I'd check it against a GPS first and see how far it actually is off. Mine is fast by 4 or 5 mph at 70 with 205 x 75 x 15's. I figured with 225 x 75 x 15 it should be right on. 235's is should be a couple of mph slower.

Speedometer Calibration

2001 Sportage, TJ 106's, Everet spacers and bumpstops, Bilstein shocks on the rear, cone filter, Crager wheels, 235-75-15 Goodyear tires. Custom front winch bumper by Dave Scott. Currently DOA, but it is a work in progress.
2011 Kia Forte EX