Speedo calibration and/or axle ratio change

[Brettfcars]

Hi,
Taking a just finished car out (it’s an Ulsteroid.) for some shake down tests.


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I found a few tweaks required, adjusted dampers, Tick over etc

A few tweaks still to do.

1. Speedo reads 20mph when GPS reads 45.  60mph max Speedo.

   

2. 1st gear not really required, pulls Away fine in 2nd.

3. 4th gear top speed about 45mph via gps.

Car is on 17” wheels 4speed synchro box, 37 ruby axle believe standard.


4. Temporary fitted a triumph gear selector to an adaptor plate and it works well, very direct and in right place. But fells a bit to triumph..

   


I have a 8.43 5.375 marked pinion and Ring gear in garage, would this be preferable?
I have a spare rear axle I could build it into....

So a couple of questions.
1. Would the change of rear axle improve things. Top speed and indicated speed ?
2. Is there different Speedo gearbox output gears that may improve things?

Thanks

Brett