a 1T with 5.14 gears and a 235 would be pretty much screaming and you'd be pushin the pedal hard to the floor unless you were on a long downgrade .... I'd suspect the car speedo too, but might be worth checking the actual rear ratio, they came out with "fuel saver" ratios for the 1/2T in 58, might have had better gears for the 1T as well, or you might actually have the 3/4T 4.57 rear
Bill