Mopar Muscle Association UK

I want to change the diff on 67 gtx clone 383 motor 83/4 axle would 3.55 be screaming on the motorway at 70

Graham,welcome aboard. There is aSticky with rpm x gear spreadsheet you can use in this section.
Tyre size will have a major bearing on this.
On my old Dart I tried 2.76, 3.23, 3.55, 3.91
Most sociable for driving was 3.23, but I settled on 3.55 as a compromise for getting the power down as well as being ok for long drives.

For current example, my Roadrunner has small rear tyres (soon to depart!) which are 25.25" diameter. I cruise at 70mph at approx 3,000rrpm which is civilised.

As Miles says gearing vs RPM would be greatly affected by the height of your rear tyres. What size rear tyres have you got?

I won't be spending much time on motorways but don't want it for drag racing so sounds like the compromise I'm looking for cheers

I agree that 3.55's are the best trade-off.

Will probably work well with a 383 that sacrifices less torque for higher rev range over a stock 440.....cruising at 4000rpm with a 440 gets very wearing...........drone, drone, drone (a bit like me )

What gears do you have at the moment? 2.76:1's???

If it already has 3.23:1 gears then I would leave alone....

It's got 275/60/15 at moment the previous owner had lsd and very high gearing I was told but the dealer I got it from must have swapped it as it's not lsd any more and gearings not bad but just spins one wheel and goes nowhere

Use Dave Robson's spreadsheet - it pretty much tells you what you need

http://www.moparuk.com/forums/viewtopic.php?t=25101

Perfect cheers

Your welcome, Dave has done the homework so we don't have to

Just spoke to Paul at southern axles to get it booked in going for 3.55

dustymopar wrote:Your welcome, Dave has done the homework so we don't have to

that is from when I could think! ;-)

You're being modest - It's really useful