What RPM at 75 mph with 37s?

Five miles an hour is worth 174 rpm here

The same tire, axle and gear at 70 mph produce 2,436 rpm; at 75 they produce 2,610. Engine speed is directly proportional to road speed within a gear, with no curve and no threshold anywhere in it, so the increment between any two speeds is fixed for a given combination. Doubling the speed doubles the rpm.

That proportionality is why a cruise-rpm figure has to name a speed. Quoted alone it means nothing.

The axle is the lever with the widest range

Holding the tire, the gear and 75 mph constant:

• 4.10:1 — 2,347 rpm
• 4.56:1 — 2,610 rpm
• 4.88:1 — 2,793 rpm

Nearly 450 rpm across the ratios a 37-inch build might plausibly run. The tire contributes one term to this calculation — 545.1 revolutions per mile — and the drivetrain contributes the rest.

And in a direct gear

3,107 rpm, if the transmission is in a 1.00 ratio rather than the 0.84 overdrive assumed above, at the same 75 mph on the same 4.56 axle. Deep overdrives are what make a heavily geared truck tolerable at highway speed, and they are the first thing a rule-of-thumb rpm figure leaves out.

Why the tire's contribution is so small a term

545.1 revolutions per mile is the whole of what a 37-inch tire brings to the equation. Against a 285/70R17's 616.6 that is 11.6% fewer wheel turns per mile — a large change by tire standards, and still smaller than the 19% swing the three axle ratios above produce between them. People habitually credit a cruise-rpm figure to the tire; on a build of this kind the drivetrain usually deserves more of it.

What these inputs are

4.56:1 and 0.84 are plausible values, not measurements from any particular vehicle — this site does not attribute unsourced ratios to named transmissions. Put your own in and every gear is computed at once, or see transmission gear ratios for why the top-gear figure is the one that varies most between vehicles.