Fuel Economy Update for the 1993 Diesel Conversion Suburban

The time was right for a road trip with my converted Suburban, so we took a family trip to visit my brother and sister-in-law and my brand new nephew. They live in Carbon County PA, right outside of Jim Thorpe PA. Right before my trip , I swapped the H.S.F. (Horsepower Sucking Fan) for an electric one from an Olds Silhouette van with dual AC. I installed it with redundant temperature senders actuating a relay (one set for 190 degree turn on, the backup set for 205 degree turn on) and an A/C input to operate the fan whenever the AC is operating. The result is cool operating temps that never exceed 195-200 degrees. Also a considerable boost in horsepower getting to the wheels. So with this considerable improvement made, I double checked the tires (all aired up to 44 PSI), loaded up the kiddies and we headed out on a highway trip. 90% of the running was at steady cruising with the cruise control engaged at either 55 MPH or 65 MPH, depending upon the speed limit. The highway miles run at each speed were approximately 60% at 55 MPH, 40% at 65 MPH. about 10% was in rural and stop and go driving on hilly terrain. All driving was with A/C. The overall results blew me away. The results are as follows:

296.7 Miles

10.938 Gallons

27.125 MPG

That is right! Pick your jaw up off the floor! OVER 27 MPG in a Suburban with a roof rack and cold A/C!

Now just to give insight as to how I conducted my test: I filled up before leaving and DID NOT top off the tank once the fuel pump clicked off the first time. I dosed the tank with some Power Service, and then I reset the trip Odometer. I then drove home to pick up the family. I used a GPS to double check my speedometer accuracy (within 0.5 MPH of GPS reading at 55 MPH). I also used the GPS to log the miles traveled to verify odometer accuracy. Upon returning home, I filled the tank at the same pump and DID NOT top off the tank after the nozzle clicked off. I then dosed with power service, and calculated the results seen here. As another note, other than the tire pressures, use of the cruise control, and strict adherence to the speed limit, I did nothing to gain an advantage. No drafting, coasting in neutral with the engine off, etc. I am really satisfied with these results. I am very happy I dropped a diesel under the hood of the “Burb!

Man, thats great! I love blowing people minds when telling them about 6.2 mileage. Congrats on your diesel stimulus package! :P I know of a guy getting 32mpg in his 85 Burb.

Glad to see others getting good fuel economy numbers out of their older 6.2's. When tuned and set up correctly, they can do surprisingly well and have relatively good performance. If fuel prices continue to rise as predicted, demand for light trucks that get good fuel economy will most likely increase.

Thanks for the positive responses. I am very satisfied with the way it runs, but I still want to do a gear swap to 3.08s. I am really surprised at how much this engine reminds me of my Ford 300 in my F-150. Except that this engine sounds cooler.

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Originally Posted by argo

but I still want to do a gear swap to 3.08s.

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IMHO, way to long. You'll have to put your foot deep into the pedal or run in 3rd instead of OD. I bet it's a net loss in efficiency!

Having a 3.08 rear end works well on my 82 Suburban and it does well on fuel economy. I’m using 30x9.50 tires and have a free flowing exhaust. However, if the truck were 4x4, had oversize tires, or was ¾ ton, I agree that shorter gears might be preferable.

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Originally Posted by JohnC

IMHO, way to long. You'll have to put your foot deep into the pedal or run in 3rd instead of OD. I bet it's a net loss in efficiency!

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I agree. A 3.42 in 3rd gear would be about equivalent your current 3.73 in OD. 3.08 would be a long reach, and would probably drop your RPM at highway speed well below the efficiency and power range in OD, and would be like your current gears starting out in 2nd. The 700R4 has a deeper 1st gear than other GM autos, but the 3rd/OD gears are about the same as any other (direct/~.70).

Perhaps OK for a blazer or shortbed/cab pickup, if they rarely/never see a load on them. There were a few models that had 3.08 gears (from the factory), but performance with them was very anemic for all but the most conservative daily driving. There's only so much you can do with the stock power of a 6.2L.

For an inexpensive "test" of what the gear ratios my net you, change the rear tire size (temporarily). An 8% change in tire diameter will be about equivalent to the gear ratio spreads (3.73 - 3.42 - 3.08). For example: If your current tires are 245/75/16 (30.25") with 3.73 gears, swapping to a 255/85/16 would be like changing to 3.42 gears. Keep in mind though, the speedo will also be off by an equal %. Just for perspective, going from a 3.73 to 3.08 would be about like switching 28" tires (215/75/16) to 33" tires (255/85/16).

3.73x0.70=2.61:1 final drive, 1670 RPM@55MPH, 1973 RPM@65MPH

3.42x0.70=2.39:1 final drive, 1531 RPM@55MPH, 1809 RPM@65MPH

3.08x0.70=2.16:1 final drive, 1379 RPM@55MPH, 1629 RPM@65MPH

So after doing the math, I think that maybe 3.73 gears would be best, since they spread the RPM around the 1800 RPM "sweet spot" at the cruising speeds I normally drive at, 55 and 65 mph. If I had a lighter and/or more aerodynamic vehicle, the 3.08 gears might be a more attractive proposition. I might use them in the 6.2L Station Wagon that I am intent upon doing next.

That sounds reasonable. You can make fine adjustments over time with the tire size.

For the station wagon project, I wouldn't be opposed to even a 2.73 G/R. You couldn't tow with it in OD, but the power requirements, weight and aerodynamic differences between the two vehicles is astronomical.

Just got back from a fairly long trip myself. My calculations were not as exact as yours as I filled up at two different stations at either end, but my results were 435 miles and 19.14 gallons for 22.73 mpg.

I've corrected the mileage for 285/75-R16 tires and the axle ratio is 3.42.

Most of the driving being between 65 and 70 on I-81 between western Virginia and Southcentral Pennsylvania; some rural driving at the Virginia end and urban driving at the other; 650 lbs of occupants and gear; and no A/C.

I'm pretty pleased considering that my Toyota would get 24 to 26 mpg on the same trip with a gutless 4-cylinder that would barely maintain 50 mph in the mountains on I-81.

For a 4x4 3/4 ton, that is awesome!

Thanks! Actually, I was a bit disappointed because I hadn't noticed that you were talking 2WD. I was hoping for a bit closer to 24 mpg.

Well I know I dredged this one up from the deep, but I wanted to give a 1 year update on the fuel economy for my Suburban. This is an aggregate average of a full year's worth of driving and fuel use. Since the conversion was completed, it has traveled 18,485 miles, consumed 740.258 Gallons of fuel, and averaged 24.971 MPG. This is overall MPG, with around town and highway. I am very satisfied.

Thats some very impressive mileage.

Brings back memories of the 82 K5 I had. Little critter would do 24-25 Hwy all the time.

Glad to hear all is well.

Missy

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Originally Posted by argo

Well I know I dredged this one up from the deep, but I wanted to give a 1 year update on the fuel economy for my Suburban. This is an aggregate average of a full year's worth of driving and fuel use. Since the conversion was completed, it has traveled 18,485 miles, consumed 740.258 Gallons of fuel, and averaged 24.971 MPG. This is overall MPG, with around town and highway. I am very satisfied.

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Wow, that's very impressive fuel economy and good to know you're getting it with 3.73 gearing. I'm getting ready to do a very similar conversion on a 2 wheel drive 99 Tahoe. It has a 3.42 rear end so it'll be interesting to see how it compares to the Suburban with 3.08's. Right now, I'm looking for a serpentine belt setup before I get started.

I also tend to drive slow (my wife can't stand it) but I usually go 55 MPH in the 55 MPH zones, and 60 MPH in the 65 MPH zones. I have proven to her 8 ways from Sunday that you don't get anywhere appreciably faster by going 70, but she can't grasp it. I even showed her the GPS which said that we would get to my parent's house (95 miles away) in one hour and thirty three minutes (about right given Philadelphia traffic) and within about 5 minutes, that is what it takes, no matter how fast we go. Sure, If I were driving cross country to California, maybe 70 beats 60 by a good bit, but I am not sure that it wouldn't balance out when the 70 MPH me has to stop for an extra tank of fuel (or two), compared to the 60 MPH me. Oh, and I keep my tires pumped up to 50 PSI all the time.

My worst tank was when I buried the speedometer for an 80 mile trip to Wilmington to get to the Birth Center when my Wife was in labor. The 6.2 will push the speedometer past the 80 on the speedo. I dont know how much faster than 80, but I coasted for about 10 seconds before it dropped below 80. So maybe 100? Maybe that doesn't count since I didn't need a complete fill up, but since I am such a dork, I filled up anyway (later, after the baby came, of course) to find out what flying did to my MPG.

My daily driver is my 82 C10 Shortwide 6.2L stock (except non-egr now).

It's a 2.73 geard truck, but with 26" tall tires because it's lowered 6".

I run 80MPH down the highway routinely and it still returns 27mpg.

Acceleration is kind of a lacking, but not really all that bad by other early 80's 6.2's I've owned.

My former 1991 2WD C10/R10 6.2 Suburban would only get 22 max on the highway with 3.42 gears 235/75 R15 rubber running 70 on flat ground. So I'm really surprised that some are getting 25 or better. Must be at 55mph.

J

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Originally Posted by john8662

My daily driver is my 82 C10 Shortwide 6.2L stock (except non-egr now).

It's a 2.73 geard truck, but with 26" tall tires because it's lowered 6".

I run 80MPH down the highway routinely and it still returns 27mpg.

Acceleration is kind of a lacking, but not really all that bad by other early 80's 6.2's I've owned.

My former 1991 2WD C10/R10 6.2 Suburban would only get 22 max on the highway with 3.42 gears 235/75 R15 rubber running 70 on flat ground. So I'm really surprised that some are getting 25 or better. Must be at 55mph.

J

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John,
What transmission do you have behind your 82 C10 Shortwide 6.2L? The lower stance must help with aerodynamics to achieve the 27 mpg at 80MPH.

TH700R4 like it should be (previous owner had a TH350 in it, it went out with the scrap metal).

What!!! You do realize that there are still folks who haven't upgrades from the powerglide to a 3 speed auto right? I am sure you could have sold it so someone for more than dirty aluminum scrap price! :D By the way, what tire size are you running on the rear?