Easiest is work out what you have now, and multiply the ratio by 0.8 to find out what you need.

Without knowing your tyre size, 2500rpm at 0.85:1 5th gear equates to about 3000rpm in 4th gear, so if you have say 235/60/14 tyres my guess is you have a 3.36. Multiply by .8 and you have 2.78. But you'll have to do your own sums. Work out the tyres circumference in metres, convert 100kM/h to metres/min and work out how many revolutions the tyre will do in one minute, multiply this ny the diff ratio and this gives you tailshaft revs. Multiply this by the gear ratio (1 for 4th and 0.85 for 5th) to get engine revs. Or work backwards