Speedo problem

[Cooper1999]

I'm building a Mini Marcos currently. I presented the car for IVA and one of the things it failed on was the speedo - at an indicated 35mph (on the speedo) it was actually doing 37mph. (The car must not read low - it can read high by a surprising amount, but not low)
I thought I might have got my speedo spindle/drive mixed up (and that might still be the problem), but are there any other methods of adjusting the speedo reading, or is it only through the speedo drive gears?

Some more info:
Currently running 145/70x12 wheels/tyres.
2.7FD fitted (as fitted to an MPI mini)
The 3-clock dash/speedo unit is definitely from an older (than MPI) car.

I was originally going to fit a 2.9FD and may have gotten the spindle/drive gear mixed up when I ultimately decided to go 2.7FD (I have a correct yellow spindle (7 tooth) and black (15 tooth) drive gear for a 2.7FD to fit if required, when I pull those currently fitted out to check), but if this isn't the problem I'm wondering if there are differences in the actual speedo head unit itself that could be throwing the recorded speed out.
And if I do have the 2.9FD speedo gears fitted, does that sound correct - a recorded 35mph would be slower than the actual speed?

Hoping someone can help. Cheers,

Edited by Cooper1999 on Sunday 2nd July 16:37