What RPM at 70 mph with 33s?

The tire took away 99 rpm

Before the swap, a 265/70R17 on the same 3.73:1 axle and the same 0.84 top gear turned 2,333 rpm at 70 mph. On 33x12.50R17 it turns 2,234. Ninety-nine revolutions per minute, from nothing but a taller tire — the drivetrain is untouched, and the engine is doing less work per mile because each wheel rotation now covers more ground.

Re-gearing overshoots slightly

The nearest available replacement ratio for this swap is 3.91:1, and on that axle the same tire at the same speed turns 2,342 rpm. That is nine above where the truck started rather than back at 2,333, because 3.91 is marginally deeper than the 3.89:1 the arithmetic asked for. A small overshoot, and worth knowing before treating a re-gear as a restoration.

Which gear you are counting matters more than anything

Top gear on modern transmissions is very deep. At 0.84 the answer is 2,234; in a direct 1.00 gear the same tire, axle and road speed give 2,660. Nothing else changed. Quoting a cruise rpm without naming the gear is quoting one number out of however many the gearbox has, and the tool below runs the whole set at once — on its own ratios or on the ones you type in.

Where the 33 enters the calculation

Only once, and only as a rotation count. 33x12.50R17 turns 611.2 times per mile; the 265/70R17 it replaced turned 638.1. Multiply either figure by the road speed, the transmission ratio and the axle ratio, divide by sixty, and the engine speed falls out. The tire's width, its wheel diameter and its construction are all absent from that line, which is why a 33x12.50R20 gives exactly the same answer as a 33x12.50R17.

The ratios used here

3.73:1 is a common truck axle and 0.84 is a representative overdrive top gear rather than a figure taken from any specific transmission. Both are inputs, not facts about your vehicle — enter your own in the calculator and the arithmetic follows. The transmission gear ratios page explains where the numbers come from and why they are not published consistently.