Advice on diff ratios

[Leeroy]

Had a bit of an inident this week, rear wheel departed the slick whilst doing 100km/h.

Luckily only damage is what you see and one stripped stud. Not sure what's happened that caused all the lug nuts to unwind, but it did! It's either been tampered with or just not tightened correctly.

I've decided to replace the studs in both axles just to be on the safe side and whilst I do that I want to change the diff ratio to something more highway friendly.
Think it's a 3.7 in there now, but it's in 2nd by 10mph and 3rd by 25mph under normal load. Thinking about going to a 3.00 or 3.25 at most.
I run 17" wheels with 275/55 tyres and a 351c/fmx.

Don't do much towing (none cause there's no towar yet!) but may tow a small trailer and maybe another car+trailer on very rare occasions ie 1-2 times a year.

Just wondering what other people have done in this areas, I'd be interested to hear your experiences.

Leith

[64 f100]

I assume you have a 3 speed standard tranny, and are running an FE engine or something roughly equivilant. I believe 3.5 gears may be as far as you can go with your wheel and tire combination. 3.25 is a common gear setup from Ford with the three speed in 65/6. 3.73 is a towing gear and was mainly used with the four speed. You might consider going to a 3 with overdrive in the form of the mid late 70's 4 speed look alike car transmission, or a 5 speed. Other options are the 3 speed with borgwarner overdrive or a car four speed with gera vendor overdrive behind it. Your tire/wheel size has already changed your ratio and may already be equivilant to a 3.25 ratio. I would not consider going any taller than 3.25. At that you will need a very good clutch to pull anything from a standing start.
I am surprised that you didi not notice a vibration before the wheel came off, and suggest you not trust someone else with torque on the lug nuts. A genereal rule of thumb is that the lug nuts should be torqued about 100 to 120 ft/lbs. on aluminum wheels. Just remmember you may not have the benefit of a long handled torque wrench when removing the wheels on the side of the road, so don't get to rough or you may end up regreting it.
One other thing, don't get rid of the 3.73 hog head, just buy another in the ratio you want. It doesn't take to long to change the hog head and you may want to switch from time to time.

Have a great day,
Rich

[Truckrat]

I agree with 64 on keeping the 3.7 diff, but a 3.25 or 3.0 for highway driving when you don't plan on doing any towing for a good while will really reduce the rpm and give you better fuel mileage.
Not sure about how tall your tires (2.75/55 x 17) are so I am guessing around 28 inches. Plugged it into my rpm calculator with 70 mph as highway speed, 28 '' tire, and 3.7 axle ratio with final drive being 1 to 1. Came up 3,130 rpm. Next I tried 3.25 and 3.0 as the axle ratios and they came up 2,732 and 2522 respectively. That is almost like having another gear in your tranny. Now, if you wanted to try something a little different without all the gear swapping you can swap for a set of taller tires for the rear. Say something like a 32 - 33 inch tall tire. Does the same thing swapping diffs does, but not near as much work. I have a pair of 32 inch tall tires for my 65 with a 3.5 rear gear for highway driving, and some shorter 28 or 29 inch tires for pulling a trailer or car hauler. I can possibly get a little closer on the rpms you would be running on yours if you posted the exact tire diameter. 28 was a WAG (wild assed guess). TR

[R Pope]

So, now you know why Chrysler and pretty much all big trucks use left-hand threads on the driver's side! Especially with aluminum wheels, they have no "spring" to them like steelies do to keep some pressure on the nuts. I've had the bleft rear wheel come loose on just about every car I ever put mags on. You just have to keep tightening them all the time.
My slick has 3.83 gears, with the M5 OD trans it should work out good with the 351W engine.

[STOFFER]

I'd leave the 3.7 gears in place or step up to 4.11's