Final drive calculation.

[Aram]

Hi guys,

Since I put the Altezza hardware in my car the speedometer has quite an indication gap.
When driving 100km/h on the gps the speedometer indicates 130km/h.

I don't want to change my tyre size (205/40/16) so I will change the ring gear.

But how do I calculate this??

Altezza (SXE10):
1: 3.874
2: 2.175
3: 1.484
4: 1.223
5: 1.000
6: 0.869
Final Gear Ratio: 4.100

AE86: (for comparison)
1: 3.587
2: 2.022
3: 1.384
4: 1.000
5: 0.861
Final Gear Ratio: 4.300

Thanks!

Aram.