Gearbox & diff ratio?

[Sil-80 WB]

I got a Kouki silvia K's edition.

When doing 120km/h on speedo (which is just 2-3km/h off compared to gps) in 5th gear, I'm at 3600-3700rpm.
This is on 225/45/17 tires.

I think that is a bit high.

Can it be that I have a shorter diff, like a 4.3 from a skyline or something?

What rpm's do you guys run at about 120kmh/ 75mph?

[kees]

For an S14:

SR S14
1 3,321
2 1,902
3 1,308
4 1,000
5 0,838
R 3,382

Diff:
S14/S15 SR20DET/Z32TT M 3,69
S14/S15 SR20DET A 3,92

120kph should be ~3100 RPM
A R33 GTST Auto diff (4,36) does give you 120kph at 3650