How to find out rearend gear ratio in my truck?

 

The most precise way is too crawl under rear and look for a stamped tag on one of the bolts on the pumpkin. Could be 3.55, 3.73, 4.10 etc.
Write it down and compare with list.
13- Ford 9" 2.75 ratio 1981-83
13- Ford 8.8 4.11 ratio 1985-86
H3- Ford 9" ls 2.75 ratio 1981-82
H3- Ford 8.8 axle 4.11 ratio with limited slip 1985-86 (3800 pound)
14- Ford 9" 3.00 1981-1986
H4- Ford 9" 3.00 Limited slip diff 1981-86
15- Ford 9" 3.25 1981-82 and 2.73 in 1983
H5-Ford 9" 3.25 limited slip 1981-82 and 2.73 in 1983
16- Ford 9" axle 3.50 ratio (3750 pound)verified
H6- Ford 9" axle 3.50 ratio with limited slip (3750 pound)verified
18- Ford axle 8.8 3.08 ratio (3800 pound) 1983-86verified-still accurate
H8- Ford axle 8.8 3.08 ratio limited slip (3800 pound) 1983-86verified
19- Ford axle 8.8 3.55 ratio (3800 pound) 1983-86verified
H9- Ford axle 8.8 3.55 ratio with limited slip (3800 pound) 1983-86verified
23- Dana 60-3 axle 3.54 ratioverified
24- Dana 60-3 axle 3.73 ratio verified
29- Ford Semi Float Sterling 10.25, 3.55 ratio
B9- Ford semi Float Sterling 10.25, 3.55 ratio limited slip diff
B5-Ford semi Float Sterling 10.25, 4.10 ratio limited slip diff
39- Ford full float Sterling 10.25 3.55 (single rear wheel)
C9- Ford full float Sterling 10.25 3.55 (single rear wheel) with limited slip diff
35- Ford full float Sterling 10.25 4.10 (single rear wheel)
C5- Ford full float Sterling 10.25 4.10 (single rear wheel) with limed slip diff
33- Dana 61-1 3.54 ratioverified
C2- Dana 61-1 3.54 ratio with limited slip diffverified
52- Dana 70-1 HD 4.10 ratio (8700 rating)verified
53- Dana 70-1 HD 3.54 ratio from 1983-86 (4.56 in 1982) (7400 rating)verified
E2- Dana 70-1 Hd 4.10 ratio with limited slip diffverified
49- Ford Sterling 10.25 dual rear wheel 3.55 ratio (7400 rating)verified
D5- Ford Sterling 10.25 dual rear wheel 4.10 with limited slip diff (7400 rating)
69- Ford Ford Sterling 10.25 dual rear wheel 3.55 ratio (8250 rating)
F5- Ford Ford Sterling 10.25 dual rear wheel 4.10 ratio limited slip (8250 rating)
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if it is single rear wheel then it has 3.73s otherwise read the axle tag on the diff cover