Byrdman--Gear Venders has made an over/under drive for many years. I have used them on trucks and RV's. They are available for most vehicles, with a little adaption in some cases. You can put the appropiate rear end ratio and get a good boast in performance.
Unfortunately many of the more recent diesels are getting worse mileage (read emissions control--hopefully this will be solved at some point). Dodge Sprinter down about 20%, I hear the same for latest Dodge trucks. Some of the other light trucks have relitatively "new" engines, so may not be comperable.
A couple of years ago, I saw a rig with an Isuzu cab foreward with a relitatively small diesel, which had a utility body and was pulling a 40 foot 5th wheel. The owner told me that it was a little slow over the grades but never dropped below 45 mph--not bad. As I recollect he was getting over 15 miles a gallon.
Unfortunately many of the more recent diesels are getting worse mileage (read emissions control--hopefully this will be solved at some point). Dodge Sprinter down about 20%, I hear the same for latest Dodge trucks. Some of the other light trucks have relitatively "new" engines, so may not be comperable.
A couple of years ago, I saw a rig with an Isuzu cab foreward with a relitatively small diesel, which had a utility body and was pulling a 40 foot 5th wheel. The owner told me that it was a little slow over the grades but never dropped below 45 mph--not bad. As I recollect he was getting over 15 miles a gallon.