Replacing final drive from beetle gearbox?

[a14blue]

I'm in the middle of replacing the rear main!
I picked up a beetle gearbox AM code as the bus only does 50 (sat nav) and its screaming do it, at first thought it must be a lack of power so I fitted a pair of drla 36's and now it just gets to 50 quicker? It's a USA import so I thought it might have a mountain gearbox. But it has a CO code? So it's normal. Could be contaminated clutch plate but it pulls up hills and it doesn't feel like it's slipping.
The AM gearbox is an unknown quantity where my gearbox changes well. is it possible to swap the final drive from the beetle as this will help top speed. I'm not greedy 60 would be bliss and keep me out the way of the HGV's

 

[*Sam*]

It should do more than 50 as stock. What size tyres do you have on the rear

 

[a14blue]

Hi I'm Currently running 195/50 15's at the rear!

 

[megaaircooledvw]

What is the final drive ratio on that gearbox

 

[a14blue]

It has a CO code gearbox which is the same as I think a CA which has a final drive of 5.375
The beetle AM gearbox has a final drive of 4.375

 

[Trikky2]

Hi I'm Currently running 195/50 15's at the rear!

There's your problem. 12% undersized.

12% doesent sound much but it's about the same as the gearing difference between a 1600 and a 2 liter gearbox.

195 70 15 would be pretty close to stock diameter for example.