Gearing advice

Graham Martin · Post by **Graham Martin** » Mon Nov 11, 13 11:14 am

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

MilesnMiles · Post by **MilesnMiles** » Mon Nov 11, 13 11:21 am

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.

Guy · Post by **Guy** » Mon Nov 11, 13 11:30 am

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

Graham Martin · Post by **Graham Martin** » Mon Nov 11, 13 11:30 am

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

Pete · Post by **Pete** » Mon Nov 11, 13 11:46 am

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

)

Pete · Post by **Pete** » Mon Nov 11, 13 11:47 am

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....

Graham Martin · Post by **Graham Martin** » Mon Nov 11, 13 12:22 pm

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

dustymopar · Post by **dustymopar** » Mon Nov 11, 13 1:55 pm

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

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

Graham Martin · Post by **Graham Martin** » Mon Nov 11, 13 2:53 pm

Perfect cheers

dustymopar · Post by **dustymopar** » Mon Nov 11, 13 3:20 pm

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

Graham Martin · Post by **Graham Martin** » Mon Nov 11, 13 3:46 pm

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

Dave-R · Post by **Dave-R** » Mon Nov 11, 13 5:48 pm

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

that is from when I could think! ;-)

dustymopar · Post by **dustymopar** » Mon Nov 11, 13 6:43 pm

You're being modest - It's really useful