why does a bigger tire read slow?

The truck counts wheel turns and assumes each one covers the stock tire's circumference. A taller tire covers more ground per turn, so you go faster than the speedometer says. Going from a 31.6 to a 33.8 inch tire, the speedometer shows 60 at a true 64.2.

Does it change the odometer and fuel economy?

Both, by the same percentage. The odometer logs fewer miles than you drive and the trip computer shows worse mileage than you get. Service intervals come up later than they should until you fix it.

Do new gears change the speedometer?

Only when the speed sensor is on the transmission or transfer case. Deeper gears turn the driveshaft faster at the same road speed, so the speedometer reads high. Many newer trucks read the ABS wheel sensors instead, and those only see the tire.

What number do I program?

Usually the tire's diameter or its revolutions per mile. Using the measured diameter beats the size on the sidewall, and a GPS check beats both. If your truck asks for a tire size from a list, pick the one closest to your diameter.

Use at your own risk. These calculators are here to help you plan and to inform your decisions, not to sign off on a build. Many numbers are estimates worked out from factory figures, and some factory figures are not yet confirmed. Check every number against your own truck, its manuals and a qualified shop before you buy parts, cut, weld, lift, regear or drive on it. Long Range Desert Tools is not liable for anything you do based on these calculators or their results. The full terms are in our policies.