I put a 6.2 in my 85 K5 blazer a few months ago and have yet to get more than 14.5 mpg which is better than the 350 got but not what I was hoping for. It has a 700 R4 recently rebuilt with a hydraulic lock up that seems to be functioning, the Tv cable seems to be adjusted about right and it has new filters. I have 33 inch tires on it and it has the stock 2.73 rear end ratio which makes driving under 65 in OD virtually impossible. Any ideas for why the mileage is low? Is it the ridiculously high gears with the tall tires or is there something else I should check?

the gearing......... its getting the crap lugged out of it and has to work like hell to move. sooner or later it will kill engine, tranny or both.

Hopefully your transmission doesn't let go like mine did in spectacular fashion! Split it clean in half :D

Do you really mean 2.73 --- or 3.73?

Anyway, diaric is right. Smaller tires or lower gearing should help. I'm going for stock size tires next time I need new ones ...

I put a 6.2 in my 85 K5 blazer a few months ago and have yet to get more than 14.5 mpg which is better than the 350 got but not what I was hoping for. It has a 700 R4 recently rebuilt with a hydraulic lock up that seems to be functioning, the Tv cable seems to be adjusted about right and it has new filters. I have 33 inch tires on it and it has the stock 2.73 rear end ratio which makes driving under 65 in OD virtually impossible. Any ideas for why the mileage is low? Is it the ridiculously high gears with the tall tires or is there something else I should check?

Are you sure about that gear ratio? Standard is 3.08, and with that and the 700R4, the 6.2 barely has enough power to maintain 65 MPH on the highway with standard 235/75-15" tires. Can't do it with 33" tires except on flatland and a wind behind you.

If you put standard 235/75-15" tires on it, it ought to get around 20 MPH on flat highway trips. You're going to lose 2-3 MPGs just from those wide and high 33" tires.

he is most likely correct. it was a gm high milage joke. my 81 had 2,73's and a 305 with a lock up th350. good milage new, horrible when older as the motor had to work so hard. either way, 2.73's and 3.08's aren't any good for that tire tranny set up

Yea i double checked, I have the build sheet and it says 2.73, you can tell because when you get on it, it doesn't shift out of first till almost 20 mph. I think I'll opt for some smaller tires, it's about time for some anyway. Thanks

Yea i double checked, I have the build sheet and it says 2.73, you can tell because when you get on it, it doesn't shift out of first till almost 20 mph. I think I'll opt for some smaller tires, it's about time for some anyway. Thanks

Even with 31" tires (31 X 10.50), the diesel Blazers with 3.08s and 700R4s can't maintain 65 MPH on a hilly highways unless you unhook the wire harness from the transmission. I've got one that I put a switch on the harness. I leave it off all the time around here, but turn it on for long flat highway trips. By unhooking, the trans still shifts fine but the converter won't ever lock up. So, you wind up with somewhere around 15% slippage at high speeds. Makes the rig much a much nicer driver with big tires, but drops MPGs a bit.

And, please - anybody out there that thinks that'll overheat the trans - tell it to somebody else. I've done it with several, for many years with no problems. Doesn't run any hotter with the harness unhooked, than a T350 or T400 that never had any lockup.

even with stock tires. its still geared to high for 4th gear. 3.73's is what you need with 33's. possibly 4.10's if you want more get up and go

I would like to change ratios, without changing axles, can I just buy a ring and pinion gear set for it?

Yes you can, or you if you don't mind getting messy, you could buy a set of used axles and just swap your gears with those ones, it's still less work than swapping axles.
Plus you can resell those axles and hopefully recoup your costs.

you need a different carrier, i'm pretty sure. the 2.73's don't interchange swapping diffs would be way easier