tandem position for best mpg

If you're pulling a van or reefer and the 5th wheel position, keeping the gap between the back of cab and nose of trailer as minimal as possible is always going to help improve fuel economy by keeping air turbulence to a minimum. Heavy or empty, makes no difference.

As far as the trailer tandems, I guess this depends on wether you have trailer skirts or not. If so, and if weight will allow, I'd imagine keeping them moved up closer to the rear of the skirt panels will help with MPG, but you have to position them where weight distribution will allow.

As far as temperature differences, with non-turbo gasoline engines, it's generally accepted they will get better MPG in hot temps versus cold temps, however I'm not sure if it makes so much different with today's charge-air cooler systems in high HP diesel engines. And based on past discussions on the matter, the matter will probably generate a lot of different attitudes

 

better to have more weight on your drives than on tandems, that way your not "pulling"the weight over hills if its on your drives, but im no expert.

 

Right, it's better for fuel consumption to "carry" the weight rather than "pull" it. Move tandem position wherever you need to keep the weight high on the drives.

 

It doesn't matter where the weight is, you are pulling it.
You might carry more on your drives or the tandems, but you are pulling the same weight. The engine does not know the difference - does it?

 

Ride is better with the weight on the drives. You can really tell when the trailer is tail heavy.