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Question: I have a '47 Ford truck with 12 inches of homemade suspension and Bronco axles. I have figured out the final gear ratio of the dual T-case setup I want to run (203-205) to be 145:1 using my NP435 and 5.43 gears in the axles. I would like to run 39.5s and was wondering what this would make my final gear ratio. Would this tire and gear setup allow me to do 70 mph without blowing my engine, a fuel-injected 302?
David
via 4wheeloffroad.com
Answer: As you seem to know, the final gear ratio is different in every gear. Your crawl ratio is determined by multiplying your lowest low-range transfer case ratio by your lowest transmission ratio by your ring-and-pinion ratio. Your cruising ratio is determined by multiplying your high-range transfer case ratio by your transmission high-range ratio by your axle ratio. Though the tires affect your final drive, they are not considered part of you gear ratio. In your case I feel 5.43 is just a bit too low for 39.5-inch-tall tires when you are cruising, but it will be great if you are rockcrawling.
Remember your NP435 transmission has no overdrive, only a 1:1 high gear, and thus at 70 mph your V-8 engine will be running at just over 3,200 rpm. Here is the equation:
| RPM = | MPH x Axle Ratio x High Range x Top Gear x 336 |
| Tire Diameter (in) |
Now with your numbers:
| 3,233 = | 70 x 5.43 x 1 x 1 x 336 |
| 39.5 |
The issue is that I do not know your vehicle's powerband, most economical rpm, or redline for your engine and truck to help determine what gear ratio would work best. The powerband is usually defined as the most efficient engine speeds. These speeds often range from the max torque rpm to the max horsepower rpm but also covers all rpm that result in 75 percent or more of max torque. The most economical rpm is the point where your engine, truck, and load get the best fuel economy while moving the maximum amount of miles. This depends on everything from aerodynamics to engine build. The redline of your engine is the rpm at which parts like pistons, valves, and bearings get upset due to lack of balance, and the excessive speed causes the engine to start to self-destruct.
So what does this all mean for you? Though I think 3,200 rpm sounds a bit high for a Ford 302 V-8 to be spinning while cruising at 70 mph, I doubt it will blow up. It's probably a fair bit under redline rpm. Plus, back in the '80s Ford trucks were available with the 302 V-8, and they could also be had with 4.10 gears, a nonoverdrive transmission, and 29-inch-tall tires from the factory. If all these options were chosen the truck would be spinning 3,300 rpm at 70 mph-though other gear options had 70 mph seeing 2,500 or 2,800 rpm, which sounds more pleasant to me.
To get back to factory specs simply do the following equation with 2,500 or 2,800 as your rpm:
| Final Drive Ratio = | RPM x Tire |
| MPH x 336 |
Then just try to shoot for that. However, you could do some trick machine work and such to allow your engine to rev at high rpm, and then 5.43s are probably not a problem. The best thing you can do is talk to your engine builder to determine where the powerband is and where it should be when cruising for economy.
This is a great question and qualifies for Tech Question of the Month. Since you'll be doing some gearwork I'm sending you a copy of the new book Differentials: Identification, Restoration, and Repair by Jim Allen and Randy Lyman. Allen is a longtime tech writer who specializes in 4x4s, and Lyman is the namesake of Randy's Ring & Pinion, the largest aftermarket supplier of ring-and-pinions in the U.S. The 380-page book covers more differential tech and info than the average human could ever wish to know, but truck guys like us can't get enough of it. Whether you want step-by-step instructions for setting up gears or identifying your favorite truck axle parts, this book is awesome. All our other readers can order a copy from Randy's Ring & Pinion, 800.209.2881, www.ring-pinion.com.