Final drive ratio choice on a VX tarmac car

[Grundo]

Hello,

I have a VX tarmac rally car and I went in a tarmac rally last weekend and the car went VERY well, but, I barely used 4th let alone 5th using the standard VX gearbox.

So this weekend I found and bought a 2litre box and an 1800 box with the intent to use the final drive (and pinion). The VX has a drive ratio of 3.2, the 2litre approx 3.8 and the 1800 4.1 ( I can't remember the exact numbers).

What is the combined wisdoms view on what I should put in?

The car revs higher than a standard VX  (up to 6,500 rpm where I've set the rev limiter which I hit).

I've modelled it using gearbox cascades which while was a neat exercise was hopeless in being helpful

Any help and suggestions would be helpful. I also have a 4.3 final drive which would just be silly.....

Thanks,
Adam

[WestonE]

Adam

Put the complete 2000 box in as this is the next nearest and the VX torque would blow through the gears on the 1800, but you may want larger diameter tyres if you find it under-geared. As the pinion is attached to the base of the primary shaft fitting the complete 2000 box is simplest although your speedo might miss-read afterwards. 

Eric

[Grundo]

Thanks Eric, it is the ratio I was erring towards. I don't need to worry about the speedo being out as I have an LSD in the existing box and the speedo is a sensor on the driveshaft now. Changing the box will mean changing the LSD and main gear, I don't know what condition the new box is so will have a look and work out which is better to do.

Thanks,
Adam