[jfman]

I know mpg's can and will vary but what can I expect from a diesel powered k5 with 38's.

 

[jdemaris]

I know mpg's can and will vary but what can I expect from a diesel powered k5 with 38's.

You didn't mention gearing including axles and overdrive trans. or not. 38" tires with 3.42 axles and no-overdrive would just barely have enough power to be driveable - it'd be running around 2000 RPM at 65 MPH. If you had overdrive with the same axles and tires - it would be too underpowered. Most K5s come stock with 3.08s - which also might barely work if you don't have any overdrive.

Keep in mind a 6.2 has about the same horsepower and torque as a 305 gas engine. The biggest tires I have on any of my 6.2 diesels are on my 88 K5 with 3.42 axles, 700R4 trans. and 33" tires. The tires are really too big for it and does not drive well in overdrive on the highway unless it is absolute flat. The slighest grade slows it down and it shifts in-and-out of lockup. Flat highway trip it will get 21 MPG. With the older, stock 15" tires if got 23 MPG and drove much better.

 

[High Sierra 2500]

Welcome to the forum! :welcome:

There are a couple of other factors to think about... Most notably gearing. The transmission would also be another thing, but I'm guessing you are running a 700R4 (4spd overdrive) unless it is a military blazer. What gears and tranny are you going to run?

Bigger tires shouldn't affect power output, acceleration, or performance if they are installed with the proper gears and the governor on the tranny is set up right. They will affect steering and roll characteristics, however.

Welcome again! :welcome2:

 

[D.Camilleri]

Rolling resistance is very high with 38 inch tires. To run propperly, it will need to have either 4.10's or 4.56's in the diffs. I would venture to say that 15 mpg would be a fair guess as to fuel mileage. Keep in mind that a blazer with 38's and a 350 gasser will only get around 8-9 mpg.