3.73 vs 4.10 gears - fuel economy?

[cbxer55]

That is the way I was always told, to figure out what gear ratio you have. Jack up the rear, mark the driveshaft, use the valve stem, spin the tire one full rotation, count how many times the shaft turns. There's your gear ratio in a nutshell.

 

[cbxer55]

Drove it to work today, took the long way. More varied conditions. I can tell the difference, it isn't huge, but I can tell. Especially when the OD is off. Or when driving it manually shifting it. There's one area, about 1/4 mile long, at work, where I usually just stick it in 1st and let it rev out before parking it. It's revving noticeably higher in that place, I'm more apt to shift it into 2nd now. Accelerates just a tad better now as well. Makes that high rpm cai intake work better.

 

[cbxer55]

More observations on the swap from 3.73 to 4.10. There is no more indecisiveness about which gear it's going to stay in, D or OD. When it shifts into OD, it stays there, even uphill. No more hunting around at upper 30's low 40's. On the highway at 80 mph, it accelerates up from there much better than it did before. Downshifting coming down the backside of a hill, engine braking is more effective.

Yeah, I like it.

 

[scotts90ranger]

But it does... you can jack up a locked rear*, spin the wheels 1 revolution, count the number of turns the driveshaft makes, and it WILL match the gear ratio.

*On an open diff, you only jack up 1 wheel.

Jacking up one tire on an open diff won't work, there's more teeth on the side gears than the spiders so it's going to be all wonky... I'm not going to math especially since I don't know how many teeth are on anything . On an open diff if you jack up both assuming no brakes are dragging turn the driveshaft and the axles should both turn the same, but assuming it hasn't been lost the axle tag on Ford axles is super nice to have around...

On your lack of tach thing, just find a tach cluster from a similar year range, my '90 didn't come with a tach, I put in a tach cluster, apparently it works with the third gens too if you match the right years, I need to do it on my '97...

 

[don4331]

Jacking up one tire on an open diff won't work, there's more teeth on the side gears than the spiders so it's going to be all wonky... I'm not going to math especially since I don't know how many teeth are on anything . On an open diff if you jack up both assuming no brakes are dragging turn the driveshaft and the axles should both turn the same, but assuming it hasn't been lost the axle tag on Ford axles is super nice to have around...

On your lack of tach thing, just find a tach cluster from a similar year range, my '90 didn't come with a tach, I put in a tach cluster, apparently it works with the third gens too if you match the right years, I need to do it on my '97...

Doesn't matter how many teeth on the spiders or side gears - if one tire is stationary, the other side is going twice as fast as the differential is in middle.

With 4.10 gears, the driveshaft would turn ~2 and trace (.05) while 1 wheel made a complete circle assuming the other was stationary.​

That is why 1 wheel peels are so hard on the differential gears - they are accelerating the spinning wheel to twice speed that the speedometer says it should be turning.

Note: It's better to ensure both wheels turn the exact same amount (locker/limited slip helps, and maybe turn 10 turns - then even the difference between 3.45 and 3.55 gears can be determined.

 

[4x4junkie]

If the axle has an open or limited-slip diff, the best (most accurate) way to check the ratio (short of opening the diff and counting the R&P gear teeth or looking for gear stampings) is infact to jack up ONE wheel, then rotate the wheel exactly TWO turns while counting the pinion (driveshaft) rotations. Two rotations of the wheel are needed because (as said above) the differential carrier rotates at half-speed with one wheel stationary (additionally, jacking up both wheels of an open-diff axle also introduces the potential for errors if both wheels aren't rotated together exactly the same amount).

Only if you have a mechanical locker (or a spool) should there be any need to jack up both wheels, in which case you would then rotate the wheel(s) one turn while counting the pinion rotations.