I have called the dealership to figure out the axle ratio and they told me they can only narrow it down to a 3:55 or a 4:10.

Yes I had both the serial and the rear pumpkin tag # to give them but they said 1988 was to far back to look... Isn't there a way to manualy check by the # of shaft vs. tire rotations???? If so someone please tell me the proceedure for doing that in detail so I can check to see what my ratio is...

Thanks!

 

There are 2 sources for the gearing info on these trucks, the axle tag and the axle code on the drivers door pillar. What are the numbers on your axle tag?

You can also jack up the rear axle and spin the wheels counting driveshaft and wheel rotations to confirm somebody hasn't changed it from stock in the past.

 

The rear diff tag # has the ratio printed right on it. Look for 3 55, 3 08, etc. printed on it. 4.10 gears were very rare on these trucks, 3.08 and 3.55 are very common.

 

Or something like "3L55" on the pumpkin tag for a limited slip 3.55 gear ratio but Limited slip was crap shoot on trucks.

 

If your tags are missing jack one side up see if it spins it's an open gear (make sure emergency brake isn't on and you block up one of the front tires so it won't move, AT ALL!). If open put a chaulk/soap stone/yellow lumber crayon paint or other on the bottom of the driveshaft at the pumpkin. Same with the tire where you can see and watch both. With the truck in gear spin the tire two turns exactly and count the turns on the axel....3 1/2 turns = 3.55, just past 3 = 3.08 etc. I'm not sure the method works the same with a limited slip or locker. Or take off the cover and count the teeth on the ring and pinnion Big / little = gear ratio. Probably need to change the gear oil anyway.

 

33homerdoodle explained about the easiest way to tell.
Just be sure and put the truck in Neutral. It doesn't need to be in gear, unless you are superman and can turn the engine too.

 

FlaredFun, Yep, it's been a while. Thanks for the catch.

 

Is it the same amount of spins for a 4:10??? The guys at Ford said that I have a 3:55 or a 4:10. I'm just trying to figure out which one. And NO actually I do not need to change my rear diff oil! I just changed it less than 5,000 miles ago when I did a complete rear break job with rear axle seals... I would rather not fill the pumpkin again!

 

No problem man! I had to do the same thing with my Mustang when I got it. I knew that it had lower gears than stock. So I did the same thing to find out my gear ratio. Turns out it had 3.73 gears in the rear. Perfect for Fox body.

 

Yes it works the same way for any ratio.

If you have 3.55, then your driveshaft will turn appx. 3 1/2 turns per 1 turn of the wheel.
4.10 gears, the driveshaft will turn slightly more than 4 turns per 1 turn of the wheel.

Take a piece of chalk, or crayon, and put a small but visible mark on the top of the tire, and the top of the driveshaft. Then slowly turn the tire while watching the driveshaft. Count the number of times the mark on the driveshaft passes the top (12 o'clock position.) Then look at the driveshaft and see where the mark is when you make 1 full turn of the tire.

This is not a perfect way to do it, BUT it is a very quick way to tell you about what gears it has in it. It is actually pretty close as long as there isn't a lot of slop in the driveline. Good luck with it!