Hello all, I would like to know what diff ratio would give a higher top speed and what ratio would give faster acceleration with a standard black top Zetec and type 9 gear ratio. What sort of figures can I expect with a 3.92 diff.?
Thanks for looking.

I may be wrong and some one will correct me if so,

I believe a 4.11 will give better acceleration and a low 3. something will give better top speed. I would think that 3.92 is a good compromise with a standard type 9.

Yep, thats about it. But get the ratio too far wrong and it can make the car unpleasant. Actual times will vary more with power than diff ratio. But remember these cars are rarely about top speed, more to do with agility and acceleration. I suspect a sensible range would be 3.5-4. Cruiser to higher revving.

If you go bike engined then another factor is the cruise RPM.

For me it feels wrong to cruise at 8000RPM, so I have a 3.133 diff (giving 5.5kRPM at 70mph), even though that gives an irrelevant theoretical top speed of 140mph.

Matt

I have a 3.62 in wit a 1.8 silvertop. will be changing back to the 3.92 as the 3.62 seems to struggle from 90

It's all about 1st gear in a type 9 oh and the tyre rolling radius.
1st is 3.6 in a type 9 3.3 in a V6 type 9
You have a choice of a usable 1st with a large 4th or a low 1st with a better 2nd 3rd 4th
I have gone for a V6 type 9 box modified to fit a Pinto with a 3.9 diff
If I had the money I would fit the box with the 2.7 or 2.5 1st