Regearing from 37x12.50R20 to 245/75R16 on 3.31 gears

37x12.50R20 → 245/75R16 on 3.31 gears

3.08: 1 axle ratio

2.73 would be exact; 3.08 is the nearest ratio actually sold

Gear ratio needed3.31 × (30.47″ ÷ 37″)	2.73: 1
Nearest ratio you can buy+13% from ideal	3.08: 1
Tyre diameter-17.7%	37″ → 30.47″
Engine rpm at 65 mph — before37x12.50R20 on 3.31 gears	1,954rpm
— after new tyres, old gears21.4% higher — this is what feels wrong	2,373rpm
— after regearing to 3.08back within 13% of stock	2,208rpm
Effective ratio nowwhat the bigger tyre has already done to your gearing	4.02: 1

Stock tire: 37x12.50R20
New tire: 245/75R16
Stock axle ratio: 3.31

Bigger tires are a gear change you did not ask for

Going from 37x12.50R20 (37″) to 245/75R16 (30.47″) on 3.31 gears leaves you with an effective ratio of 4.02:1. At 65 mph the engine falls from 1,954 to 2,373 rpm — 21.4% — which is why a truck feels lazy and hunts between gears after a tire upgrade.

The ratio that puts it back

2.73:1 exactly. But axle ratios are not continuous: you buy from a short list, and the nearest is 3.08:1, which lands 13% from ideal. That brings 65 mph back to 2,208 rpm.

Both axles, not one

On a four-wheel-drive vehicle, regearing means front and rear together. Mismatched ratios bind the transfer case on any surface with traction, and it is an expensive way to discover the rule.

Regearing does not fix the speedometer

The speedometer, odometer, ABS, stability control and the transmission shift schedule all read off wheel speed and are calibrated to the original rolling diameter. Restoring engine rpm at the driveshaft does nothing for any of them — that is a separate recalibration, and on most modern vehicles it is a software change rather than a part.

Geometry only

No manufacturer fudge factors here. Real moulded sizes vary a few millimetres between brands, and a worn tire is up to ⅜″ smaller in diameter than a new one — both smaller than the difference this page is showing.

Notes

Fitting 245/75R16 on 3.31 gears raises effective gearing to 4.02. The engine spins 21.4% faster at 65 mph — more responsive off the line, worse on fuel and noisier on the highway.
Axle ratios are not continuous. 2.73 is exact, but you buy from a short list: 3.08 is the nearest, and it lands 13% numerically higher than ideal. That is far enough to be worth checking the next ratio up or down against how you actually use the vehicle.
Regearing means both axles on a four-wheel-drive vehicle, not just the one you notice. Mismatched front and rear ratios bind the transfer case on any surface with traction, and it is an expensive way to find out.
Everything downstream reads off wheel speed. The speedometer, odometer, ABS, stability control and the transmission shift schedule are all calibrated to the original rolling diameter — a regear that restores engine rpm does not by itself fix the speedometer, which is a separate recalibration.
Geometry only, no fudge factors. Real moulded sizes vary a few millimetres between brands, and a worn tyre is up to ⅜″ smaller in diameter than a new one.

Nearby sizes

What Is a 33-Inch Tire in Metric? Seven Answers, RankedSeven common metric sizes get called 33s. They span half an inch in height and nearly two inches in width, and the width is what decides whether they fit.
Reading a Tire Sidewall: Every Marking, in OrderThe size is the part everyone reads. The characters after it are the part that decides whether the tire is legal on your vehicle.
Metric to Inches Tire Size: Both Directions, WorkedMetric to inches is arithmetic with one answer. Inches to metric has many answers, because you are choosing among sizes that happen to land near your target.
Comparing Two Tire Sizes: The Four Numbers That ChangeComparing sizes by diameter alone hides the change that most often causes the problem. Four numbers move when you change a tire, and they move independently.