Engine speed at road speed is set by three things multiplied together: how far the tire travels per turn, the axle ratio, and the gear you are in. Change any one and the others have to be re-thought.
Where the formula comes from
A mile is 63,360 inches. Divide that by the tire's circumference and you have revolutions per mile — 720 for a 28-inch tire.
At 70 mph the tire turns 720 times per mile times 70 miles per hour, which is 50,400 turns per hour, or 840 per minute. That is driveshaft-side wheel speed.
Multiply by the axle ratio to get driveshaft speed, then by the gear ratio to get engine speed. At 3.73 and a 1:1 top gear that is 840 times 3.73, about 3,130 rpm.
What the axle ratio actually does
A numerically higher axle ratio — 4.10 rather than 3.42 — means more engine turns per wheel turn. That multiplies torque at the wheel and raises rpm at any given speed.
So higher numbers give better acceleration and towing and worse fuel economy and more noise at cruise. Lower numbers do the reverse. There is no free lunch; it is a trade between the two ends of the rev range.
Overdrive gears complicate this usefully. A gearbox with a 0.65 top gear lets a truck run a short 4.10 axle for pulling and still cruise quietly, because the top gear divides the ratio back down.
Tire diameter is a gear ratio
This is the part people underestimate. Fitting a tire 10 percent taller has the same effect on cruise rpm as changing the axle ratio by 10 percent — it is a gear change whether you intended one or not.
A truck that came with 31-inch tires and a 3.73 axle, fitted with 35s, is effectively running 3.30. It cruises quieter and accelerates noticeably worse, and if it tows, it now works much harder.
That is why regearing is the standard companion to a big tire fit. The axle ratio calculator on this site works out what ratio restores the original behaviour.
Reading the result
Most modern cars cruise at 1,800 to 2,200 rpm at 70 mph in top gear. Diesels often sit lower, small engines higher.
The mph per thousand rpm figure is the most portable way to compare setups — it does not depend on the speed you picked. Around 30 to 35 mph per 1,000 rpm is typical for a car in top gear.
Peak torque rpm is the figure worth cruising near for towing, and it is usually well below peak power. That is why a tow rating assumes a gear low enough to sit there.
What this does not account for
Tire growth at speed. A tire's rolling diameter increases slightly with speed as centrifugal force pushes the tread outward, which is a fraction of a percent but real.
Torque converter slip on an automatic. Below lockup there is no fixed relationship between engine and driveshaft at all, so the calculation only holds in a locked converter or a manual.
Loaded versus unloaded rolling radius. A tire under load flattens slightly, so the effective radius is a little less than the free diameter suggests.