Data sheet TS-05 · Calculator

Gear ratio calculator

By the formula, engine speed is 2,372 rpm at 65 mph with a 3.73 axle, a 1.00 transmission gear and a nominal 34.4 in tire. Use the calculator to compare engine speeds or find the axle ratio that restores engine speed after a tire diameter change.

Result

Engine with 265/70R17

2,578 rpm

at 65 mph

Engine with 315/70R17

2,372 rpm−8.0%

at 65 mph

Axle ratio for stock RPM

4.06

your 3.73 × new diameter ÷ old diameter

Effective ratio now

3.43

your 3.73 turns like this on 315/70R17

Overall diameter: 265/70R17 is 31.61″, 315/70R17 is 34.36″ (+8.72%). A taller tire covers more road per turn, so the engine turns slower at the same speed.

Axle ratios come in fixed steps: pick the one your axle maker offers closest to this number.

Engine RPM by speed

Engine RPM at road speed for both sizes
Speed	265/70R17	315/70R17
45 mph	1,785	1,642
55 mph	2,182	2,007
65 mph	2,578	2,372
70 mph	2,777	2,554
80 mph	3,174	2,919

Nominal diameters, axle 3.73, transmission gear 1.00. RPM = speed × axle × gear × 336.13 ÷ diameter in inches.

§ 1

How is engine rpm calculated?

Engine rpm = mph × axle ratio × transmission gear ratio × 336.13 / tire diameter in inches. Enter the transmission ratio for the gear being compared, not just the axle ratio.

The conversion factor is 336.13 = 63,360 / (60 × π). It combines the distance and time conversions with the tire circumference calculation.

At the same road speed and transmission ratio, a larger tire diameter lowers calculated engine rpm. A numerically higher axle ratio raises it.

§ 2

How do tire size and gearing compare?

The nominal diameters are 31.6 in (803 mm) for 265/70R17, 34.4 in (873 mm) for 315/70R17 and 33.8 in for 285/75R17. The following engine speeds are by the formula at 65 mph.

With a 3.73 axle and a 1.00 transmission gear, engine speed is 2,578 rpm on 265/70R17. It drops to 2,372 rpm on 315/70R17 and 2,409 rpm on 285/75R17.

With the same 3.73 axle in a 0.69 gear, the results are 1,779 rpm, 1,636 rpm and 1,662 rpm, respectively. With a 4.10 axle in that 0.69 gear, the results are 1,956 rpm, 1,799 rpm and 1,827 rpm in the same tire order.

Changing from 265/70R17 to 315/70R17 lowers calculated engine speed by 8.0% when road speed and gearing stay unchanged. That comparison describes the gearing change, not whether the tire is suitable for a vehicle.

§ 3

What axle ratio restores engine speed?

Use new axle ratio = old axle ratio × new tire diameter / old tire diameter. This holds road speed and transmission gear constant while finding the ratio that restores the original calculated rpm.

Starting with a 3.73 axle and 265/70R17, the calculated target is 4.06 for 315/70R17. For 285/75R17, the target is 3.99.

Starting with 3.55, the targets are 3.86 and 3.80 for those larger sizes, respectively. Starting with 4.10, the targets are 4.46 and 4.39.

These are mathematical targets, not claims that those exact gear sets are sold. Axle ratios are sold in steps, so an available ratio may differ from the calculated target.

§ 4

What does the calculator leave out?

Torque converter slip, tire growth and transmission shift points are outside this calculation. The result describes engine speed at the entered road speed, gear ratio and tire diameter.

Restoring calculated rpm does not establish which axle ratio a truck needs. Tire fit depends on the vehicle, wheels and suspension.

A tire diameter change also affects the relationship between indicated and true road speed. Use the speedometer calculator to examine that separately from engine rpm.

Q&A

Common questions

Do bigger tires lower engine rpm?

Yes, by the formula, when road speed and gearing stay unchanged. Changing from 265/70R17 to 315/70R17 lowers calculated rpm by 8.0%.

Can I calculate axle ratio from rpm?

Rearrange the formula: axle ratio = rpm × tire diameter / (mph × transmission gear ratio × 336.13). Use tire diameter in inches.

Does a 4.10 axle restore rpm after larger tires?

From a 3.73 axle on 265/70R17 to 315/70R17, the calculated target is 4.06. That is an rpm-matching result, not a recommendation for a particular vehicle.

Why does transmission gear matter?

Transmission gear ratio is a multiplier in the rpm formula. With a 3.73 axle and 265/70R17 at 65 mph, calculated engine speed is 2,578 rpm in a 1.00 gear and 1,779 rpm in a 0.69 gear.

Notes

Method and sources

Each number and rule comes from a source with the date it was checked.

Engine RPM = mph × axle ratio × gear × 336.13 ÷ diameter (in)
Ratio for stock RPM = old ratio × new diameter ÷ old diameter
336.13 = 63,360 ÷ (60 × π)

Diameters are nominal, from the size code. Pick the axle ratio your axle maker offers closest to the result.