1988 GMC with 3406B CAT - Estimated Fuel Mileage?

Looks like some of the responses on here are from people that weren't even born when the 3406B was around.
Had a 88 Pete with 3406B. Coast to coast fully loaded going east usually light going west 5.5 - 6 mpg round trip.
Empty it depends if city or highway driving. On the highway you'll be getting 8.5-9 from what I remember pulling an empty reefer on mostly flat ground.

 

You have to look at with the GVW is for each. Most of the single axle Fords like that were 33,000 if they were the really heavy-duty ones. So if the truck weighed 15000 that gives you about 18,000 that you can carry.

The tandem auto car could be a Heavy spec truck. A regular tandem generally is about 52000 lb, a heavy one could be 58000 or more depending on what year state will allow. So if it weighed 25,000 that would give you at least 27000 with 52000 GVW. Or more if it's a heavier spec.

If it were me I would go for the caterpillar or even better I would find a truck with an l 10 or a big cam Cummins. The difference between those engines and the 6.6 and the 3208 cat is that those little engines are throw away engines. In other words the caterpillar when you rebuild it you can pull the liners out and just rebuild the engine unless you have a problem with the block. If anything happens to those little engines that they need to be rebuilt you have to pull the whole engine out and either get another one or send it to machine shop to press cylinders in it and and that's expensive and a lot more work compared to rebuilding the truck engine. Some may tell you differently but I think that the caterpillar is much more expensive to rebuild and even work on them than the big cam or L10.

If you're not going that far I wouldn't worry as much about the mileage. A smaller truck fully loaded down moving like a slug will probably get a similar mileage as a larger engine pulling a smaller load.

What I would really be concerned about is the automatic transmission. They tend to get worse mileage and are crazy expensive to repair if you have a problem. A regular manual transmission is better on fuel and much simpler and cheaper to repair when you need to.

 

I bet you get better than 6. I get 6.7 pulling an empty cattle pot. They are nothing less than a parachute