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Discussion Starter #1
I'm getting 13.5-14mpg with my '97 Suburban. I think this is a bit low, what do you guys think?


This is mostly around town driving with 2wd driwe. I think I should be getting better mpg. I'm very gentle on the gas pedal...
 

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1997 6.5 TD extended cab long bed 3.73, I get 17-19 mpg on the highway, and I'm not especially gentle on the gas pedal. I can get 20 if I keep it at 70-75.

What rear end do you have? You may have a 4.11. To get an idea, what are your revs on the highway at, say, 70 mph? When I go drive my truck today I'll get some numbers for you and you can use that as comparison. If you have a 4.11 that would probably make a noticable difference in mileage.

You may also have a clogged cat. Mine was very clogged, so I gutted it and that brought my mileage up to 17-19 from 16-18. However, I got my 20 mpg before going a steady 75.

Good luck.
 

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Discussion Starter #4
I checked the glove box sticker for the production option codes. And found GU6(AXLE rear 3.42)... Something wrong?


Or did I read the codes wrong?
 

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My book has a GU6 as a 3.42 axle and this would account for poor fuel milage.


My 97 2500 totaling 16,540# gets 12+ at highway speeds of 65 to 70 mph
 

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How many miles on the Suburban? Does it smoke much black smoke when you puch hard on the accelorator? How is the power?
 

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16gaSxS


If your question about power and smoke was directed to me, the truck has 130,000 miles on it runs clean and showed 192hp last year on the dyno when it had 106,000 miles.


The truck is running with more power than ever right now. I relocated the FSD to under the front bumper, see my page, just before my last trip down south. On hills that I would have used propane in the past i ran at road speed. Prior to that my towing milage was running between 10.9 and 11.7. After the relocation it had more power and the fuel milage jumped to 12.7.
 

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Discussion Starter #8
I have 153 000km on my Sub right now. No black smoke, runs real fine. The IP has been changed earlierly under warranty...
 

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I'm having the same problem with poor mileage. Just had the Injector Fuel pump replace. Only thing I have seen is that with this cold I get a white smoke on start up for a second or two then runs fine. Didn't get it when it was a bit warmer. No smoking that I can tell after that. Was told that it has 4:11 gearing but not sure how to read the codes in the glove box. Thanks for any Help.
 

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For RPO Codes look in glove box for the letter schemes below to find out what options you have, these are listed in front of the GM Service manual. IMO an essential piece of your toolbox is the GM manual, especially with the 6.5 as the dealers no longer school their techs in 6.5, it's all DMAX now.


The manual is pricey $150 new set avg www.helminc.com can also be found online at ebay and book seller sites for less sometimes, $150 is about 2-3 hrs GM troubleshooting time, you can get your codes scanned for free at Autozone & others if you don't have a scanner $75 I think is GM avg scan charge, won't take many trips not made to GM to recoup investment, plus gives a sanity check if your dealer lacks integrity and is feeding you a line.


C5I -- GVW RATING, 8050 LBS
C5M -- GVW RATING, 6100 LBS
C5P -- GVW RATING, 6250 LBS
C5Q -- GVW RATING, 6300 LBS
C5R -- GVW RATING, 5800 LBS
C5S -- GVW RATING, 6600 LBS
C5U -- GVW, 6800 LBS
C5Z -- GVW, 7200 LBS

C6P -- GVW RATING, 8600 LBS/3900 KG
C6U -- GVW RATING, 9000 LBS
C6W -- GVW RATING, 9200 LBS
C7A -- GVW RATING, 10000 LBS



F44 -- CHASSIS, HEAVY DUTY
F60 -- SPRING FRONT, HD


GQ1 -- AXLE REAR, STD RATIO
GT4 -- AXLE REAR, 3.73 RATIO
GT5 -- AXLE REAR, 4.10 RATIO
GU4 -- AXLE REAR, 3.08 RATIO
GU5 -- AXLE REAR, 3.23 RATIO
GU6 -- AXLE REAR, 3.42 RATIO
G80 -- AXLE REAR, LIMITED SLIP (POSITRACTION)


HC4 -- AXLE REAR, 4.56 RATIO


JB6 -- BRAKE, POWER, DISC/DRUM 7200 LBS
JB8 -- BRAKE, POWER, DISC/DRUM 10000 LBS
JD5 -- DUAL POWER, DISC/DRUM 6400 LBS
JD7 -- BRAKE, HYD POWER, DISC/DRUM 8400 LBS


KNP -- HD TRANSMISSION COOLING
K05 -- HEATER, ENGINE BLOCK
K34 -- CRUISE CONTROL, AUT0MATIC, ELECTRONIC
K47 -- AIR CLEANER, HIGH CAPACITY
K60 -- GENERATOR, 100 AMP
K68 -- GENERATOR, 105 AMP
KC4 -- COOLING SYSTEM, ENGINE OIL


L49 -- ENGINE, DIESEL, 8 CYL, 6.5L
L56 -- ENGINE, DIESEL, 8 CYL, 6.5L, TURB0
L57 -- ENGINE, DIESEL, 8 CYL, 6.5L, HO
L65 -- ENGINE, DIESEL, 8 CYL, 6.5L. TURB0, HO
L76 -- ENGINE, DIESEL, 8 CYL, 6.6L, V8, DI, TURB0, ISUZU
LH6 -- ENGINE, DIESEL, 8 CYL, 6.2L


MM4 -- MERCHANDIZED TRANS, MAN 4 SPD PROVISIONS
MT1 -- TRANSMISSION, AUTO 4 SPD, HMD, THM R2 (4L80-E)
MW3 -- TRANSMISSION, MAN 5 SPD, NVG, 109MM,, 5.61 1ST, O/D
MX0 -- TRANSMISSON, AUTO PROVISIONS, O/D
M20 -- TRANSMISSION, MAN 4 SPD, 117MM, 6.55 1ST
M30 -- TRANSMISSION, AUTO 4 SPD, HMD, 4L60-E, ELECTRONIC


NA1 -- EMISSION SYSTEM, GVW < 8500 LBS
NA5 -- EMISSION SYSTEM, FEDERAL, TIER O
NB8 -- EMISSION SYSTEM, CA/NY/MA/CT EMISSIONS OVERRIDE (REQ FE9)
NC7 -- FEDERAL EMMISIONS OVERRIDE (REQ YF5 OR NG1)
NF2 -- EMISSION SYSTEM, FEDERAL TIER 1
NG1 -- EMISSION SYSTEM, NY/MA/CT
NG5 -- REGISTRATION, EMISSION SYSTEM (50 STATE)
NK3 -- STEERING WHEEL, SPORT, SOFT RIM, SIMULATED LEATHER
NP1 -- TRANSFER CASE, ELECTRONIC SHIFT CONT, TWO-SPEED
NP5 -- STEERING WHEEL, LEATHER WRAPPED
NRQ -- EXHAUST, CLOSE COUPLED



PF4 -- WHEEL, 16x7 ALUMINUM
PO6 -- RALLY WHEEL/TRIM RING


RO4 -- WHEEL, SINGLE RR
RO5 -- WHEEL DUAL RR
 

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Here's my two cents on fuel mileage. I have a '97 GMC K2500 6.5 TD Suburban. I have the L80 tranny and a locking 4.10 rear. Had it since it was 16000 miles old. It runs the same today as it did back then and I'm one of those nuts
that checks the fuel mileage with almost every tank full.


Around town: Between 15 and 16 mpg. usually towards the high 15 range but usually just shy of 16 mpg. Strickly highway: running 60-70 mph, I get around 18. Towing a 6000lb. trailer on the flats will yeild around 12 mpg. Towing with lots of up-hill stuff can get me less then 9mpg. On my last trip out west, NJ to Glacier Mt. I did 6131 miles round trip. I used a total of 572.49 gallons of fuel. The cost was $763.74 for fuel. The average cost per gallon was $1.33 (the good old days) and I had a grand average of10.71 mpg. Theres lots of hills between here and there. Told you I was one of those fuel record nuts!!!!Edited by: MFParker
 

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One of the reason I got the Suburban was to tow a trailer. Need something that could hold a growing family. As we are into camping most travel trailers are around the 6000 pound mark. While a gasser would have been able to do the job I need to be able to use it everyday and a gasser would have cost more in fuel was my thinking. So we went diesel. Well the family loves the truck. Just need to get that MPG up. Just to let you know after I got a diesel is when fuel prices started to rise in NJ. It's like they knew. Thanks for all the help.
 

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Couple of thoughts on poor mileage, low ratio on axel 4.10's , oversize tires, bad injectors, no boost. Heavy right foot.


I normally get 15-16.5 around town that went down with no boost and end of life injectors.Edited by: 16gaSxS
 

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Well at the last fuel up MPG was at 14.5 not great but better then my first tank full. I'm hoping that by April when summer fuel comes back it will go up. Gearing is 3:73. Tires are a little on the big side, 265 instead of 245. I think the injectores were replaced around 10 to 20 thousand miles ago. Need to check paper work. Explain boost?
 

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Diesel Grinch said:
Well at the last fuel up MPG was at 14.5 not great but better then my first tank full. I'm hoping that by April when summer fuel comes back it will go up. Gearing is 3:73. Tires are a little on the big side, 265 instead of 245. I think the injectores were replaced around 10 to 20 thousand miles ago. Need to check paper work. Explain boost?

Boost is what the Turbo charger does, it "boosts" air pressure to higher level than ambiat air pressure. Factory boost is 5-7 PSI. If your boost isn't working you do not get as much air to burn the fuel so you waste fuel and get black smoke with reduced power.
 

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Well no Black smoke. I do hear the whine of the turbo but I don't have a boost guage. Went on a highway trip and MPG went to 15.1
 

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Discussion Starter #17
Could the winter fuel be the answer to my bad MPG?


I'm only getting 13,5-14MPG at the moment. My tire size is 265/75/16 and the sub now has 155 500km, and the rear axle is 3.42. I run 99% of time using 2WD... Something must be wrong here...
 
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