So I'll tip in some info about the 6.5
Mine is a K2500, 6.5 TD, auto and 4:10.
I average around 14-15 combined. Somewhere around 16-17 highway. Towing our 35 foot travel trailer drops it down to around 12-13 mpg highway.
A dual rear wheel truck will be slightly less due to more road contact and pushing a bit more wind on the highway.
Power?
Factory rating for the early TD's is around 195 hp, 330 ft/lbs (that's off the top of my head guys, don't "bench racer" me to death with numbers)
My 98 has a lot of work done to it and last dyno'd 271.9 rwhp @ 3200 rpm / 446.3 tq @ 1900 rpm on a dyno dynamics chassis dyno. So I'm tickling somewhere around 300 HP crank and 475-ish ft/lbs. Basically, right around stock LB7 levels. I'm probably right at the limits of what you can expect out of the 6.5 IDI design and still have it reliable.
Of note: I do not have a GM block and heads in my truck. I have a fresh new General Engine Products (GEP) Optimizer 6500 direct from the Franklin, Ohio assembly plant. It shares very little with a GM 6.x except the basic architecture. The block and heads are cast by Navistar (revised thickness, geometry, metallurgy and high quality control in final machining) and the engine is finished and assembled by hand At GEP. Everything is a "select fit" right down to individual pistons in individual bores. This is the production line for them:
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No robots there, all hand built.
All the engines that go into the Up-armoured HMMWV's come off this line as well as the ones they sell to GM as crate replacements. Mine came with QC paperwork that had signatures from everyone that laid a hand on or passed eyes over my block. To say they take their jobs seriously is an understatement....
I would not recommend driving a GM block 6.x up to my power levels, they frequently calve at stock output let alone chucking another 100-ish hp and 145-ish ft/lbs at it.....
Now, the really bad stuff:
The 6.5 in stock form is not a reliable beast. Back when they were pumping them out you had about a 75% chance of getting a lemon.
At the bare minimum, you need to relocate the Pump Mounted Driver out of the engine bay. Plan on dropping $500 on that.
The blocks crack, the cranks break, the heads crack, starter mount pads crack off, lift pumps die, oil pressure switches fry, harmonic balancers squirt their rubber out, Stanadyne electronics leave much to be desired, turbo vacuum systems fail and then you toss in all the other things that go wrong with a vehicle 25 odd years old.
God help you if you get a rust pinhole somewhere in your fuel feed lines and introduce air into the fuel injection system. That will drive you around the bend in very short order and lead to an impromptu late night "barbecue" in the driveway....
and I've only scratched the surface in things that can go wrong. You would think that a 6.5 still running at this point would be one of the 25% good ones, but nope. The 6.5 gremlins can jump up and bite you at any time. Many, many 6.X's are out there running around with cracked mains. It runs for many, many years and then it finally get to the point where the main webs break out of the block and rapidly destroy the whole thing is a painful screaming death. PMD's can and will go any time without warning. 300,000 mile IP's fail (anywhere from $500-$2,000 to fix).
You also have to learn to do mostly everything ont he engine yourself. Most shops don't have a clue what to do with them and most diesel shops are many generations past the basic "Roosa Rotary Pump Injection" and IDI technology. Common rail Di rules the day, that's where the money is so that's where they train to. Plus, the 6.5 TD's have some many specific quirks no one will know what to do with the damned thing when it doesn't run unless you have intimate knowledge of it. Even then it will stump you....
Counting the purchase price, I have right around 18 grand in my truck to make it powerful and reliable. And it's doesn't even have fresh paint!
Way less than a new Duramax, but way more than I should have in the truck.
Now, if you're lucky and run across a 92/93 6.5 TD you can sidestep a lot of problems. They have the 599 block castings which are generally accepted as the best GM blocks (even though I popped one
). They have a mechanical DB2 injection pump so you avoid all the electrical gremlins in the DS4 pumped trucks (fyi: the DS4 pump is mechanically sound, it's usually the electronic bits in it that fail and give the truck a bad rep).
Find a 1992/93 5 Speed truck and everything is pretty much mechanical.
We're talking zombie Apocalypse reliability at that point.
As long as it's sound when you find it. 1992 is a pretty old truck no matter how you look at it.
The 6.5 TD is also a noisy beast. As much as any other diesel of that vintage. You
HAVE to like diesel clatter or you won't like a GM 6.5 TD.
Well, I'll stop there for now because my fingers are getting sore from typing!
The 6.5 can be a good truck, it just takes a lot of monaaay to get it there!
