An overdrive turns its output faster than the input, so at a given engine rpm you can go faster. As others have mentioned, they are expensive (Advance Adapters has a very neat two-speed but it's $1500 plus driveshaft shortening). I paid $250 for my truck auxiliary trans but it was a lot of work to install (you have to make crossmembers for front and rear mounts, appropriate rubber bushing mounts, U-joint angles are critical, a front driveshaft has to be fabricated as well as shifter linkage, etc). Definitely not a bolt-in for the amateur. I suggest you don't try this at home


OK, JD, it is time to stop guessing! From what I'm reading you have checked your rear ratio and speedometer and they are correct? If you have a 3.50 rear you should have no problem maintaining highway speeds (in other words you aren't limited by running out of rpm, instead you don't have enough power). For a final verification you could hook up a tach and drive the truck. You should only see about 2500 rpm at 60 actual mph with a 28" tall tire, for example.

It will also be important to see if it can pull to 4000+ rpm in the lower gears, or whether it won't go over 2500 in any gear. Anyway your problem sounds like it's in the engine, not the rear gears. Good luck, let us know what you find!
-Charles

ps if your emergency brake were stuck on it would burn up pretty fast, and also the tremendous heat/smoke generated would be easy to find.

 

[font color=red size=2]I have a '73 F100 2wd with a 390 T18 and 3.50 gears. I think these gears are great. I can easily cruise at 65-70mph and only running about 2800rpm. It'll easily hit 100mph - not that I've tried it.;-) Do you have a tach? Get one if you don't. Before I put a tach on mine, I thought it was reving real high, turns out it wasn't, sounds can be deceiving.

Just a thought...

Marty
"Cleverly Disguised as a Responsible Adult"[/font]

 

For what you want, you need to rid your self of the ranny 4 speed. I run a top loader, short tail from a Mustang. It works great in my truck; behind my 428 ) That way you have a useable 4 spoeed just like in a car. It still has plenty of pulling power, the again 428 ))

Pete's Ponies
Mustang RUSToration & Performance

 

Correct me if I am wrong......but isn't the final gear ratio in both of the transmissions still 1:1?? So a toploader wouldn't really change the top speed of the truck would it?
I tend to think that either the engine is revving enough or isn't being revved enough. Just my opinion

Sparky