Rear gears

I thought I would put these up to help out on determining your rpm at a specific speed and also to calculate the diameter of a regular tire. This information has helped me to figure out what rpm's I will be turning at 60mph with a 27 inch tall tire and my 4.56 gears. It tells me the rpms would be 3405!!!! I will not be driving 60mph much with that rpm let alone to Tennessee. Lol!

Another bit of info that helps if you're trying to choose gears or tire size in order to maximize economy or find a sweet spot for cruising at highway speed, is the piston speed formula.

Piston speed = 2 x Stroke in inches x rpm / 720
http://www.gassavers.org/showpost.php?p=87202&postcount=10

16.4 to 19.8 ft/s

For example, my Torino now has a 351C engine and that has a 4" bore and 3.5" stroke. The tires are 215/70R14 and have a 25.9" diameter. If I end up with the AOD (0.67 overdrive) and keep my rear gears for now (2.75:1) then the formula works like this:

16.4 = 2 * 3.5 * rpm / 720
16.4 = (7 * rpm) / 720
16.4 * 720 = (7 * rpm) / 720 * 720
16.4 * 720 = 7 * rpm
11808 = 7 * rpm
11808 / 7 = (7 * rpm) / 7
1687 = rpm

19.8 = 2 * 3.5 * rpm / 720
19.8 = (7 * rpm) / 720
19.8 * 720 = (7 * rpm) / 720 * 720
19.8 * 720 = 7 * rpm
14256 = 7 * rpm
14256 / 7 = (7 * rpm) / 7
2037 = rpm

Then find those rpm's in the mph calculator.

http://www.currieenterprises.com/cestore/calculators/mphCalculator.htm

In my case, 1687rpm would be at 70.6mph and 2037rpm would be at 85.2mph. I know this 351C can produce enough torque at 1700rpm to maintain highway cruising. I'll probably drive the car in "D" most of the time until I'm over 60mph (2141 in 3rd, drops to 1434 in OD).

Those of you with 400's or something with a 4-inch stroke, you'll be looking at even lower cruising rpm's, like 1476-1782. Just replace the 3.5 stroke above with 4. If you're stroked more, figure an even lower rpm. Obviously if your cam doesn't open up til higher, or you have a higher stall converter, you'll need to substitute higher rpm's to cover it, and it won't be as efficient as it could be.

1800 sounds about right for an AOD lockup.

With the FMX in there, the sweet spot runs from about 47mph to 57mph with the 2.75 gears and 25.9" tires That sounds about right for the 55mph era. But I will typically drive 65-70mph because that's the speed limit. So I want the middle of the sweet spot (1867rpm) to be about 67mph. That's not easy to do with the .67 overdrive in an AOD. I would have to cut down my tire size and go with shorter gears, or go with just shorter gears. I would have to run 195/70R14 tires with a 3.00 axle or could use my 215/70R14 or the 225/60R16 I want with 3.25 gears and be at 66-67mph at 1867rpm.

I've got a lot of calculations to do. But yes, if I throw the AOD in there now with these tires and the 2.75 gearing, I'll have to run 75mph just to get above the stall speed. But won't it be in lockup anyway? Does it matter in lockup? A new 2011 Mustang V6 runs 1400rpm at 60mph with the stock gears, tires, and automatic transmission (0.691 overdrive gears). Does it slip at that rpm? Nope. It's in lockup. Is it having trouble maintaining the speed? Nope. It's in lockup. it also gets 30+mpg. Yeah, that's the ticket.

http://www.allfordmustangs.com/forums/2011-v6-mustang-tech/263327-2011-v6-mustang-auto-rpm-60-a.html

With my tires, gears, and 0.67 overdrive in an AOD, I would be running a mere 1434rpm at 60mph. But if I'm in lockup, I think it'd work ok. I just won't shift into overdrive until I'm out of town. It's 40mph then 55mph then 70mph coming out of Malone. Then drops to 60 and 50 then 60 and back to 70 in Bynum. Then drops from 70 to 60 to 50 to 40 to 35 coming into Hillsboro. I could accelerate out of town here and drop it in overdrive once I'm past the 55mph sign. Then leave it in overdrive until that 50mph sign in Hillsboro and put it in Drive then. Back into overdrive if I get on the interstate. The PRN-OD-D-1 shift pattern would be appropriate, I wouldn't need the ability to shift to 2nd for 1-2 shifting (or just 2, not sure).

This might be darn good for mileage.

An example, 2000-ish Buick LeSabres, Park Avenues, and the other big GM FWD cars get 30+mpg on the highway out of the 3800 V6 and auto overdrive. They run 1500-1700rpm at 70mph and do 30mpg easy, weighing 3600-3900lbs. I won't see those kind of numbers, but 20-22mpg isn't out of the question at all if I leave the 2.75 gears in.