To my knowledge there are two ways as far as tags to tell. Folks will tell you all kinds of ways to mark your tires and the ground and count rotations of axle vs tires and yada yada yada. To me it ws easiest to just look at the tags and look at these charts.
Second, you can look in your driver's door jam at the white tag. the bottom lines will have a part that says "axle". Under the word axle there will be either a letter and number or two digit number. Compare it to the following compliments of diesel_brad of PSN.
B9= 3.55 LS
C9= 3.55 LS
H4= 3.55 LS
H9= 3.55 LS
B5= 4.10 LS
C5= 4.10 LS
D5= 4.10 LS
F5= 4.10 LS
H5= 4.10 LS
Should you have opposing codes between the door and diff cover, go with the diff cover or do the mark and spin thing to verify. The door tag will only be accurate if the diff has not been changed. Like in parts truck I have a code on the door for 3.55 LS but the diff tag says 3.55 open diff.
Also, if you have near stock size tires or even slightly larger you can tell by rpms at 75 mph. 4.10s will have you right at 2500 rpms with an auto tranny and slightly higher with 5 speed. 3.55s will have you closer to 22-2300 IIRC with auto and again higher with 5 speed.
Hope that helps.
1996 F-250 Super Cab XLT 4x4 7.3L PSD: DP Tuner PCM, E4OD-->5 spd, Valair clutch, 3.55 LS, Dana 60, Rosewood stage I's, IDM-120, eFuel, 6637, no EBPV, 3" DP, gutted kitty 205k
2006 Explorer Eddie Bauer 4.0L 5r55s K&N 105k (hers)
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