I pulled this from my reply from this other thread, and added some additional info (
http://coloradok5.com/forums/showthread.php?t=222793)
I have 1990 K10 Burb, 4WD, 6.2L, TH700R4, 3.42s, 235/75/15 tires. All stock other than 4-core radiator (stock was 2 core) and added aftermarket trans and oil cooler (oil cooler no longer part of radiator). Has crappy dual exhaust with rust holes in it.
I've been driving it since Jan 2005 and logging miles/fuel religiously. It gets 18-19 mpg driving easy, not towing, no inner city driving, mostly highway, but some rural town stoplights, etc.
It weighs 6000 lbs as I drive it every day (I scaled it at a truck stop for fun). It is the Scottsdale model with manual locks and windows, vinyl /plastic interior - only options are electric tailgate window and A/C.
Towing 7000 lbs trying to maintain 65mph (only did it once, not fun) I got 12 mpg.
Towing 4000 lbs or less (enclosed U-haul or open flat-bed) and sticking to 55-60 mph I get 16 mpg.
Keeping your highway speed down helps mpg, but you need to be going fast enough to keep it in OD. I think mine would get a little better MPG with 3.73 gears because it could stay in OD at lower speeds - now it downshifts on hills.
I plan on adding a nice 3-inch single exhaust before inspection time in March. Also need to add a non-EGR intake - the stock EGR C-code intake sucks. Having more power should also let the truck stay in OD more, because you would not have to press the throttle as far to maintain speed (so would not kick out of OD).
I love my Suburban for family trips (me, wife, and 1 or 2 Labs, and now baby due in March), especially to the NC "Outer Banks" beaches, where you can (and sometimes must) drive on the beach to fish or get to a house. I run 33x12.50s on it for these trips, and I can't use OD with these taller tires - I just leave it in 3rd. MPG stays at 16.5 and above. But forget towing anything with the 33s on it - just not enough power with the 3.42 gears and 33s. If I ever swap some 4.10 axles under it, I'll probably leave the 33's on it all the time rather than the 235s - the 33s look so much better on it!
I also like that it is enclosed (vs a trailer or truck bed) - I can fit so much stuff inside and not worry about having to tie stuff down or it getting rained on (like bags of concrete). Went to the home improvement store yesterday to get material to build a roof under my deck, took my trailer with me because I thought I'd need it, but everything ended up fitting in the Burb instead, no need for the trailer. Was also very useful when we moved twice in the past 1.5 years.
3/4-ton is a good idea, only downside is it's hard to get an older one with overdrive trans (for improved MPG). The older straight axle Burbs did not have OD trans option - TH400 only. I *think* you could get a 4L80 in 3/4-tons in 90 and 91. If you were always towing, OD would not be as important, but for family cruising, I want it. My stock 1/2-ton rear springs SUCKED - sagged in the back with no load on it. I added two more leaves each side from another 52-inch pack, and added a 2nd overload leaf to each side. This works great for most loads, but could still use airbags when loaded heavy. The 3/4 ton would have better rear springs and the stronger rear axle ad bigger front brakes to begin with.
A turbo kit would be great, but I can't justify it at this time in terms of money and time needed for install. I have a Cummins powered K30 crew cab that I tow heavy stuff with (like cars and my K5), so the Burb is OK without the turbo.
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