Regearing from 35x12.50R20 to 265/65R18 on 3.08 gears

35x12.50R20 → 265/65R18 on 3.08 gears

3.08: 1 axle ratio

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

Gear ratio needed3.08 × (31.56″ ÷ 35″)	2.78: 1
Nearest ratio you can buy+10.9% from ideal	3.08: 1
Tyre diameter-9.8%	35″ → 31.56″
Engine rpm at 65 mph — before35x12.50R20 on 3.08 gears	1,922rpm
— after new tyres, old gears10.9% higher — this is what feels wrong	2,131rpm
— after regearing to 3.08back within 10.9% of stock	2,131rpm
Effective ratio nowwhat the bigger tyre has already done to your gearing	3.42: 1

Notes

Fitting 265/65R18 on 3.08 gears raises effective gearing to 3.42. The engine spins 10.9% faster at 65 mph — more responsive off the line, worse on fuel and noisier on the highway.
Axle ratios are not continuous. 2.78 is exact, but you buy from a short list: 3.08 is the nearest, and it lands 10.9% 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.

Bigger tires are a gear change you did not ask for

Going from 35x12.50R20 (35″) to 265/65R18 (31.56″) on 3.08 gears leaves you with an effective ratio of 3.42:1. At 65 mph the engine falls from 1,922 to 2,131 rpm — 10.9% — which is why a truck feels lazy and hunts between gears after a tire upgrade.

The ratio that puts it back

2.78:1 exactly. But axle ratios are not continuous: you buy from a short list, and the nearest is 3.08:1, which lands 10.9% from ideal. That brings 65 mph back to 2,131 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.

Nearby sizes

Load Index and Speed Rating — The Two Codes at the End of the SizeEveryone reads the size and stops. The two characters after it are the ones with a safety consequence.
Speedometer Error From Tire Size — One Ratio, and It Runs Both WaysA one-inch increase in diameter on a 30-inch tire is a 3.3% error. At an indicated 70 you are doing a bit over 72.
Recalibrating a Speedometer After a Tire Change: Four Routes, and How to Verify It WorkedOn most modern vehicles the tire size is a stored number, not a mechanical ratio. Where that is true, correcting it takes minutes; where it is not, the options get physical.
Plus Sizing Chart: Wider Tire, Shorter Sidewall, Same Rolling DiameterPlus sizing trades sidewall for wheel and keeps the overall diameter where it was. Done properly nothing on the vehicle notices — which is the entire point.