It is time to get the 9" rear axle I purchased set-up for the cat, and I was looking for advice on the ring gear ratio.
The existing open diff will be replaced with an OEM posi unit, but the current ring gear is a 2.75, and I am wondering if something in a 3.25 or 3.50 would be better.
Currently, the 428 (Aluminum Edelbrock CJ heads and Performer dual plane intake) in the truck has an open diff and 3.25 gears and I can spin the 15" 195 tire (no ‘s’) at will, well into 2nd gear. I imagine the posi will tame that some.
Now most of the thrill riding will be in the 0-60mph range rather than the 60-100mph, and I still want good 2 wheel smokey burn outs, but not a big fan of breaking it loose in 2nd gear.
3.25 if you will be doing a fair amount of street and any highway. 3.50 would be the max for street IMO and highway would be tough. Both of these assume no overdrive, OD gearing is a whole different story. The equation changes too if your engine is built with a non street power band (like 3000 - 6500 for instance), more gear there (and less driveability).
I had a 2.79 ratio in a 390 G car with a fresh stock rebuild and it was a great driving ratio. The low end torque of the stock 390 would light up the 215 70 14’s just fine. It probably would get beat in a real drag race, but it was not built for that. It would cruise with traffic any where.
Happy days, my differential guy found a 9" 3.25 Yukon Gears R&P set without all the mark-up @$260. He likes them better than the Richmond’s offered up at MU.
I was just going to stick with the 2.75 and set up a second diff like Royce sometime in the future, but he found that the posi unit I got from Cougar collector Walter had new clutch plates in it, so with the money I saved, I decided to get the Yukon set.
Before removing your 2.75, if you drive in 2nd it would simulate if you had a 3.85 gear in the rear. It gives you a feeling of what it would be like to run with a 3.91 or 3.73 gear. It’s quite a bit more than 3.25. But, kinda gives you an idea of rpms in 3rd gear with a larger rear gear. Personally, I like to cruise more than drag race. So, if the vehicle has a 3 speed, I run a 2.75. My 70 Cougar that has a 460 with a C-6 uses a 2.75. Plenty of torque from the 460, even with the 2.75 is moves pretty good. My 69 Sport Special has an AOD trans, so I run a 3.50 gear set.
In my opinion it depends on your transmission. A C-6 auto has a 1:1 3rd gear ratio, so a 3.55 gear would be a pretty hot gear for long highway trips. Probably run around 3300rpm (give or take) at 75 with that combo. However if you’re running a 4 or 5 speed your final gear in the trans will probably have more like a .84:1 or so ratio. Then a 3.55 would be perfect. The 2011-2014 Mustangs run a 3.55 ratio, with option for 3.73, but they have the 6 speed trans. A while back my ex-brother-in-law and I put a 91 Mustang LX together, primarily for drag racing, we ran a C-4 with 4.11 ratio, and it was not horrendous on the highway, but way too much gear for a daily driver. Straight out of highschool I ran 3.73 in my Camaro and that was just fine with the 5-speed trans… so, there are many factors to consider. 3-speed auto trans 3.55 may be a bit “hot” for the daily drive and run your motor at the 3300rpm area at 75 (give or take). On a 4-speed manual it might feel perfect. It also depends on your preference for what you want it to feel like. Running an engine at over 3000rpm for long distance won’t hurt it, just burn more fuel. As far as being able to smoke your tires in 2nd, any gear over 3.00:1 will probably do that with the CJ motor. One thing I like to say is “rather you’re running 200hp or 1000hp, the car will only accelerate at the rate you tell it to.” I personally am thinking about 3.55 or 3.73 for my 67 cat. Neither of these will be too “hot” for my preference of feel, and I commute 42 miles highway too and from work each day…
The guy building my 428 likes 3.00:1. I’m currently running 3.5:1 and it is too low for cruising. He suggested that 3.0:1 and a higher stall converter provides a better launch combination and comfortable freeway cruising.
I have 2.75:1 in the 69 and it is a really comfortable highway gear
A 428 is a long-stroke torque motor, not a high-rev motor like a 427. Use that low-end grunt to your favor and use no higher than a 3.25. Actually, a 3.00 would be better. Better yet would be to upgrade your trans to a 4-speed (overdrive) automatic like a AOD/4R70, or E4OD with an adapter plate or a Gear Vendors unit on your C-6 and have the best of both worlds.