I recently bought a 1993 f700 with a 5.9 Cummings and a 5 speed manual transmission. I was wondering how I could increase the highway capabilities. I have been on a trip to Kansas from Ohio to donate hay and I have been going about 55 the whole way. I was wondering if swapping a six speed or a higher geared rear end or even a two speed rear end would work, and which is the cheapest way. Thanks, Matt

 

Welcome to FTE.

There is no "cheap way".

If you post a picture of your data/warranty sticker, or just post the info, we may be able to get it decoded and find out what trans (direct or OD) and rear axle you have. Then you can decide to look for a trans or an axle.

An auxiliary transmission is also an option.

I found a 6 speed Fuller for $1200 in 5 seconds but it's a direct trans.

 

When loaded an overdrive won't gain you much. You'll be down in direct or lower I doubt your engine would have the torque and horse power keep you rolling at 60 or 65. Sure, you can crank up the pump. That runs the risk of other problems.

 

There are some transmissions that will get you overdrive, but it's almost always going to be cheaper to swap the back axle. If in driving it you feel like you have more than enough power, you can go with a single-speed axle. If you might do some heavy hauling and need the gears then a 2-speed might be just the ticket. Of course it's worth noting the 2-speed axles have a poor reputation for reliability compared to a single speed and if your truck wasn't set up for that you are going to need the controls from a donor truck so it gets complicated pretty quickly.

It's not a substitute for correct gearing but you can bump the fuel and governor on a Cummins safely as long as you keep it modest, you should add a pyrometer for safety if you go that route.