I have an RD28T + 5 speed fitted in a classic car.

The gearing is all wrong though, 3.9 diff(there is no other diff ratio available) means it is revving at 2600rpm at 100 kph.

I want it 2000rpm at 100kph 

Am running 205/70 14" tyres.

Is there a 6 speed that will bolt on to the RD28, apparently the RD28 is the same as the RB range of engines.

The engine i have is ex  Nissan Patrol but gearbox is Laurel or Skyline 1989ish. 

Hi....welcome to the Forum

I don't think there is much else you can do to lower the RPM at 100kph.....you've already gone for higher ratio tyres, so the only other thing is to fit another gear ratio(s).'

I'm not sure which box will fit though, so may be a case of living with the higher RPM.

Let us know what route you go with it and would love to see some pics of the Laurel