Gear Ratio Calculator: RPM, Tire Size and Road Speed
Enter tire diameter or sidewall size, axle ratio, transmission gear and engine rpm; get road speed, rpm at your cruise speed, and the effect of new tires or gears.
Road speed at 2,000 rpm
49.5 mph
79.6 km/h
Tire: 787.4 mm, 97.39 in around, 650.6 revolutions per mile.
With 33.00 in tires and a 3.73 axle
Equivalent axle ratio on the old tires: 3.50 (−6.1% effective gearing).
At 2,000 rpm: 52.6 mph. Rpm at 65 mph: 2,470.
Uncorrected speedometer: reads 65 mph while you are doing 69.2 mph (+6.5%).
TREMEC aftermarket transmission ratios used by the presets (tremec.com, published 2025-03-11)
| Transmission | 1st | 2nd | 3rd | 4th | 5th | 6th | Reverse |
|---|---|---|---|---|---|---|---|
| T-5 5-speed (Ford, 10-spline) | 2.95 | 1.94 | 1.34 | 1.00 | 0.63 | — | 2.76 |
| TKX 5-speed, 2.87 set (Ford or GM, 26-spline) | 2.87 | 1.89 | 1.28 | 1.00 | 0.68 or 0.81 | — | 2.56 |
| TKX 5-speed, 3.27 set (Ford 10- or 26-spline, GM 26-spline) | 3.27 | 1.98 | 1.34 | 1.00 | 0.72 | — | 3.00 |
| TR-4050 5-speed | 6.16 | 3.11 | 1.71 | 1.00 | 0.76 | — | 6.03 |
| Magnum / Magnum-F / Magnum XL 6-speed, 2.66 set | 2.66 | 1.78 | 1.30 | 1.00 | 0.80 | 0.63 | 2.90 |
| Magnum / Magnum-F / Magnum XL 6-speed, 2.97 set | 2.97 | 2.10 | 1.46 | 1.00 | 0.80 or 0.74 | 0.63 or 0.50 | 2.90 |
Formula: mph = rpm × tire diameter (in) × π × 60 ÷ (axle ratio × gear ratio × 63,360); km/h = mph × 1.609344. Tire diameter from a sidewall size = 2 × width (mm) × aspect ÷ 100 ÷ 25.4 + rim (in). Comparison lines assume the speedometer is driven from the transmission output and not recalibrated. Sources (accessed 2026-09-14): TREMEC gear ratio calculator (same formula, using 0.002975 = 60π ÷ 63,360), Crawlpedia engine rpm calculator (rpm = axle × mph × gear × 336.13 ÷ tire; 336.13 = 63,360 ÷ (60 × π)), NIST Handbook 44 Appendix C (1 in = 25.4 mm; 1 mile = 63,360 in = 1,609.344 m), Performance Plus Tire: tire dimension formula, TREMEC aftermarket manual transmission gear ratios and TKX service manual (p. 12).
How It Works
Every engine revolution turns the driveshaft 1 ÷ (transmission gear ratio) times, the axle turns the wheels 1 ÷ (axle ratio) times more slowly, and each wheel revolution covers one tire circumference (diameter × π). Multiply by 60 minutes and divide by 63,360 inches in a mile and you get road speed: mph = rpm × tire diameter (in) × π × 60 ÷ (axle ratio × gear ratio × 63,360). Run it backwards and you get the rpm at any cruise speed. TREMEC’s own gear ratio calculator uses the same relationship with the constant 0.002975 (60π ÷ 63,360); Crawlpedia writes it as 336.13 (63,360 ÷ 60π).
Tire diameter is the overall diameter, not the wheel size. If you know the sidewall size, the tool computes it: sidewall height = width (mm) × aspect ratio ÷ 100, converted at 25.4 mm per inch, and diameter = 2 × sidewall + rim. Real tires run a little smaller than the arithmetic once mounted and loaded, so use a measured diameter when you have one.
The transmission ratio defaults to 1.00 (direct drive, which is 4th gear in every TREMEC preset). The presets are TREMEC’s published aftermarket ratios (T-5, TKX, TR-4050, Magnum) and are only examples; type any ratio, including your factory automatic’s overdrive.
The comparison block shows what a tire or axle change does. The equivalent ratio is new axle × old diameter ÷ new diameter, the ratio your truck would need on the old tires to feel the same. The speedometer line assumes the speedometer is driven from the transmission output and has not been recalibrated: bigger tires make it read low, a numerically higher axle makes it read high.
Worked Example
A 31 in tire with a 3.73 axle in a 1.00 gear at 2,000 rpm: 2,000 × 31 × π × 60 ÷ (3.73 × 1.00 × 63,360) = 49.5 mph (79.6 km/h). At 65 mph the same setup turns 65 × 3.73 × 336.14 ÷ 31 = 2,629 rpm, which is Crawlpedia’s worked example (they get 2,628 with the rounded constant). Move to 33 in tires without changing the 3.73 axle and the gearing acts like a 3.50 (−6.1%), 65 mph drops to 2,470 rpm, and a speedometer that reads 65 is really doing 69.2 mph (+6.5%).
FAQ
What axle ratio does my Chevy truck have?
Check the RPO code sticker in the glove box and look up the axle code in your owner’s manual or with a dealer. If the sticker is gone, count driveshaft turns per wheel turn with the other wheel held, or read the tag on the differential cover.
Do I use the transmission ratio or the overdrive ratio?
Use the ratio of the gear you are in. For highway cruising that is usually the top gear, which is below 1.00 on any overdrive transmission (0.68 in a TKX, 0.63 in a Magnum; check your automatic’s published ratios). The tool multiplies it by the axle ratio to give the overall ratio.
Why is my measured speed different from the calculator?
Loaded tires deflect, so the rolling diameter is a little less than the sidewall arithmetic; a lockup torque converter that is not locked adds slip. Enter a measured rolling diameter (chalk mark, roll one revolution, measure) for the best answer.
Do bigger tires change my speedometer and odometer?
Yes, unless the truck is reprogrammed. The tool’s speedometer line shows how far off the reading is. Bigger tires make both read low, so the odometer under-counts miles too. A dealer or a handheld tuner can correct the tire size in the computer.
Where does the formula come from?
It is unit conversion, not an estimate: 63,360 inches per mile and 25.4 mm per inch come from NIST Handbook 44, and the relationship is the same one TREMEC’s gear ratio calculator uses. The Sources list links every constant and the published ratio tables.
Sources
- TREMEC: Gear ratio calculator - computes rpm = axle × gear × mph ÷ (tire diameter × 0.002975), i.e. the same speed formula, and tire diameter from the sidewall size
- Crawlpedia: Engine rpm calculator - rpm = axle × mph × gear × 336.13 ÷ tire diameter, with 336.13 = 63,360 ÷ (60 × π); worked example 3.73 × 65 × 1.00 × 336.13 ÷ 31 = 2,628
- NIST Handbook 44 (2026), Appendix C: General tables of units of measurement - 1 inch = 2.54 cm exactly; 1 mile = 63,360 inches = 5,280 feet = 1,609.344 m (Units of Length table, p. C-9)
- Performance Plus Tire: Formula for calculating tire dimensions - sidewall = section width × aspect ratio; diameter = 2 × sidewall + rim; width (in) = mm ÷ 25.4; example 235/35R19 = 25.48 in
- TREMEC: Aftermarket manual transmission gear ratios - published ratio tables for the T-5, TKX, TR-4050, Magnum, Magnum-F and Magnum XL (2025-03-11)
- TREMEC: TKX service manual - TKX ratio sets by part number (p. 12), cross-check for the preset table