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Old 10-08-2020, 08:41 AM   #1
Seth Swoboda
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Default 1967 Mustang rear end gear ratio

I have a '67 Mustang with a terrible 2.79 rear end gear ratio. I'm thinking about changing it up and was curious as to what ratio you guys thought would be good for the street and have the ability to break a wheel loose. If I change things up I'd go with a 9 inch, posi-traction as well. What about 3.25's? Are 3.50's too much? I'm not terribly concerned with gas mileage, it's already poor with the 2.79's.

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Old 10-08-2020, 09:09 AM   #2
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Default Re: 1967 Mustang rear end gear ratio

Automatic or manual? I'm a 3.55 with a 1993 or earlier AOD. Visit the yards for an Explorer/Ranger 8.8. The 3.73s in the Explorer were T-locks, the Ranger (V-6s only) 8.8 will be a 3.23 or 3.55 and be really close to a bolt in with removal of the spring pads nad welding new ones on for the axle on top of the spring
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Old 10-08-2020, 09:13 AM   #3
Seth Swoboda
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Default Re: 1967 Mustang rear end gear ratio

Quote:
Originally Posted by Ford blue blood View Post
Automatic or manual? I'm a 3.55 with a 1993 or earlier AOD. Visit the yards for an Explorer/Ranger 8.8. The 3.73s in the Explorer were T-locks, the Ranger (V-6s only) 8.8 will be a 3.23 or 3.55 and be really close to a bolt in with removal of the spring pads nad welding new ones on for the axle on top of the spring
C4 automatic/289.

I will add that I have a 1974, 351w that I'd like to build and replace the 289. I'd also like to eventually go with the AOD transmission.
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Old 10-08-2020, 01:45 PM   #4
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Default Re: 1967 Mustang rear end gear ratio

A family member swapped the 3.00 ring-and-pinion in his 65 Mustang (302/w C4) for 3.50 gears. He now wishes he had kept the 3.00 gears. If you do much highway driving, you probably won't like anything higher than 3.00 with the C4. 3.50 gears/w AOD transmission would be better.

Dynamic Racing Transmissions has a C4 wide-ratio gear set. 1st 2.90/2nd 1.60/3rd 1:1. The stock C4 ratios are 1st 2.44/2nd 1.49/3rd 1:1. The wide-ratio gears will help off the line, while your current rear gears good for highway. I guess it comes down to what you want and what you can afford.
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Old 10-08-2020, 05:32 PM   #5
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Default Re: 1967 Mustang rear end gear ratio

What percent of high way miles do you drive? I have 67 Galaxie with 3.25's from factory and had 72 Cougar with same. Both cars were manual transmission and seems it was a comfortable cruise. I've been driving for so long with the old 2.70's gears and now with the modern overdrives that higher rpms kind of get on my nerves. I guess it depends on what you want to do with the car but you can't go to far wrong with 3.25's.
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Old 10-08-2020, 06:10 PM   #6
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Default Re: 1967 Mustang rear end gear ratio

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Have a 260 with early C4. 3:25 gears are great for around town and on the highway, they are not too much. With new cars having 5 and 6 speed automatics, we think that any RPM over 1800 at 65 MPH is too much. Sure 1800 RPM helps gas mileage, but do you really care with your car.
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Old 10-08-2020, 07:59 PM   #7
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Default Re: 1967 Mustang rear end gear ratio

I have a 48 Ford with a 302, C4 and a 3.50 (3.55?) rear. I don't find the engine working that hard when doing 65 or so. Don't have a tach in this car, so ,I don't know what the rpm is at 65.



My TBird has a 3.31 rear gear, no O/D. 2800 at 65 which I don't think is excessive.


If you don't do a lot of highway driving, I would suggest a 3.50, which is fairly easy to find for an 8 inch rear. 3.25 may be a little better on gas, and would be a good comprise between 2.79 and 3.50.
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Old 10-08-2020, 11:27 PM   #8
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Default Re: 1967 Mustang rear end gear ratio

This might help: strangeengineering.net/gear-ratio-calculator/
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Old 10-08-2020, 11:39 PM   #9
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Default Re: 1967 Mustang rear end gear ratio

I didn't like the OEM 3.31 gears in my 1955 Thunderbird/w Fordomatic on the highway, and yes, I'm used to modern cars/w overdrive. I had 2.72 gears and an Auburn limited-slip differential installed in the Dana 44 rear end, swapped the Fordomatic for a C4, and installed the wide-ratio gear set. With 215/70R15 tires (26.9" diameter), I'm looking at approximately 2200 rpm at 65 mph-600 rpm lower than the 3.31 gears. Whatever you choose, make sure it's a good match for your camshaft and torque converter stall speed.
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Old 10-09-2020, 10:54 AM   #10
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Default Re: 1967 Mustang rear end gear ratio

2.90 1st x 2.72 =7.888/stock C4 1st 2.44 x 3.23=7.8812.
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Old 10-09-2020, 06:21 PM   #11
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Default Re: 1967 Mustang rear end gear ratio

For most cars with large diameter tires something in the low to mid 3s would generally be fine but with small diameter tires the taller gears like 3.0:1 or 3.25:1 would be OK with an automatic transmission.

If it's built and you want to get off the line quick then the higher ratio numbers would apply but it won't do much for top end or gas mileage. If you don't care about getting off the line but still want freeway speeds then don't go for higher ratios.

You might try to find a mustang forum to ask your question just to see if anyone has experience with changing the 8" type ratios. With 14" tires & wheels you may want to try a 3.25. It should be a good all around ratio with that wheel size. If you tires are 15" or just really high aspect ratio then you would be OK with 3.55:1.
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Old 10-09-2020, 06:36 PM   #12
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Default Re: 1967 Mustang rear end gear ratio

Lots of good advice, but if you're going to pull the 289/C4 combo and go with the 351/AOD, the rear gears are going to be a compromise. A good torquey 351 with 3.5's or 3.9's would really be a nice driver in a light car like that. The o/d would give a final ratio equivalent to 2.6, and make a great highway gear. And the 3.9's should let you "break a wheel loose". Better buy the posi unit.
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Old 10-10-2020, 06:57 AM   #13
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Default Re: 1967 Mustang rear end gear ratio

My '68 with a 351W and C4 had a 3.90 rear. I ran tall 15" tires on it, 60 MPH was 3000 RPMs. I could burn the tires off it from a dead stop. I would say that anything from a 3.55 to 3.90 would make you happy.

The gas mileage with my car was still decent and I drove the car all over, state to state. A great car for getting somewhere fast.
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Old 10-10-2020, 11:25 AM   #14
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Default Re: 1967 Mustang rear end gear ratio

If your engine is stock, it's pretty much all done at 4500 rpm. It may rev higher but not make more power. Unless your engine is set up to make power in the upper-rpm range, it doesn't make sense to me to use a numerically high gear ratio. Serious drag racers don't want to burn rubber after the lights turn green, and they do their racing on a track. There's no sense in spending a lot of money on tires if you're just going to burn them off.
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Old 10-13-2020, 03:29 PM   #15
Seth Swoboda
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Default Re: 1967 Mustang rear end gear ratio

Thanks for all the advise fellas. This was all very helpful. I was away from the computer for a while. I'll probably sit tight for now. I do want to build a 351W for this car and I'll keep researching the most sensible combination of engine/transmission/rear gear ratio for that.

Have any of you built a 351w with the Edelbrock 2092 top end kit? Thoughts?
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Old 10-13-2020, 06:46 PM   #16
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Default Re: 1967 Mustang rear end gear ratio

If I'm not too late with another gear ratio story...
Back in '71 I bought a new Mercury Comet, a slightly upscale version of the Maverick.
It had a 2bbl 302 with single exhaust, optional 3spd floor shift with a 2.79 axle gear and small 14 inch tires. Not too quick from a stoplight but no other real complaints as I worked out of town a fair amount, spending lots of time on the hiway.
After the warranty was up the engine had some parts changed: a 4bbl carb & intake, dual point distributor, cam, hi-po Mustang heads of some sort, higher compression pistons and a set of headers. Also similar height but wider tires & wheels.

Being young and foolish at the time (as opposed to much older and only slightly less foolish now) I put a close-ratio 4 speed in it, just because "4spd" sounded so much better / cooler than 3spd.
The engine had a nice wide rpm operating range but the close-ratio gears were all too much the same, still no pull at the bottom and not as much speed at the top. My out of town job had ended and I compounded the earlier questionable transmission change by swapping in a 3.25 axle gear. It had slightly better low end pull but of course even less hiway speed.

In hindsight, a wide ratio 4 speed with an overdrive high gear and the 3.25 axle gear (or maybe 3.00 ?) would have been a better 'all around' transmission and axle gear choice for me as I prefer 'road' driving and corners to 1/4 mile.

Last edited by dmsfrr; 10-14-2020 at 04:20 PM.
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