I want to beef up my 83 K5 4WD diesel Blazer for use as an on and off-road motorhome. I'm hoping to get some decent fuel-mileage with it. I'm hoping for perhaps 16-18 MPG on the flat highway. I'm a little fed up with my F250 ex-cab 4WD turbo IDI 7.3 Ford diesel and slide-in camper combo. Last 2000 mile trip I averaged 12.5 MPG and I want to do better. Thus the Blazer idea. I'm making some guesses based on past experience. Any body got suggestions or ideas? I put 500,000 miles on my 87 3/4 ton 4WD Surburban with 6.2 diesel - many of those miles pulling a 4000 - 6000 lbs. trailer. It had the T-400 trans and 3.73 axles. Pulling power was fine except for steep-hills. When pulling a long steep hill, besides a 35-45 MPH top speed - you really have to watch the engine temp. I had a few occasions over the years that I got worried - and pulled off to the side for a few minutes before finishing a long hill. So, I don't know for sure what WOULD of happened. If you've got restrictive aftermarket mufflers the heat problem gets worse with heavy towing. The engine finally seized going down the highway - and I later found the crankshaft broken into three pieces and two main-bearing webs tore off the engine-block. Seems to be from metal-fatigue of some sort. So, my son put another engine in it and took it home to Colorado (I live in New York). For general highway driving, 2000 RPM at 65 MPH is a good target for a diesel. After 2000 RPM, fuel mileage takes a nose-dive. I've got a K5 4WD diesel Blazer with 3.08 gears and the 700R4. It will not tow anywhere near as nice as my Suburban - and I put a shut-off switch on the lockup torque-converter which does help. When climbing a hill with a trailer behind, even when in third gear (not overdrive) - the thing pulls too hard with the converter locked up - runs much nicer with it unlocked and slipping a bit. I've had it that way for 70,000 miles (since the last trans rebuild) and it's been fine. When I need to pull something heavy, i.e. over 5000 lbs., and I know there will be a lot of hills, I use my 94 Ford 7.3 IDI turbo - it handles the pull nicer than my 6.2 Suburban - but it's murder on fuel-mileage overall. When worked hard, the Ford IH diesel isn't too bad -but empty?? It's has 4.10 gears with overdrive and turns 2000 RPM at 65 and only gets 13.5 MPG empty. My non-overdrive Suburban with 3.73s - which turns around 2550 RPM at 65 MPH gets up to 19 MPG, and my Blazers (I've got four) with 6.2 diesels, 3.08 gears, and 700R4 get up to 22 MPG. Better yet, my 82 Chevy truck, 1/2 ton, 4WD, 6.2 diesel, 3.08 gears, and four-speed manual trans. with overdrive has gotten 24 MPG empty - but it's not much of a tower. And good thing it's 4WD so it's got a low-range. I'm building an 83 K5 diesel Blazer - or I should say converting - for use as a small motorhome - so I've been thinking about trans and axle options. It's an 83 Silverado 4WD 6.2 diesel - I've had it for 15 years - but due to rust it's been mostly a snow-plow vehicle lately. I'm going to install a Blazer-Chalet motorhome camper-body on to it. So, I'm trying to figure what changes to make. It presently has 3.08 gears and a T-400 trans. with a mechancial governor shift (convertered years ago from a bad 700R4). It gets around 20 MPG tops. Now - with the added weight - probably an extra 1200 - 1500 lbs. when I get done, and some added wind resistance - I'm trying to guess at changes. I have an 89 GMC diesel Suburban up in my field. I'm thinking of putting the springs and axles from it, into my 83 Blazer. That will give me 16" load-range E tires, 3.73 axles and heavier springs - along with a larger u-joint at the rear-axle hookup. But - the trans choice? I'm still up in the air. At first I was thinking a 700R4 built as heavy as possible - but I don't now if I trust it. I also have a New Process 833 overdrive manaul 4 speed setup. I've also been thinking about a 4L80E if it can be done - but I know little about it. I love the T-400s - great transmissions - but I want some-sort of overdrive - and I dont' want to spend $3000 for a Gear-Vendors unit. And besides, even if I had a Gear-Vendors overdrive - I still would not have a lockup converter. On a sidenote - I also thought about installing a turbo - but I know it will NOT increase the fuel-mileage - just the power. So far - this is what I've figured. 3.73 axle with 16" tires and 700R4 overdrive results in 2000 RPM at 65 MPH. 3.73 axle with 16" tires and T-400 trans results in 2550 RPM at 65 MPH 3.73 axle with 16" tires and 4spd OD trans results in 1876 RPM at 65 MPH I haven't accounted for torque-converter slippage with the T-400, but the figures are close enough. The 700R4 has a .78 overdrive, whereas the New Process 833 four-speed overdrive has .73 overdrive. 16" tires are standard 235/85-16" tires with 31.7" diam.