At the moment my car is running a 3.9 diff with 185/60-14 wheels/tyres, with the type 9 5th gear ratio this allows me to cruise quite nicely at around 80-85mph without the engine revving it's nuts off.
My new tyres/wheels will be 195/50-13, obviously the revs are going to be quite a lot higher for the same speed, do you guys think it will be worth changing to a 3.54 diff ratio or will this kill the acceleration

Shooter

If your running a 3.9 I assume from this its a Sierra diff therefore your next higher option is a 3.6 (3.54 is a live axle ratio).
There are some great gear ratio calculators that let you try different wheel/tyre/gear ratios
Much better than guessing

Look up the Westfield speed calculator and just put in diff ratio tyre size gear ratios and it works it all out for you

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Originally posted by richard munnis
Look up the Westfield speed calculator and just put in diff ratio tyre size gear ratios and it works it all out for you

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