How much speedometer error do 35s cause?

The error runs in the direction the rules do not allow

Speedometer regulations in most markets are one-sided. An instrument may read high — showing more speed than the vehicle is doing — but it may not read low, because a driver misled downward is a driver travelling faster than they believe. Manufacturers therefore calibrate with a deliberate margin on the safe side of that line. A taller tire spends that margin and then keeps going: at 7.01% the needle is firmly on the side the standard was written to prevent.

7.01% in numbers you can check

An indicated 70 mph is 74.9 mph over the ground. An indicated 30 is 32.1. Across 100 miles of odometer, 107.0 have been covered. Those figures assume 285/70R17 came off, which is the usual case for a 17-inch truck moving to 35s — the diameter grows by 2.29 in, and 1.15 in of that lands under the axle.

Start somewhere shorter and the number moves a long way

The percentage belongs to the pair, not to the 35. From 265/70R17, which is a full inch shorter than 285/70R17, the same 35x12.50R17 produces 10.74% instead of 7.01%. That is the difference between an indicated 70 being 74.9 mph and being 77.5 mph, decided entirely by two starting sizes 1.10 in apart. Swap either size in the calculator if neither pairing is yours.

What the error does not touch

Wheel-speed data feeds more than the dial — cruise control, the transmission's shift points, and the traction and stability systems all reference the same rotation count. Recalibrating the speedometer readout corrects the display. It does not change the diameter that produced the discrepancy, and anything the vehicle computes from raw wheel rotation stays keyed to the tire that is fitted. The speedometer calibration reference sets out where the reading is generated and what a correction can and cannot reach.