Rear End Change Question

Most of my experience is academic. These guys ☝have actual experience. Refer to them first

 

Contrary to popular belief they’ll turn RPM and not hurt them. I used to run a narrow rod B model 2100 all night long. Had 3.70’s and an RTO 13 so it was slow. That said I understand where your at. 3.55’s would put you where you want to be rpm wise. 3.36 if you had a 15 over be the way to go as they don’t have as much overdrive.

 

What you might gain in mileage would take a looooooong time to hit the breakeven point on the cost of a re-gear.

Only way I'd consider changing the gear ratio is if you needed to replace the diffs for some reason anyways.

 

I only switched mine because I had a center section that was on borrowed time and I had these 3:36 on hand.. @wore out is spot on in his posts..
I run a 15 against the dash. .. I wanted alittle more on the top end I was getting in problems because I run alot of 2 lane in hill country and I like the 62 -65 ,mph speed and when I got hung up behind Grandpa heading to town at 50 mph I was screwed because as wore out put I was gear bound.. I was constantly shifting the 50 -58 mph was out of my comfort zone
Iam not a found lover of the 15 speeds anyhow but my thinking was I would try the 3:36s and see how it goes .. if it's not what I like then I'll swap out the 15 for the 18 I have sitting in the shop..
,@Swine hauler is right your motor is making xxx hp it is what it is..your just going to do things in a different gear

 

This truck was a 15 over and 3.36 on 11R24.5 from factory. I thought I would hate it however I friggin loved it. 9th for anything 2 lane up to 70 mph and the big hole for the big road. So at a million the 15 was giving trouble so I took the 18 out of my wrecked truck since it was almost new. (Didn’t last long probably due to being towed with driveline in truck). Ran it a few months and the front diff gave up. Not liking the 3.36 and 18 combo and already having 3.70 in the wrecked truck I swapped them. That suited me fine. One reason I went with what I had however was a cracked front housing and 402 housings of any sort are like hens teeth. A friend of mines truck burnt and he had low air same as mine with super 40’s with really recent 3.55’s. So a deal was struck and I rolled housings suspension and all under my frame.

Sounds like a lot of work and it was but it’s part of running older iron with hard to come by parts. Sometimes you do what you have to to keep moving till you nail down the permanent fix. With a cracked housing it could have went multiple ways. My frame is too nice to need a cut off and at 275 WB it’s all I can do to get on a 70ft scale so stretching it was out of the equation as well. What it did do is make me realize a few hard truths that even I before all that would have argued the point.

It’s top speed which doesn’t matter is actually faster now than the 3.36 and 15 over. The 3.70 and .73 overdrive was actually #### near dead nuts the same as 3.36 and .79. But there was the 1 situation in a hard cross wind you were gonna slow down or spin the sheet out of it. With the 3.55’s it’s better there.

MPG is as always 4.8-5.2
Top speed on Beaumont just east of witchita is 65 usually as always. Wind can affect that.
I will agree it does sound better at 75mph at 1500RPM.
For a 31 year old sheet box pulling a spread axle bovine bus it does ok.