Getting close to putting the limited slip into my rear axle. Ford 7.5 inch North american alxe. Just wondering on which rear gears would work well. I am probably looking at 700 kg for the car when finished and a minimum of 225 rwbh and at least that much torque. The gears I have already are 3.08s and that will put me just about to shift to 3rd gear at 60mph. Does it seam to be roughly in the right area or is the gear range too long.
Dale

seems odd, those figures. What engine/box and tyres/wheels are you using?

I would've expected a 3.08 diff to give you a very high top speed, but if you are into 3rd by 60 then im guessing something around 100+ ish flat out? Probably adequate, but 3.08 is low. Imagine how slow it would be with a 3.9 diff.

I assume by a 7.5inch ford diff you mean the diff fitted to cosworth sierras/merkur? And granadas etc.

id stick with the 3.08 as you have a torque multiplier bolted to the side of engine. if you run 15"wheels and 50series tyres the gearing will be the same as the 2.8 5 speed capri which iirc would be 2750rpm at 70mph in tophth

Sorry I did not give enough info. The engine is a 2.3 ford turbo with a t5 from a thunderbird. and thunderbird rear axle.
From gear calc
Gear Mph per 1000 RPM Mph @5800 RPM Mph @6500 RPM
--------------------------------------------------------
1 5.93 34 39
2 10.08 58 66
3 16.04 93 104
4 23.90 139 155
5 29.50 171 192

Mph RPM (in Gears)

The car that this came out of was a 13 sec 1/4 miler with a 300lb driver- even I am lighter than that. The car was around 3000 lbs.
Dale

192mph!

imho i'd gear the car for around 130mph, your current gearing from the above info shows max revs in top gear @ 192mph?! it's never going to pull that and all you'll be doing is compromising acceleration through the gears. a 3.62 or 3.92 if available over there would be better ratios imho.

Ned.

I dont think it will ever pull those speeds - certainly not with me in thats for sure. I am not going to be doing any racing except maybe a few trips to the strip to get some certified results.
If I go to the 3.9 or 4.11 I dont think I will ever keep traction. It would probably pull harder in the higher gears though. I may have to look around for a set of 3.27 or 3.62 or so. The 3.08 puts me at good rpms for cruising but I guess it will all come down to where my power hits in the rpm range. I should be able to have full boost by just under 3000rpm.

Thanks
again
Dale

if i were you, id try it with the 3.08, but id really expect it to be more fun with whatever ratio is closest to 3.5. Do you have to select one now to put the LSD in, or could you test it first with a few different gears?

Pretty sure the lsd is independant as to the gearing.
Dale

In the 'old days' you geared a car so that straight through the box at max rpm = the top speed that you wanted, 5th then becomes an overdrive.

For outright performance 5th at max rpm is the fastest part of the circuit with the torque curve falling in line through the cogs... ie when you change up approaching max revs the torque in the next gear gave you better acceleration than hanging on to the cog.

I,d say 139mph @ 5800 rpm in 4th would not be far out (maybe a little high) given that your engine should have a huge torque spread with the turbo and its for driving not racing.

Regards Mark

As you have a 5 speed box, make use of it, if you gear too low, first is a total waste of time. I think is is best to have fifth as a cruising gear, 2500rpm at your ideal motorway speed. Most of my accellerating is done between 30 and 90 so 2 gears covering this range is ideal IMHO

Have a look at the diff calculation program here, or Google Bowling and Grippo if i dont get the link to work.Bowling and Grippo