I’ve had my 92 blazer now for about 4 months, use it for my daily driver, and have found it to be rather fuel hungry. When I purchased this truck it was getting approx. 12-14 MPG, since then I have replaced most all the fuel system components. I am confident the fuel system is working within tolerance but still the best MPG the truck has shown since the R&R is 14-16. I know many people can get higher MPG but my driving most of the time is 100% in town, no speeds higher than 45 MPH. I Have noticed a considerable increase in MPG when my driving habits included highway with speeds of 55 or higher. For this experiment I will be calculating MPG on routes that are mixed city driving and highway

The first significant change I made to this truck was give it some mud tires, then cold air intake, new coil, new spider unit, new fuel pump, new filter and recent nut kit from the PO. (Tires - 31x10.5R15) Also, I purchased this truck with a 3" body lift, with the lift the aerodynamics changed a little bit. The spare tire and rack have been removed also. Here are some of the future changes I have planned to improve my MPG. Replacing the AC compressor with a bypass pulley for less parasitic drag on the engine. Removing the rear window wind deflector for less drag. I know these seem like small changes but I hope with improved driving habits they add up to something worthwhile.

For my driving habits there are several things I can change. I have been noticing the shift points for the transmission are at 2k rpm. My plan is to keep engine rpm at or around 2k, this is a little hard to do all the time but with a little pedal work I can get the trans to shift at 2k most of the time. Next thing is fuel weight, gallon of gas is about 6 lbs. The idea is to carry only a half tank of gas instead of filling up. That should shave about 50 lbs off the rolling weight of the truck. Removing the AC compressor should also reduce the vehicle weight.

As for calculations, there are couple things to consider. First would be recalculating for the larger tire size.

Stock Tire - 235/75R15 __________On truck - 31x10.50R15
Circumference: 90.69 in __________Circumference: 97.29 in

(97.29 / 90.69 = 1.07) This is the factor to recalculate the odometer

So for every 100 miles of odometer reading, real miles traveled is 107

Here’s the link I used to calculate tire size.
Tire Size Calculator - tire & wheel plus sizing

Calculating MPG without filling up is going to be a challenge but I think there is a way to do it. My plan it to keep a total of the number of gallons I purchase and then use that number with the adjusted odometer reading to get the MPG. I’ll start this with a 1/4 tank of gas and run through multiply half tanks of gas ending back at the 1/4 tank mark. Not perfect but should be close.

Blazer specs:
92 blazer 4D, 4WD
4.3l vin (W)
Automatic
Gear ratio 3:42
216K miles

Any ideas, comments are welcome.

 

An easy way to reset your speedo is GM DRAC/VSSB Calibration I did that with mine and after a couple tries and comparing it to a GPS, I dialed it in nicely. At least if you change your speedo, its one less thing to worry about with your calculations. I'm sure you've already thought about this but, if you REALLY want MPG's, consider getting rid of those tires and the lift. A good set of road tires will boost things up quite nicely. And by the way, I've never seen anything higher than 18MPG in my truck (same everything)

 

Update:

Since my last post I have been logging my mileage. I filled the tank 4 times @ 5gallons. Here are the numbers, (rounding up).

20 gallons gas
223 odometer miles
223 X 1.07 = 239 miles actual
239 / 20 = 12 mpg

I started a second round tonight and there are a couple of changes on this test. I have changed the front (head) knock sensor and connector. Swapped out the distributor and ignition module with low mileage parts. Re-timed the distributor to TDC and set engine timing to 4 degrees BTD.

 

Update:

20 gallons gas
283 odometer miles
283 X 1.07 = 302 miles actual
302 / 20 = 15 mpg

No new changes to report. This last test included a 180 mile road trip with alot of hill climbing.

 

Update:

20 gallons gas
264 odometer miles
264 X 1.07 = 282 miles actual
282 / 20 = 14 mpg

Changes this time were new plug wires. This might have been the best test of all-around driving conditions. During this round I did some light trailer hauling, city stop and go and a few highway miles. Probably no more than 30 miles of trailer hauling and on the rest 30% highway and 70% city.

 

My dad has a 92 with the vortec. he's getting 22 mpg. But its 2wd with 308 gears and 225/70/15 tires. you should be getting at least 17. even with the 31" tires. My tbi 4wd gets 18 with 30" tires 3.42 gears.

 

Yeah I agree chris. Right now I am in progress of getting the tools, cable and computer, to do some brain surgery on this beast. Once I can get a look at the computer, maybe I can get an idea of what is going on. One thing I did notice is your blazer is a 2 door and mine is a 4 door. Looks like there is a 240lbs difference between the 2 and 4 door models. Don't know if that would translate into a 4 MPG difference but it does mean more gas per mile. At any given time I could have 100+ pounds of gear, tools and stuff, in the truck so that's going to add to the problem. I would be really happy if I could get 16-17 in city but I have this feeling that's going to take an act of god.

 

ah thats right. I had a 4 door model years ago. with the 4.3 tbi,3.08 gears 2wd and it only got 15 mpg. but it had 400k on it,original motor. haha 2 cylinders only had 70lb of pressure. but still ran. i love these things.

If i was you,id empty everything out of it possible(tools etc) and drive it and see whatcha get. Also,try some full synthetic oil,as well as for the rear diff. I changed over to synthetics in my jeep and gained 2 mpg. I wont do it to my blazer though,i have a leaky rear seal. dont want it to get worse.

 

Update:

20 gallons gas
283 odometer miles
283 X 1.07 = 302 miles actual
303/ 20 = 15 mpg

This go around was 50/50 city / highway.

 

Update:

11.5 gallons gas
189 odometer miles
189 X 1.07 = 202 miles actual
202/ 11.5 = 17.5 mpg

This go around was 25/75 city / highway. I am starting to log my MPG's a little different now, doing the fill up method. I filled the tank and drove to approx half tank to take my readings. I should mention the truck was fully loaded with gear, as I took a camping trip.