Final drive ratio - OD X axle ratio is final drive (.64 * 4.11 = 2.63)
Okay, my gears are 3.55 because that is what was in the parts car. I also have the 9 inch from the convertible that came stock with a 390 and cruise-o-matic. Not sure what the gears are because there is no tag on it. I think the door tag has that info but i have not looked it up in a few years. Whichever rear end I use, I'll go through it and might end up buying a new 3rd member anyway.
The only reason I was looking at the TKO-600 and the .64 was i want to cruise on the freeway at 75. That being said, I also need to beat my son in his 63 Comet with a 302 and toploader. That is the only car i need to beat. I went to modern driveline and looked at their calculator for speed and gears and stuff. I thought the 600 with .64 was what I was after. If this is not what i want, I'd value opinions.
How do I do that math for the 12:1 thing?
......remember, you can go too little gear too and tire size matters My Mustang with the .64 OD, 3.70s and 27 inch tires got better mileage with the 4.11 gear than a 3.70 because the engine was at a better cruise RPM. Smaller tires and less cam, you could go down, but I would limit myself to about 2.45:1 or so in OD. Keep in mind, mine is a relatively mild EFI 489 so no shortage of torque
1st gear compound ratio (also called Starting Line Ratio)
- (1st gear X rear axle ratio) (2.87 * 4.11 = 11.79)
First gear in a TKO 600 is 2.87:1, different than a Ford 4 speed at 2.32 or 2.78. You can go as deep as you want, but 1st gear gets a little short beyond 12:1 with a set of 27 inch tall tires. If your tires are shorter, I would crowd closer to 11:1. Ironically, the same Mustang got better mileage and was also more fun in town with 4.11s compared to 3.70s, stoplight to stoplight all the gears were happier, and hit the highway, I still drive a 2.63:1 final drive car
So, assuming you are running a shorter tire (run slightly less gear), heavier car (run slightly more gear), and a milder cam in a 390 (run slightly less gear), BUT 100 less cubic inches (run more gear), I would likely want the extra gear around town. So tall as a 3.89/3.91 with a TKO-600, with as low as a 4.11.
FWIW, we just did a 520 HP 461CJ FE, carbureted, concours resto sort of a hidden build Mustang, and we are going to go the 3.91 route with a TKO-600 because it has so much torque that it likely will blow the tires off with a small tire anyway. That will put us at 11.22 / 2.50:1. I likely could go taller, but the car will drag race now and then. If there is any concern about not going enough gear, a 390 Mustang with a 4 speed would be 9.73/3.50, looking at those numbers you can see, the car will accelerate harder (work less in traffic) then hit 5th and cruise on compared to stock.
One last comment for you, just realized you said you had a set of 3.55s A 235/60-15 with a 4.11
gear and the TKO-600 would have you at about 2600 RPM at 75 mph, 3.91 would have you at 2450
rpm or so. A set of 3.55s would have you at 2200 rpm. If the 390 is mild, it will be fine, if it's got some cam in it (more than about a 268H or 270H Comp), you might need the extra gear. FYI 10.18/2.27 compound gearing will be perfectly fine in 1st (and way more than stock
), but you can see 5th gets a little tall. If you are on a budget, I would run the 3.55s and swap gears later, the stock 9 inch is likely 3.00s
Hope this helps