620 rear gear ratio interchange??

[joshiroku]

ive got a 78 620 with a 3sp auto. ive put a smaller tire on it now and its not very freeway friendly. ive put feelers out for a 5 speed conversion but im not that mad at the auto. ive used the search function to gather info before asking the same questions over and over. is there a particular 720 h190 rear pumpkin thats a drop in for my truck?? i think a 3.90 would be a better solution at this point and possibly a faster and cheaper option to make my truck more texas hwy happy.

[datzenmike]

The '78 uses the same 4.375 differential for standard or automatic so no significant rev difference.  A 4.11 or 3.90 will work if you don't use it foe hauling loads otherwise you WILL need those gears.

 

I think the '83 and up 720 2wd with Z24 were around 3.90.

[joshiroku]

i was hoping you would reply. i dont use it for loads other than some bicycles and my daughters power wheels. if i found another h190 from early 720 would it be a drop in?

[datzenmike]

It should but you said 3.90 ratio. If you have the truck, look on inner passenger side fender below the hood hinge for the engine tag. The gear ratio of the diff is stamped lower bottom right. Should say HF39 for a 3.90

 

HF35 is 3.545, HF 37 is 3.70, HF41 is 4.11 and so on. 

 

edit: yes the 4 or 5 speed '80 720 will have 3.889 (3.90) BUT NOT THE LONG BED OR AUTOMATICS

 

the '81-'82 with Z22 engine including long bed had them. BUT NOT AUTOMATICS

[Charlie69]

I had a 1980 King Cab L20B automatic and it had a HF41 a 4.11.

[joshiroku]

i found a rearend on car-part from an 85 std cab 720 with 5spd. 2.4 engine...from what ive gathered this should be like a 3:89 ratio? they wont look at the tag for me and its 4hrs from me..so im just checking my facts