Transmission Gear Ratio Calculator

Gear ratios, axle and tires turned into road speed

A gearbox is a list of numbers until you run it through the axle and the tires. This transmission gear ratio calculator takes up to 10 forward ratios, the axle ratio, the redline and a tire size in metric or flotation form, and returns the speed each gear reaches at redline, the speed at 2,000 rpm, the rpm left after every upshift and what the engine turns at 55 to 80 mph in the top two gears. It answers the usual questions behind a regear or a tire upgrade: how many rpm at 70 mph, how fast each gear goes, and how far the revs fall between gears.

Quick start: a 10-speed with the 10R80 ratios (4.696 to 0.636), a 3.55 axle and 275/60R20 tires, which work out to 33.0 inches. At 70 mph in 10th the engine turns 1,610 rpm; in 9th, 1,744 rpm. First gear runs out at 35.3 mph at a 6,000 rpm redline.

Ratios, axle, redline, tire: the four inputs

Gear ratios - 1st, 2nd and 3rd are required, 4th to 10th are optional and must follow without gaps. Each one from 0.40 to 7.00, each lower than the one before. Up to three decimals, as printed in spec sheets (0.854).
Axle ratio - the final drive, from 1.50 to 7.00. On a truck or a rear-drive car this is the ring and pinion, such as 3.55 or 4.10; on a front-drive car it is the final drive inside the transaxle.
Redline - the rpm where you shift, from 3,000 to 12,000. It sets the speed of every gear in the first table and the shift points in the second.
Tire size - metric (265/70R17: width in mm, aspect ratio, rim in inches) or flotation (35x12.50R17: only the first number, the overall diameter in inches, is needed).

From ratio sheet to rpm at 70 mph

Tire diameter. A metric size hides it: 275/60R20 means a sidewall 60% as tall as the 275 mm width, so 165 mm, twice over (top and bottom) on a 20-inch rim. That is 330 mm, or 12.99 in, plus 20: 32.99 in. One turn covers 103.6 in, so the tire turns 611 times per mile.

Overall ratio. The gear and the axle multiply. 10th at 0.636 times a 3.55 axle gives 2.26:1: the engine turns 2.26 times for each turn of the tire. 1st at 4.696 gives 16.67:1.

Speed from rpm. Tire turns per minute = rpm / overall ratio; times the circumference and 60 minutes, divided by 63,360 inches in a mile. The shop shortcut folds the constants into one number: mph = rpm x diameter / (gear x axle x 336.14).

Rpm from speed. Turn the formula around: rpm = mph x gear x axle x 336.14 / diameter. At 70 mph: 70 x 0.636 x 3.55 x 336.14 / 32.99 = 1,610 rpm.

Revs after a shift. At the same road speed the rpm scales with the ratio, so redline x next gear / current gear. A shift from 1st (2.66) to 2nd (1.78) at 6,500 rpm lands at 4,350 rpm.

Bigger tires or a regear: 33, 35 and 37 inches side by side

The same 10-speed, a 6,000 rpm redline, and four common axle ratios. Bigger tires lower the cruise rpm and stretch every gear; a numerically higher axle pulls both back. The middle column is what you hear on the highway, the right one is how long 1st gear lasts.

Tire	Axle	Rpm at 70 mph in 10th	1st gear at redline
275/60R20 (33.0 in)	3.55	1,610	35.3 mph
275/60R20 (33.0 in)	4.10	1,860	30.6 mph
35 in	3.55	1,518	37.5 mph
35 in	3.73	1,595	35.7 mph
35 in	4.10	1,753	32.4 mph
37 in	3.55	1,436	39.6 mph
37 in	4.10	1,658	34.3 mph
37 in	4.56	1,844	30.8 mph

To keep the stock feel after a tire change, scale the axle by the diameters: 3.55 x 35 / 32.99 = 3.77, so a 3.73 is the nearest common ratio for 35s; for 37s the same arithmetic gives 3.98, close to 4.10. Whether you want the stock feel or a little more pull for towing is your call; the table shows what each choice costs at 70 mph.

Close steps or a wide spread: a 6-speed manual against a 10-speed automatic

Two real ratio sets run through the calculator. The manual is the TREMEC T-56 Magnum close-ratio set (part TUET11009), the automatic is the Ford and GM 10-speed (10R80 / 10L80). Ratio spread is 1st divided by top gear; the average step is the geometric mean, the step that would repeat evenly across the box.

Measure	T-56 Magnum, close ratio	10R80 10-speed
1st and top gear	2.66 / 0.63	4.696 / 0.636
Ratio spread	4.22	7.38
Average step	1.33	1.25
Biggest step	1.49 (1st to 2nd)	1.57 (1st to 2nd)
Smallest step	1.25 (4th to 5th)	1.08 (9th to 10th)
Rpm after the 1-2 shift from a 6,000 rpm redline	4,015	3,814

The automatic covers far more ground, a deep 1st for a loaded start and three overdrive gears for the highway, with more and smaller steps in the middle. The manual starts much taller, so its first gear alone carries a 245/45R18 car with a 3.73 axle to 52.0 mph at 6,500 rpm, and its 6th turns 2,072 rpm at 70 mph. Neither number is good or bad on its own: the spread says how wide the box is, the steps say how far the revs fall on the way through it.

Five ways the answer comes out wrong

Reading a flotation size as metric. In 35x12.50R17 the first number is already the diameter in inches. Typing 35 as a width would mean a tire 35 mm wide. Choose the flotation format and enter 35 as the diameter.

Leaving out the axle. A 0.636 top gear sounds tall, but the engine sees 0.636 x 3.55 = 2.26. Every speed in the results is built on the overall ratio, so a wrong axle shifts all of them by the same percentage.

Treating the redline speed in overdrive as a top speed. The 10-speed above would reach 260.8 mph at redline in 10th on paper. Power and drag stop a truck long before that; the gearing only sets a ceiling, and the speed the engine can really reach has to come from its power against air drag and rolling resistance.

Trusting the sidewall to the inch. The size on the sidewall is nominal and unloaded. A tire carrying weight rolls a little smaller, so real speeds sit slightly below the table and real rpm slightly above it. Worn tread does the same.

Mixing up "higher" and "lower" gears. A 4.10 axle is numerically higher than a 3.55, yet people call it a lower gear because it gives a lower road speed per rpm. The calculator only reads the number.

Gearing questions from the shop and the forum

What does a 3.73 gear ratio mean?

The driveshaft turns 3.73 times for each turn of the wheels. Multiply it by the transmission gear in use to get the overall ratio: in a 1.00 (direct) gear the engine turns 3.73 times per wheel turn, in a 0.63 overdrive 2.35 times.

What gear ratio do I need for 35 inch tires?

To match the rpm you had on stock tires, multiply the old axle ratio by new diameter / old diameter. From 33-inch tires and a 3.55, that is 3.77, so a 3.73 is the closest common ratio. Enter both setups in the calculator and compare the rpm at 70 mph and the speed of 1st gear.

Why does my speedometer read low after bigger tires?

The speedometer still counts the revs per mile of the old tire. A 33.0-inch tire turns 611 times per mile, a 35-inch one about 576 times, so at an indicated 70 mph the truck really moves at about 74.3 mph until the speedometer is recalibrated.

How can I find my axle ratio without paperwork?

With the vehicle safely raised and the transmission in neutral, mark the driveshaft and a tire, turn the tire one full turn and count the driveshaft turns. About three and three quarters means a 3.73. With an open differential, lift both wheels on that axle and turn them together, or the count comes out half.

Does a torque converter change these numbers?

Only while it slips. An unlocked converter lets the engine spin faster than the transmission input, so the real rpm is higher than the table shows, most of all when pulling away. With the lockup clutch engaged the two turn together and the figures apply as calculated.

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