GM 84 6.2 Diesel MPG ??????

[wcuhillbilly]

I am looking for a cheap means of running the western N,carolina roads about Franklin from Jan till April while attending National Park Service Training. thus not working a job.... I have been offered, really cheap,(5-6 hundred) an old Pensylvania DOT crew cab chevy dually 6.2 diesel w/ TH/400 and 14 bolt rear running 4:10 ( I think,, havent checked).... will this set up be feasible to run mpg related for the short ride to the training facility in the winter. the truck has dual tanks, and I was thinking about installing a tank in the bed, thus filling up whenever all three were emty or when ever the cheapest price came along.
I have heard that these rigs can get 22-25 mpg w/ the stock wheels and tires,( 235/85/16) and then have heard that they only get 10-14 mpg...
Can anyone shed some light on this subject, as my 1965 GMC 3/4 ton w/ 340hp 350 will be killing me on gas, and the motorcycle will be a bit drafty in jan.

[u2slow]

That is the LEAST economical combination for the 6.2L diesel :(

My '84 C20 Suburban 6.2L also has the TH400 trans (aka bloodthirsty-power-sucker trans). I got 12-13mpg with 4.10 gears, and get 15mpg with 3.21 gears. It would be better still if I did more highway miles.

The people that pull in 20+ mpg with the 6.2L diesel are usually 1/2-tons. Lighter vehicle, taller gearing, and OD or manual trans make all the difference.

[pcorssmit]

My Blazer used to get 20 mpg, seems like it would do it in town or on the highway. Truck had a 700R4, 31s, and 3.73s; by the time I pulled the motor it was getting 14 in town, with 35s and 4.56s (also burning lots of oil, etc). I would guess that truck would be ~15, but I don't have any real experience to base that on. Cruising at reasonable speeds will definitly help you with milage (ie 2-lanes vs. interstate).

Pete

[redcheetah]

I had a 1986 CUCV (1 1/4 ton) with 42" tires and 4.56 gears. It got about 15mpg no matter what. However, I was very light on the gas after thrashing one th400 in a few months. Before I got the 42's the truck was quite efficient. It only had about 16,000 actual miles on it though.

[black jimmy]

I got about 12 mpg with mine towing to AL. I don't really keep track of it around town. The only difference between mine and yours is a four speed and two extra doors it sounds like. What is a short run?? I drive mine around all week on one tank of fuel.

[wcuhillbilly]

A short run may be 5 miles it could be 10, as I have not found a residence yet.. the main thing will be that it is in town and stop and go traffic. the truck has 2 tanks each holding approx 15 gal apeice and I was considering if I end up w/ it, putting a tank in the bed and just filling all 3 in Jan and run till there empty or if the price comes down considerably, filling one or all three back up. biggest thing is I dont forsee myself having a job during this time as the class is 5 days aweek 8-10 hrs a day.

[black jimmy]

Coutn on using at least a tank a week. That is my .02.

[thecarman]

I have a 1990 Suburban 1/2-ton 4x4 with the 6.2 diesel and TH700R4. 3.42 gears and 235/75/15 tires. I average 17-18 mpg commuting 45 miles one way to work, using the A/C, and this includes 70-75mph on the highway and a little city driving. The best I have ever gotten is 19 mpg. My truck is totally stock, including the crappy EGR intake manifold that comes on the 1/2-tons, which hurts power and MPG a little. (You don't have to worry about that on a dually). My Burb weighs 6000 lbs every day - I weighed it at the truck scales.

Towing my 2600 pound dual-axle car trailer empty, I get about 15 mpg.

Towing a total trailer weight between 4250 and 7000 pounds, I get 12-13 mpg. Worst was 11.74 mpg towing my TJ (6550 trailer weight, plus terrible aerodynamics of TJ on trailer).

I might be interested buying that cheap crew cab dually if you decide to pass on it. I need a truck. Please let me know if it is available! :)

[wcuhillbilly]

I found the gear ratio for the rear axle today... turns out the original 14 bolt is still in it. w/ 4:56 gears and a limited slip locker since it was an old PA DOT truck. again the drive train is 6.2, TH 400 (2wd) crew cab and narrow 14 bolt( only as wide as a standard fleetside bed) I was told by a freind of mine that the truck got 12 -14mpg back when it was used to tow a 24ft car show trailer of used/new car parts, interior stuff etc. the body is in rough shape as the driverside floor pan in no longer w/ us. and the Driver side rocker is on its way out. I found a guy w/ a (I believe to be) 2:73 gear for a 14 bolt who would be willing to trade ring and pinions for this 4:56, my ? now is will this truck be able to get out of its own smoke w/ a 2:73 gear under it as most of my usage will probly be empty...... A 3:40something or 3:73 In my opion would be ideal but I have never dealt w/ the 6.2 before...

[u2slow]

my ? now is will this truck be able to get out of its own smoke w/ a 2:73 gear

:laughing: :laughing:

Probably not. (There isn't a 2.73 for the 14ff AFAIK).

While I have great highway legs with a 3.21 ratio, it seriously blows on hills or when I pull my pickup-box utility trailer. I also think my 6.2L is in fairly good shape - injection pump & injectors done ~10,000km ago, and easily passes emissions testing.

In hindsight I would have chosen 3.42 (or 3.54 if a Ford or Dana axle had dropped in my lap instead.)

You best donor axle candidate is from a C20 crewcab. It will fit your spring spacing, and since they are rated as a 3/4-ton they normally had lighter gearing. 3.42 didn't show up until the 80's though. Swap in 1-ton SRW rotors and you have un-duallied the truck completely. Now sell of the 4.56 C&C axle to pay for the swap, buy some SRW wheels, and you should still have money left over for :beer:

[pcorssmit]

I found a guy w/ a (I believe to be) 2:73 gear for a 14 bolt who would be willing to trade ring and pinions for this 4:56

You will also need to swap carriers, the OE 4.56 gears used a different carrier. Make sure the donor axle has a 4 pin carrier, and your detroit should work ok in it.

Pete

[thecarman]

You will also need to swap carriers, the OE 4.56 gears used a different carrier. Make sure the donor axle has a 4 pin carrier, and your detroit should work ok in it.

Pete

Would that rear axle definitely have a Detroit in it? Or could it be Gov-Loc? I have seen lots of 4.10 14-bolt FF's with Gov-Loc. And I have seen people advertize 4.56 with Gov-Loc (but have not had my hands on one). My guess in a 2WD truck would be Gov-Loc.

[pcorssmit]

Would that rear axle definitely have a Detroit in it? Or could it be Gov-Loc?

Could very well be. I just saw "locker" in his post, and missed the "limited slip" part.

Pete

[wcuhillbilly]

sorry for the delay guys.... the build sheet in the glove box said HD (14 bolt) locking Diff. it didnt denote between locked or limited.. I have run this truck around the junkyard for years and can tell you that it definatly isnt completly locked... a call to the local dealer confirmed that the code on the housing matched the one in the sheet as a 4:56.
If I change the gear and carrier is there a set range to look for.... Im thinking a 3:73. but the D60s Ive dealt w/ in the past denoted that the carrier was differant, from 3:73-4:10 was the same, and 4:56 was another carrier

[u2slow]

The GM build sheets I have read say something like "G80: Locking differential" and that means a gov-lock (which is a governed limited-slip). When you call the parts department, they also say 'locker', even when you're ordering the clutches and steels to rebuild it :rolleyes:

Swap in a complete used 14FF with 3.73's... its gonna be cheaper and easier than buying only the guts and installing them.

[wcuhillbilly]

Update.... Talked to my boss today... and I got the truck for a song and dance.... Now have to catch up w/ the mechanic on the other side of the county and see what the ratio of the 14FF that he had and was wanting to get rid of in place of a 4:56 like mine.. if its possible I'll swap gears and carriers, as I'd like to keep the narrow 14FF thats in the truck, but will see how it unfolds. Heck if the parts start raining from the heavens I may even slip in a front axle and a T-case.....With the dual tank system the conversion to SVO waste oil will be a snap. Any ideas and input will be appreciated.

[72fj40]

I'm pretty sure that drw and srw widths on 14boltff's were changed by the hub offsets. I think the housing widths are the same. Might want to take some measurements.

[wcuhillbilly]

here we go.... the build has started... cobbled in a dr side rocker and floor pan today, swapped off some doors from a burb. this thing is rusty from the get go.. so Im not being very careful to keep lines straight nor hide my welds.
anyhoo. DRW and SRW axles are differant... if any one needs photo proof let me know. the Standard DRW axle is a good 8-12 inches wider, not only in the hub placement. SRW rears are known in some circles as "shaved pigs" (narrow 14 bolt FF)..
The axle that the mechanic had was a 3:73 and I will be going to pick it up in a day or so... if the locker in my 4:56 will swap then yeeehaah!!!! we shall see.
Parts are starting to fall from the sky.. got a deal on a SM 465/205 with the CV front drive shaft today... trans and stick were in great shape, for $325, crossmember and all the boots plates knobs shifters, and bushings. picked up a clutch pedel converstion for $35. complete w/ Z bar too... so now all I need is an 8 lug front axle and the front suspension.....

[PAToyota]

I know that it's a somewhat different animal, but my '93 K2500 has the 6.5TD, NV4500, and 3.42 gears and I can get over 20 mpg on the highway. The higher gears definitely make a difference. People with the 4.56 typically say they are getting 14-15 mpg. Also, a crewcab dually is never going to be something you'd consider "fuel efficient" as there is a lot of weight there. If you're looking for a commuter vehicle and don't need to tow/haul stuff I'd say look at an old beat up Subaru. I've seen those going for under $1500 and they seem to never die while turning in respectable milage and having 4x4 or AWD.

[wcuhillbilly]

I hear ya PA.... I would love to run across an NV4500 just begging to be installed.. However at the moment I have $350 in this truck and the SVO conversion will save some $ at the pump. I do haul stuff quit often as I ride an 86 Honda Shadow on days that weather and cold permit, which wont fit in the back of a suby, Believe me I thought about finding one before I started this and was really tempted to buy the old Trooper in a feild across the road from the shop I work at... but the deal turned out to be if I helped the boss build a gate for junk yard then I could have the truck. The rear end is an even trade my 4:56 gears for his 3:73. tranny/Tcase have been the only thing I have bought aside from the clutch pedel set up. the doors and floorpan and rocker panel were on a scrap burb heading for the crusher. and heck w/ this peice of rust I aint afraid to scratch or dent it.. and considering my 65 GMC is a restored workhorse I would like to leave her in the mothballs.
Thanks for the info guys, and keep it coming.

[PAToyota]

Somewhere I lost that it was only 2WD, that will get you a bit better milage. When you mentioned that your other vehicle was a motorcycle, I just wondered if you needed the size and weight of the crew cab dually. I spent a long time looking for the truck I got. I specifically wanted a '93 as that had the combination of the NV4500 with 6.5TD and still have the mechanical injection system. Seems like 95% of the ones out there were automatics.

I'm having fun tearing into it and getting everything working right. Amazing what stupid things the PO always seems to do when you buy a used truck...

[wcuhillbilly]

Let the building begin... Scored big at the crusher today...got a whole front suspesion from a 7? suburban 3/4 ton, for $100. two front CV driveshafts 2 crossmembers and the skidplate. and torched the framerails just behind the shackles.. so Everything suspesion related is there. including the str box... the axle was a Dana 44 8lug. and at the moment Im not concerned about the gearing as I have a lot of friends w/ spare danas laying around that I can either swap axles or ring and pinions w/. total cost as of now is $450 and all I need now in order to put this thing together is the clutch and flywheel... which will be around$270 or a little less if I can find a few more groundhog holes to go fishing in... total cost at that time will put me near or a little over my aloted build budget of $750-$800
take care guys and thanks for all the info.