Hey there gang. I need some rear axle gear help. I have a 331 cad thats going to be backed by a Borg Warner T10 H. The T10 has following gears: 1st: 2.73 2nd:2.04 3rd:1.50 4th: 1 This is going in a model a with 750-16 or 700-18 rear wheels. 750-16 diameter:31 700-18 diameter:32,5 highway cruising speeds probably 60-70 miles. But also want a good Get up and go green traffic light additude. what kind of rear Axle gear should i look for? Is there a happy medium gear? Ideas?
You need to decide what rpm you want to achieve at cruising speed. I think the 331 is at peak torque at 2700 so that would be a good starting point. As a rough rule of thumb, cruising is at max torque and shifting at max power. Those two figures for your motor will help you work out what that means in terms of RPM and road speed in the gears. Usefull calculator for working out engine rpm to road speed.... https://spicerparts.com/calculators/transmission-ratio-rpm-calculator
I run a 29" tall tire (235/75-15) with a 3.89 rear. My 1st was a 2.56 but is now a 2.52 (Muncie M20). From a dead-stop, acceleration is good. At 65 I'm at 3000 RPMs, at 70 around 3200. Passing at 60-70 is excellent.
Light car, excess of torque, tall tire, no O/D. I think the recipe for power you can feel in any scenario is 3.31 or in that range. Some late model ratios are 3.27 or 3.28, some older axle ratios were like 3.37. That should have you cruising just below peak TQ at 65-70 and with such light car even a stomp in 4th should really get your attention.
Is the engine going to have a bigger cam, higher compression? You don’t want too high (lower numerically) geared rear end that your slipping the clutch and keeping the engine going just to putt through a parking lot.
notice that the answers are all over the place? That's because we each have our own idea of what's "right"....and you gain that with experience. Pick a ratio, try it out, see how you like it. Somewhere in the mid 3s is a good place to start...if you don't like the cruising RPM, you can go to a milder gear, and if you don't like the lack of acceleration, you can go to a steeper gear. Good luck, it's a really personal choice.
It’s all going to start with your desired engine RPM at cruise speed and the average length of cruising on the highway. You pick that either off personal preference or engine requirements/limitations. When I pull out of my driveway it’s min 55 mph for 30 mins any road any direction before I get anywhere. My old place was 30 minutes any direction max 35 mph before I ran out of places to go or got to the highway. Is it 15 minutes on the highway or 3 plus hrs on the highway? Squirrel said it best, but maybe you can ride in a few other buddies cars and gain a little experience/ insight without spending a bunch of money. Knowing what you don’t want is good too.
I wouldn't go more than a 3.56 rear gear myself. 331's are not high rpm engines, and a 3.73 gear will have your engine cruising about 3000 rpm's at 70 mph, and it gets old fast. Even a 3.31 or 3.23 gear would likely be better yet with a 1:1 high gear in the trans.
Here are two Speed, rpm, tire size , gear ratio calculators that I use quite often this one you put in speed in mph, Tire diameter, rear gear rato and trans output gear ratio and get the rpm Calculate RPM for Given Speed(MPH), Rear Gear Ratio, and Trans Gear Ratio (purperformance.com) This one works best with 1 to 1 transmission ratio in high gear rather than overdrive. The good is you can plug in three factors and get the fourth and that lets you play with things like tire diameter and get both rpm and speed. Tire Size, RPM, Speed, and Differential Ratio Calculator (advanced-ev.com) I'd have to agree that between 3.4 and 3.7 should work pretty well.
The Jalopy Journal
