Diff Ratios

Mustang Australia

Author Topic: Diff Ratios  (Read 2260 times)

Offline SGB

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Diff Ratios
« on: January 16, 2023, 03:08:55 pm »
I have 64.5 with a standard 289 and am fitting a T5Z manual gearbox behind it. I am looking for some advice on what differential ratio I should be runing. The car is currently running an 8" diff I dont know what ratio at this stage but I suspect it is a 3.1. Am also considering buyingh a diff from a later vehicle that has disks etc. I anyone has any ideas on suitable alternate diffs that I could get from a wrecker I would apprciate your comments

Offline FB.65.68.70

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Re: Diff Ratios
« Reply #1 on: January 16, 2023, 04:52:38 pm »
Hi Steve,
try downloading this app for your phone. 'Gear Speed Calculator. or there is also a 'Tremec' app. If you plug in your tyre size and intended diff ratio they will show you your speed in each gear against engine RPM. really handy in working out what diff ratio to look for. Then work out what diff you want or to change gears in your 8inch and add disc brakes


Offline GEOFF289

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Re: Diff Ratios
« Reply #2 on: January 29, 2023, 01:30:38 pm »
I just noticed this thread and thought I'd chime in. I have a T5 in my '66 behind a mild 5.0 roller cam engine.

You are very wise to think about this. A lot of people focus on the overdrive when doing a T5 swap and don't give any thought to the other end of the equation. First gear in a lot of T5's is pretty low and you run the risk of rendering first gear pretty much redundant if you go too low (numerically high) in the diff. I do think the Z's have a 2.95 first gear, which is what I have in mine - I got it changed from the 3.35 my transmission had originally for this very reason.

Tyre diameter is the other variable to consider and one of the calculators Mark pointed you to is a good idea.

I have 3.25's in my diff with 17" wheels and 225/45/R17 tyres. I find this a pretty good set up with good acceleration through the gears, its happy a bit over 80 in fifth on the flat and cruises at 100 in fifth at about  1,900 rpm (0.68 overdrive). Depending on your tyre size I don't think I'd go much lower in the diff unless you want to routinely change into second after about 2 metres, or you want to routinely wring the car's neck.

I wouldn't bother with discs on the back. The fronts do most of the braking.

If your car was originally an auto its probably got 2.80's in the diff but you can determine what it is easily.  enough. Jack the car up at the rear, mark the pinion or tailshaft on the bottom, then put your phone under it on video record. Turn one of the wheels two full turns, then see how many rotations your mark on the pinion made - you'll have to guesstimate fractions of a turn but if its three and just a little bit it probably confirms your 3.10 suspicion, but I think 2.80 or 3.0 is more likely. Its unlikely but if you turn one wheel and the other one also rotates in the same direction (don't worry about it going the other way) you have an LSD of some sort and only one rotation of the wheel is needed to give you the diff ratio.

If this suggests you have 3.0 or 3.10 I reckon just put the trans in and see how you like it before spending the money on a different ratio, particularly if you have smaller tyres than mine. You can always change it later.

Offline AussiePhil

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Re: Diff Ratios
« Reply #3 on: January 30, 2023, 07:15:14 am »
Hey Steve,
If you do find a diff housing with large bearing, new small flange you can pick up Ford Explorer UN, UP UQ disc brakes with drum park brake at a wreckers for almost nothing.
Cheers Phil.
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