Join Date: Jan 2010
Location: Splendora Texas
Bronco Info: 1989 4X4 351W C-6 3.54/3.55
In simple terms, your axle ratio refers to how many times the drive shaft turns to make the tire(s) turn one complete circle.
There should be a tag bolted to the inspection cover on your 8.8 Axle. It'll tell you the gear ratio.
There's also an Axle ID number on the door plate.
If you can't find either of those, pull the inspection cover and count the pinion and ring gear teeth, divide the number of pinion teeth into the number of ring gear teeth to get the gear ratio. Unless you bought the truck new, this is the most reliable way to tell the ratio anyway. Ya never know what a previous owner may have done before you ended up with the truck.
Things to note:
Ford's 9" rear is far superior to their 8.8"....and not only because the guts of the thing is bigger overall. Compaired to the 8.8, the 9" is very easy to work on when nessessary, it has a rear support bearing for the pinion and has a better axle bearing/bearing retainer design. Having said that though, it will NOT have the exact ratio as a simular 8.8. What that means is that you will NOT be able to use your 4-wheel drive unless it is on a surface that allows the rear tires to slip a bit. That is because your front axle and rear axle must match as to ratio (as does tire size) or something in the drive train will either slip (or break). As she sits now, your axle ratios are PROBABLY 3.55(R)/3.54(F). You swap in a 9" and they'll PROBABLY be 3.50/3.54.....which is even more of a miss-match than Ford put in to start with.
Do your homework before plunking down your money.
1989 Bronco 4X4 351W EFI C-6 3.54/3.55 Gears.