My 64 F100 has a 8.8 rear end with 3.08 ratio. It has a 302 and Aod. What rear end ratio and tire size would be best for cruising and decent mileage? Thanks
Pick the tire size that you think has the look you want...THEN run the math, because tire size is part of the final equation. That said, assuming you use a tire of about 28" diameter or so, probably a ratio in the are of 3.55 would work well. You could use a 3.73 as an alternative, but I wouldn't go that high (numerically) unless I wanted a strong performance vs cruise outcome.
All depends on what rpms your 302 wants to run best at. 308 and OD isn't usually the best combo unless you are making peak torque well below 2000 Rpms or running well above the legal speed limits.
The 302 in my F250 runs just under 3k rpm cruising on the highway at about 65-70, running a C4 (1:1 in 3rd gear), 245/75r16 tires and 3.73 in the rear. I would prefer a shorter tire once I lower it, but that will put me over 3k cruising. Good to see you're doing your homework before making changes, it's worth it!
mph x gear ratio x 336 divided by tire diameter= rpm's example... 70 mph x 3.00 x 336 divided by 30"= 2352 rpm's
^This. It depends on how mild (or radical) the engine is. A stock engine will have decent torque at lower RPM's than a screaming high horsepower monster that wails like satan's chainsaw. You need to take everything into account before attempting the math.
You have a real economy gear ratio now, especially for a pickup. I doubt you would gain enough from a change to make it worth the trouble and expense. The gears you have now should be fine if you don't carry heavy loads or pull a trailer. The 302 is one of the best motors for economy and decent power. You should be getting 18mpg on the hiway, maybe a little better. You are never going to beat the Hondas. If you baby the gas pedal and get significantly less than that, I would look elsewhere than the rear axle for the answer.
Not always true. With numerically higher rear gears and overdrive - You get the added benefits of moving the load easier thru leverage 1-2-3 gear. Saves gas. Then OD gives momentum and leverage the to save gas by knock the rear gears down. It's all compromise and choosing the correct components to work together. However you do it, it starts with a target rpm set by the engine.
There a lot of tire calculators online that can help you adjust your rpms with tire size. If you arent Towing or drag racing, 3.08s are great. I did tow with a set in my 66 c10 for years, no problem, although i was weak going over Donner pass, and Pacheco Pass, and on burnouts, although I hooked up well on the launch.
Just look at what Ford did when gearing there trucks. Compare weights, tire size, and engine modifications. Wind drag is a big killer of milage, so lugging the engine against the wind in overdrive is not ideal. My '37 Chevy has a 3.08 with 235/75R15 tire, about 28.5" tall, and runs 2700 rpm at 65-70 mph. This is perfect for the little 250 inline engine. Fuel milage drops drastically going against the wind. Joe
Sorry, your thread starter reminded me of the infamous Talking Barbie that said, "Math is hard- let's go SHOPPING!", and got the women's lib movement all upset years ago Okay, sorry again for the OT Brain Fart rant, PLEASE go on
If you run a thirty inch rear tire, the rear gear numbers will tell you how many rpm you will need at 90 mph. For example, a 3.00 overall gear ratio will turn 3000 rpm at 90. The setup you have will turn about 2000-2100 rpm at 90 mph. There are a lot of modern cars that are geared that tall, but I think you are better off to go with a 3.55-3.73-4.10 rear gear, which will give you a fast cruise rpm at 75-80 mph between 2100 and 2500 rpm. By the way, I seldom run at 90 mph; that's just where the numbers are equal.
Had a crown vic with a 4.6 and electronic AOD and 3.08 gears. Worked great for cruising. But also I once installed a 302 and AOD in a guys 56 ford truck with 3.00 gears. Sucked for cruising. Installed 3.55s and it was great. I think the car got away with the longer gear because of the better aerodynamics or something.
Missed the AOD part. I had an O/T 86 Chevy 4wd with 3.08s and o/d. I was running 32 inch tires. I switched to 3.73's, and gained more power, and my mileage went up . I don't remember how much, but I definately noticed.
You really need to pick one (tire size) or the other (rear end gear) to figure this out. You can't solve for X if you don't have the rest of the numbers in the equation!
I just ran the numbers through the Randys R&P calculator, I get 2588 at 90 mph with an AOD, 3.73s and a 30" tire. Fred, have you missed the OD?
My OT cars with OD have "rear" gears somewhere around a 4.10 ratio. In my 2012 Malibu, 70 mph is somewhere around 2000 RPMs on the tach. On a recent trip my gas mileage was 34.5 at 75-80 mph. So the 3.55-4.10 gearing is in the ballpark, depending on tire size. Bob
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