would 373s work for this?

[bigscooter]

hi guys, i can't stand this low geared rear end any longer(456s probably). it's a '91 r3500? 2wd crew cab. it is a small block, the tranny is the manual with the low and 3 speed. my rear end is not dually. I'm hoping 373 gears won't be too high. i value economy more than strong hill climbing. i don't tow more than 5k (usually just motorcycles and VWs around town). i do dream of some day installing a 5-speed. Am i barking up the wrong tree?

Thanks!

[mrolds88]

I don't think you're barking up the wrong tree. I believe you'll do just fine. Especially with the manual. The low 1st gear will help out with that. When you do go after a rear. Take a tape measure with you. And note where your shock mounts are. IIRC the 3/4 an 1 tons are different. Same rear just different mounting locations.

[wilkin250r]

I would suggest that your first order of business is to determine what you currently have. There are a couple methods.

You could drain the fluid, pull the cover, and count teeth (most definitive method).

Or you could jack the wheels off the ground and spin them while counting driveshaft rotations, but if you use this method you have to make sure both wheels turn the exact same amount, OR leave one on the ground and spin the other twice.

Once you know what you have, you'll know what you're up against. If your 4.56 turns out to actually be a 4.10, changing to a 3.73 won't be all that significant.

[Bigstevex4]

if you have 456s you would be turning 4 grand at 75 most likely 410s or 373 and 373 is low not high unless your running 38s. And what he said above.

[mongocanfly]

Mine has 4.10 gears which I like for towing so I went to a nv4500 5 spd...it doesn't have the exact gear ratios the 4 spd had but the overdrive sure is nice. And keeping the 4.10 is even nicer..I still got low gears to get loads moving and overdrive to keep the engine rpms down. But a 5 spd swap is more expensive than a gear swap. And like stated 3.73 and 4.10 is not much difference. I have been told if you do the math that the ratio difference between 4.10 and 3.73 is about the same difference as the o.d. difference of a new set of tires vs. a worn out set.

[greg64]

How tall are your tires? Or, failing that, what's the tire size? Then we can work backwards to figure out a suitable rear gear ratio for a reasonable engine rpm.

[bigscooter]

i have the cover off now and i cant see any markings on the gears. i can't see the pinion well enough to count the teeth. my tires are 32" tall, at 50 mph it's at 2300 rpm though.

[Jake Wade]

Based on the information you provided, 3.73's would drop your RPM at that speed by approximately 400 RPM and 3.42's approximately 600 RPM.

[v30crewcab]

theres numbers stamped on the outer part of the ring gear. also you can look at the numbers in the glovebox and find the diff code and check that way.

[TwoFiftyShifter]

2300rpm at 50 could be either 4.10 and 4.56. Need to pull the cover to know for sure. My 87 V20 was a 350 4 speed stick and 3.42s... Pretty good ratio. 2500 at 65.

[bigscooter]

Originally Posted by v30crewcab

theres numbers stamped on the outer part of the ring gear. also you can look at the numbers in the glovebox and find the diff code and check that way.

I wondered what those numbers were about. Looks like code HC4 is 4.56.

[bigscooter]

I'd like to do 70ish mph at 2300-2500 rpm. with this size truck, am i dreaming?
I do have a lowering kit for it. if 3.73 is too sluggish maybe i could run smaller tires?

[mongocanfly]

3.73 would be ok..my 4.10s with 5spd will run 70 all day long.

[wilkin250r]

3.73 will not be sluggish. It might even be still a tad low, but not unreasonably so. I'm running 3.73 with an inline 6 right now (5spd), a 350 could easily pull taller gearing, especially with the low first gear to get loads moving. And according to my calculations, 3.42 would put you at 70mph, 2500rpm.

No doubt that 3.73 will work just fine, but I think you could go 3.42

[Jake Wade]

Originally Posted by bigscooter

I'd like to do 70ish mph at 2300-2500 rpm

You want 3.42's

[v30crewcab]

yep without overdrive 3:42's is the best.

[bigscooter]

I'm hoping to find a complete rear end to swap out. I'm likely to find a 3.73. a 3.42 might be a tough find in my configuration.