⚙️ Speedometer Gear Calculator

How to Use This Calculator

Enter your transmission's drive gear tooth count (usually 7, 8, or 9), your axle ratio (on the door sticker or differential tag), and your new tire's overall diameter in inches. Hit Calculate Gear Teeth. The result is the ideal driven-gear teeth count; round to the nearest whole number and buy the closest available gear. The details show tire revolutions per mile and the nearest whole-gear options so you can judge the compromise.

The Method: The 1001 Formula

The constant 1001 is the cable-revolutions-per-mile standard the speedometer head expects; everything else scales the tire's actual revolutions into that expectation. The worked example: an 8-tooth drive gear, 3.73 axle, 28-inch tires. Revs per mile = 20,168/28 = 720.3. Driven teeth = 8 x 3.73 x 720.3 / 1001 = 21.47. Available gears near it: 21 or 22 teeth; 21 reads about 2 percent fast, 22 about 2 percent slow. The formula is linear, so a 10 percent taller tire needs 10 percent fewer teeth; sanity-check any result against that proportionality before ordering.

Worked Example

A classic Mustang swaps to 26-inch tires with a 3.55 axle and 7-tooth drive gear. Revs per mile = 20,168/26 = 775.7. Driven = 7 x 3.55 x 775.7 / 1001 = 19.26, so a 19-tooth gear. Before the swap with 28-inch tires it needed 7 x 3.55 x 720.3 / 1001 = 17.88, an 18-tooth gear. The shorter tire added about 1.4 teeth of need. Enter 7, 3.55, and 26 above to reproduce 19.26. If only 19 and 20 tooth gears exist, 19 is nearly perfect.

Finding Your Numbers

Drive gear teeth: stamped on the gear or in the transmission's specs; pulling the cable housing to count is a 15-minute job on most classics. Axle ratio: the door-jamb sticker, a tag on the differential cover, or the build sheet; as a last resort, jack up the rear, mark the driveshaft and a tire, and count driveshaft turns per tire turn. Tire diameter: measure a mounted tire, or compute from the size code: diameter = wheel inches + 2 x (width mm x aspect / 100) / 25.4. A 245/45R17 is 17 + 2 x 110.25/25.4 = 25.7 inches.

After the Swap: Verify

Drive a measured mile or compare against GPS at a steady 60 mph: GPS 60 with the needle on 62 means 3.3 percent fast, about one tooth too few on a 30-tooth gear. Odometer check works too: 10 indicated miles against 10.3 actual is the same 3 percent. Fine-tune by one tooth at a time; gears are cheap and the housing swap takes minutes. On electronically-driven modern speedometers, none of this applies: recalibration is a programmer or dealer setting, not a gear.

Electronic Speedometers: The Modern Method

Vehicles from the mid-1990s onward largely replaced the cable with a vehicle speed sensor and an electronic signal, so there is no gear to swap. Calibration becomes a software setting: the engine computer holds a pulses-per-mile value, and changing tire size means reprogramming it. Handheld tuners do this in minutes on many trucks and SUVs; some vehicles need dealer tools. The math is the same proportion as the gear formula: new pulses per mile equals old times (old tire revs per mile over new). GPS speedometer apps give a quick error reading before you commit to the reprogram. One caution: odometer fraud laws apply to electronic tampering too, so recalibrate to truth, not to flatter the odometer. And on vehicles with advanced driver aids, tire size changes can confuse systems calibrated to expected wheel speeds; keep diameter changes modest.

Tire Size, Gearing, and Performance

Changing tire diameter does more than skew the speedometer: it re-gears the whole car. Taller tires act like a taller final drive, dropping cruise RPM (good for fuel economy) but dulling acceleration; shorter tires do the reverse. A 3 percent diameter change shifts effective gearing about 3 percent, noticeable in both directions. Off-roaders who fit much larger tires often re-gear the axles to compensate, which is when the speedometer gear math in this calculator becomes essential: new axle ratio plus new tire diameter both enter the formula. Keep total diameter changes within about 3 percent for street cars to avoid rubbing, gearing weirdness, and driver-aid confusion. Measure the actual mounted diameter, not the nominal size: a "33-inch" tire often measures 32.5 mounted, and the formula wants reality.

Worked Example: Truck With 33-Inch Tires

A truck runs 33-inch tires, a 4.10 axle ratio, and an 8-tooth drive gear. Tire revs per mile = 20,168 / 33 = 611.2. Driven teeth = 8 x 4.10 x 611.2 / 1001 = 20.02, so a 20-tooth driven gear. Before the lift it wore 31-inch tires: 20,168 / 31 = 650.6 revs per mile, driven = 8 x 4.10 x 650.6 / 1001 = 21.3, a 21-tooth gear. The bigger tires slowed the cable, so the speedometer reads low until the gear is corrected down a tooth. Enter 8, 4.10, and 33 above and the calculator confirms 20.

Check the error from ignoring it: with the old 21-tooth gear still installed and 33-inch tires, the speedometer reads about 5 percent slow. At an indicated 60 mph the truck is actually doing 63. That is a speeding ticket waiting on every highway, plus odometer fraud in the slow direction that understates mileage. One $15 gear fixes both.

Finding Your Numbers

Three inputs, three places to look. Drive gear teeth: count them on the transmission output shaft gear, or check the build sheet; common counts are 7 and 8. Axle ratio: the tag on the differential cover, the door-jamb sticker, or the RPO/option codes; 3.55, 3.73, and 4.10 cover most trucks. Tire diameter: measure a mounted tire from ground to top, or compute from the size code (a 265/70R17 is about 31.6 inches). Measure the actual tire, not the nominal size, because worn tires run smaller.

Electronic speedometers skip the gears entirely: they take a sensor signal and apply a programmed pulses-per-mile value, corrected with a tuner or dealer tool. If your vehicle is 2000s or newer, check for an electronic speedo before ordering gears; the mechanical formula in the calculator covers the classic cable-driven era that still fills the restoration market.

Verifying With GPS After the Swap

After installing the new gear, verify with a GPS speedometer app on a flat straight road at a steady 60 mph indicated. GPS speed is accurate within about 1 mph at steady cruise. If the GPS reads 61 against 60 indicated, you are within 2 percent, which is better than factory accuracy on most older vehicles. If it reads 63, recheck the tire diameter measurement; a half inch of error in the 20,168 formula shifts the result by nearly a full tooth. The gear gets you close, the GPS confirms it, and the combination beats either method alone.