Fitting taller tires is a gear change. The engine turns fewer times for every mile, which feels like a taller final drive — quieter at cruise, and noticeably slower to accelerate or pull. Restoring the original behaviour means a numerically higher axle ratio by the same proportion.
The arithmetic is one multiplication
Divide the new tire diameter by the old one. That is how much taller the gearing became. Multiply the current axle ratio by the same figure and you have the ratio that undoes it.
Going from 31-inch to 35-inch tires is a factor of 1.129. A 3.73 axle times 1.129 is 4.21, so the nearest common ratio is 4.30 — which is exactly the swap most people doing that tire change end up making.
It works because both terms are simple multipliers on the same chain from engine to road. Nothing about it is approximate.
What it feels like before regearing
The immediate effect of taller tires is that everything feels lazier. The engine has less mechanical advantage at the contact patch, so acceleration drops and the transmission holds lower gears longer.
An automatic is the worst affected. It shifts by load and speed, and with the effective ratio changed it hunts between gears on gradients, which generates heat in the converter.
Fuel economy usually gets worse, not better, despite the lower cruise rpm — because the engine is operating further from its efficient load point and the transmission is downshifting more.
Ratios come in fixed steps
Ring and pinion sets are manufactured in specific ratios, and which ones exist depends on the axle. Common steps are 3.08, 3.23, 3.42, 3.55, 3.73, 3.90, 4.10, 4.30, 4.56, 4.88 and 5.13.
So the calculated ideal is almost never available and you pick the nearest. Erring slightly numerically higher than the calculation is the usual advice, because it favours the low-speed behaviour most people wanted the tires for.
Not every ratio exists for every axle, and very high ratios sometimes need a different carrier. Worth confirming before budgeting.
Both axles have to match
On a four wheel drive, front and rear ratios must be identical. Even a small mismatch makes the transfer case wind up against itself on pavement, which destroys it.
That means regearing is two complete jobs, two sets of gears, and two setups. Gear setup is a skilled operation — backlash and pinion depth have to be measured and shimmed — and a bad setup howls and eventually fails.
This cost is the main reason people choose a tire size that does not need regearing, and it is a legitimate reason.
The speedometer comes along too
Taller tires make the speedometer read low and the odometer under-count. Regearing puts the driveshaft speed back where it was, which on many vehicles corrects the speedometer as a side effect.
On vehicles that take wheel speed from the wheels rather than the driveshaft, it does not — the sensors still see the new tire. Those need a software or module correction separately.
Either way, worth checking after the job rather than assuming. A speedometer reading 5 percent low is a speeding ticket waiting to happen.