720 differential upgrade

[bryant2482]

is there a bigger diff. gear set that i can throw into a stock 720?

[Laecaon]

Which ratio do you currently have. There are a wide range of gears available...

[datzenmike]

You mean a larger diameter than H-190? No they won't fit. If you mean a different ratio, then yes. '85 720 2wd? Should be 3.545 or 3.889 if a 5 speed and 3.70 if auto. The '82 long bed came with 4.11 and all '80 720s had 4.375. The best way to check is to look on the engine tag on the right inner fender below the hood hinge.

 

 

.Near the bottom FS5W71B is the transmission HF is the H190 axle and 38 is the ratio 38= 3.889.

41 would be 4.11 and 43 would be 4.375

[ggzilla]

The larger C200 was fitted to some 720s, wasn't it?

[datzenmike]

Yes the Cab Chassis models and later 4x4s had them and they were 4.11 ratio. It wouldn't just bolt to your 2wd. You would have to remove the leaf spring perches and re-weld them on the under sides of the axle tubes. Also the drive shaft flanges are larger so your 2wd driveshaft won't bolt up to it.

[bryant2482]

well the tag says ive got the 3.889. but remember that i went from having the 3 speed auto and converting it to the 5 speed manual. what i would like to know is what diff set would give me a little more speed with less rpm's? i also have a extra diff. in my garage but im not sure what the ratio is on it. if i counted the teeth on the big gear would that be able to tell me what set up ive got?

[nis720]

My stock 5 sp 720 came with 3.73. I might have the 3rd member with blown spider gears.

[datzenmike]

The outer edge on the ring gear will have numbers punched in it Example .... 37:9 divide to get 4.11

 

This is 37:8 for a 4.625 ratio on a Pathfinder H-233..

 

 

 

Some models of the 720 came with 3.364, 3.545 and 3.70. Besides 3.889 there were also 4.11, 4.375, 4.625

[bryant2482]

ok the one ive got say: NH 37:10 HN 4 2 1. what exactly does that tell me?

[datzenmike]

That's a 3.70.

[bryant2482]

would a 3.70 offer me a little more speed then a 3.889?

[ggzilla]

3.70 will give the same speed as a 3.889 -- with either you can do 35mph or 85mph. But with the 3.70 your engine will rev slightly slower for the same road speed. Your speedometer will be off too, so you won't notice any change at first.

 

The biggest change you would notice, after replacing the 3.889 with a 3.70, is that your truck now accelerates a little bit slower.

[bryant2482]

o ok. i understand now. i just hate the slow take off and topping out at 15mph in 1st gear