The std diff 3.6, gives 23.2 MPH@1000rpm in top gear(0.80), with a 205/50x15 tyre
The new 3.9 diff gives 21.4mph@1000rpm top gear with same tyre as above.
Hope this gives an idea, so std diff gives 69.8mph at 3000rpm, 3.9 diff gives 64.2 at 3000rpm.
tabetha