Hi Sigurjon

Without knowing your power or terminal speed this is based on just looking at a gear calculator.

Running on 205.50.15 with the 4.1 diff ratio you will only be able to do 117mph in 4th gear (assuming 7000rpm limit) and the last thing you want to do it change into 5th as more gear changes obviously means slower time.

A 3.9 diff ratio will allow 123mph in 4th gear which i would personally recomend unless you are running over 500bhp

I personally ran my car with 225.55.15 tyres but on standard gear and diff ratio's and was crossing the line in 4th gear at about 6000rpm at 120mph so think that i had it close however a 3.9 ratio would have been better.

Also im not a fan of the Toyo 888 tyres for drag racing as it does not really matter what pressure you run in them they do not get any better traction due to the side walls being too strong.