What gear ratio for 40 inch tires?
Answer
5.01:1
Going from 285/70R17 to 40x13.50R17 on a 4.10:1 axle, the ratio that restores the original engine speed is 5.01:1. The nearest ratio sold as a ring-and-pinion set is 5.13:1.
The working
- Starting axle ratio
- 4.10:1
- Diameter change
- +7.29 in (+22.29%)
- Ratio restoring original rpm
- 5.01:1
- Nearest ratio commonly sold
- 5.13:1
- Effective final drive before re-gearing
- 3.35:1
The arithmetic lands in the gap
5.01:1. The manufactured ladder offers 4.88 below it and 5.13 above it, and 5.01 sits so close to the middle that the nearest-value rule picks 5.13 by well under a hundredth of a ratio point. In practical terms there is no near match here at all: both candidates miss by about an eighth of a ratio point, one under and one over, and the choice is genuinely open rather than arithmetic.
From a deeper starting ratio the ladder runs out
- From 4.56:1 the requirement is 5.58:1, and the deepest commonly sold set is 5.38:1 — the arithmetic asks for more than the catalogue holds.
- From 4.88:1 it asks for 5.97:1, which is further past the end again.
This is the point at which the standard ring-and-pinion range stops being able to answer the question, and it is a consequence of the size of the change rather than of anything unusual about the vehicle.
The size of the change
40x13.50R17 against 285/70R17 is 7.29 in of diameter, 3.65 in of it at the axle, and 22.29% in proportional terms. Left alone, the effective final drive falls from 4.10:1 to 3.35:1 — the engine ends up geared as though it were in a highway-ratio truck while turning a tire more than seven inches taller than the one the ratio was chosen for.
The width moves as well, and no ratio addresses it
40x13.50R17 is 2.28 in wider in section than 285/70R17. Ratio arithmetic is blind to that entirely — the formula sees revolutions per mile and nothing else — so a page that answers the gearing question has answered only one of the two questions this size change raises. The other is a clearance measurement, and it is taken on the vehicle rather than computed here.
Everything else scales too
The 22.29% applies unchanged to the speedometer, the odometer and the engine speed at any given road speed. It is one number doing four jobs, which is why the gearing question and the calibration question are really the same question asked twice. Set your own starting size and axle and the calculator will work every gear rather than only top.