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Transmission or rear end swap? thunderkiss66 11-3-03  
I am the proud owner of a 1966 int'l loadstar 1700 engine is a 392 gas tranny is a 4 speed w/creeper (any info on what this tranny is would be helpful) not sure what the rear end is but gears seem pretty high (screams just a bit when doin 60+)my question is would it be more cost effective to swap the trans or rear end ??(what type of tranny is easy swap....etc?)

thanx, craw
Re: transmission or rear end swap? Mark O. 11-4-03  
The 392 V-8 was a great engine. It had reasonable power for a gas engine and was relatively fuel efficient.

The problem with the 392 is the same as with any gas engine, no matter what you do, there just isn't enough HP to make a bus go very fast.

Almost every school bus I have ever seen that had the 392 was geared to do 55 MPH at 2500 RPM. Changing transmissions won't change your top speed because the majority of all truck transmissions have a direct drive high gear. There are some transmissions that have overdrive high gears. The downside in the OD transmissions is you would then have some really big steps on the way up and you really slow down on the hills as a result.

Changing rear ends or going to taller tires are the only ways to reduce your RPM's while still maintaining decent gear splits.

The problem you will run into is whether or not your engine has enough reserve power to pull the high speeds at lower RPM's. Most 392's are working pretty hard to acheive and maintain 55-60 MPH. The easiest way to find out if changing gear sets is worth the effort is to check out your manifold vacuum.

If your manifold vacuum is less than 10" at 55-60 MPH now, going to a high speed rear end is going to mean you are going to use more fuel, go slower on all of the hills, and run hotter.

Good luck.

Mark O.
Thanx Mark O thunderkiss66 11-4-03  
Mark O.....Thanks for your advice,you seem like a very knowledgeable person...thanks much for your input.I'm going to check into the manifold vacuum and what type of rear end i have and go from there....
Thanx again,craw
Re: transmission or rear end swap? Paul 6-7-05  
Craw- I had the same problem. I have a GMC 6000 with a 366cid engine. With the original rear it did about 4000 rpm at 55-60mph and was screaming. I swapped the rear from a ? to a ? and now do 60+ at 3000 rpm. Yeah, you feel the hills but at least you can carry on a conversation. See it at The swap wasn't too difficult if you can get a couple of friends to help. Air tools were an immense help. We only had 1 stuck nut (the yoke) that we had to call for help on.

Bus On
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