At 70-85mph, what does your overdrive button (the lower down smaller button on the inside of the auto shifter) do. As long as you are not in manual mode pressing this in and out will shift you between 3rd and 4th and obviously you'll see the rev's change.
So check/press "Manual" mode button does give a light up on the dash, if yes, then turn it back off.
Get to 70mph driving slowly, the car should then be in fourth, press the overdrive button, you should get a dash "Overdirve off" light and the revs should increase as the gearbox drops to 3rd. It'll then rev out so get to say 85 and then press the button again and it should go into 4th and the revs drop.
If it does all this ok then the gearbox is doing what it should. If the revs are high then it's looking at diff's and tyres (note generally tyre combo's if anything make the gear ratios longer ie are large rolling radius than stock so unlikely to be that giving you higher revs at a certain speed)