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Discussion Starter #1
Hey all,


I've searched this forum and seen some discussion, but I'm looking for help w my fuel economy.


Right now its 13+MPG in town, and 15+MPG hwy. Its dipped as low as 12 in town.


I use odometer reading and track fill-ups to same level in fill tube every tank.


The truck used to have a Heath PCM. I removed it and put in a stock PCM. (I thought it might be behind low MPG)



I run 235 85R16 tires, instead of stock 245 75R16. I think this accounts for a 1.04 correction to odometer miles, which doesnt change MPG much.


I bought it w straight thru exhaust, but had a local shop put on muffler, not brand name or anything.



steps I've taken (w no change):
* new PMD
* reinstall vac pump and vac turbo control (solenoid and wastegate)
* swap out 4.10 gears with 3.10


The only things left I can think of are new injectors? Could it be a transmission issue?



Can I expect to get more than 13-14 in town w K2500 Suburban? Folks list some pretty high MPG in their sigs, but I'm not sure how realistic that is...


Any ideas or feedback would help. Thx!
 

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I have a 96 K2500 Suburban and I average right around 10-12 mpg city/hwy with 4.10 gears, 35x12.5 tires and a DE exhaust. Also, have a S&B cold air intake and dual e-fans from an LS.
 

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3.10 ? your sig says 3.73. Madison is big enough to have some traffic which is never good for MPG. What sort of highway driving do you do? 70 down to Janesville? Or 80? Do you use the cruise? Are you correcting for the ratio gear ratio change too?


I had a stock 96 with 3.42 and it would hit 20 at times. My 3.73 and 4.10 don't do that.
 

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(snipped) "Are you correcting for the ratio gear ratio change too?" (snipped)
My '93 has the VSSB, (vehicle speed sensor buffer), and behind the glove compartment. I had to reprogram it with the jumper connectors when I changed tire sizes to get the speedometer/odometer correct. Don't know about a '97, but probably has something similar? And why did you remove the intercooler?
 

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I got 20 in mine frequently a few years ago.

Roof Rack - Removed;
Air Dam Modification Added (larger than stock;)
Shell T6 5w40 Oil Was Used;
Tires Were Stock & set to 80 Cold;
Cut a hole in the K&N Air Box for a cold air intake;
Used a Scangauge II for Current Fuel Economy Readouts (the calibration was about -70%.)
Never filled the tank all the way (judged by the needle. 15 Gallons seemed to last 300 miles easily.)
Differential Gearing: Unknown but I highly doubt it's a set of 4.10s. I was able to get to 70 MPH easily enough.

I'm sure with the right gearing and mods and conditions, a 6.5 could get 30 MPG HWY. I seemed to be able to get at least 25 HWY (never tested that from a tank to tank read out, the gauge just seemed to go that high) and forget that it's a 3 ton brick; it somehow just worked.

I will note that the few times I did fill the tank up all the way, the car felt like it was lugging and using more fuel (makes sense; holding an extra 20 gallons = about 139 extra pounds and more weight on the rear tires.) Then again, maybe it was just the kind of fuel I bought. No idea.
 

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Agree with Diesel pro about injectors. If they are stock OEM with over 200K miles. They should have been replaced miles ago. New injectors should help.

On truck in sig I get an honest 22 highway at 73mph 2200RPM. 16 mpg town . 15-16 highway mpg with Lance 8.5 foot SC Camper in back. All Checked with GPS. My tune is a combo of performance /economy. It is not a towing tune. Computer control Vacuum wastegate.
 

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Discussion Starter #10
Hey thx all!



Yeah my bad, my sig is correct just a typo on 3.10 gears. I put 3.73's in. Just put in used ring and pinion in rear assembly and swapped in used front diff axle carrier. I did verify and count teeth on rear gears.



Honestly not sure if I have to do anything else to correct for new gear ratios. I'll investigate reprogramming the VSSB. Also for different tire sizes. I bought it with the current tire size (non-stock).


The converter is locking up, yes...but tbh it shifts a little softer since I re-geared it. I was wondering if the converter was going. I did a rough test on a hill in drive and the truck still creeps slowly uphill at idle, so I'm hoping it's ok. I should get a scanner on it and see how lockup is working.



the intercooler was on it when I bought it and was a cheap aftermarket job that was kind of cobbled together. it was prone to popping clamps at joints and was in the way of a lot of stuff. I guess I wanted to bring it close to stock and see if I could get MPG up starting from there. I mean w the 6.5 it seemed like the intercooler wasn't gaining me a lot. Thoughts?



I'm tempted to agree w you DieselPro that injectors should be next step.


I'm a little worried that even w 3.73's it wont be enough to get up to the legendary 20MPG.


I also have a big 42 gal tank and fill all the way.



As far as driving habits, I commute about 20 mi to near Sun Prairie and mostly hwy. But yeah it is a little trafficky, so it's not great. On long trips to MN, it tops out at about 16MPG, and I'd just expect better cause I use cruise etc.


I apreciate the comments!
 

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Discussion Starter #11
Agree with Diesel pro about injectors. If they are stock OEM with over 200K miles. They should have been replaced miles ago. New injectors should help.

On truck in sig I get an honest 22 highway at 73mph 2200RPM. 16 mpg town . 15-16 highway mpg with Lance 8.5 foot SC Camper in back. All Checked with GPS. My tune is a combo of performance /economy. It is not a towing tune. Computer control Vacuum wastegate.

Agreed on injectors. I bought it with 190K and don't know when injectors were replaced. The last guy was a Natl Guard mechanic who worked on Humvees and says he went through the heads, so chances are they've been replaced at least 1x. But tbh his other work is a little off, he wasn't an ace w electrical and I've gone through and redone his mods, so not sure about brand or quality of work in head r&r



Do you all have tips on how to verify mileage w GPS? is there an app? or just phone map app? I'd like to do that.
 

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I agree with dieselpro and tommygunner about the injectors...

The fuel economy drops SIGNIFICANTLY as you pass 18/1900 rpm and beyond.

Begs the question... is it worth the loss of 10-20mpg to gain 10-20 miles within the span of an hour? Or should I just chill @ 60? (I leave earlier and chill at 60...)
 

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Agreed on injectors. I bought it with 190K and don't know when injectors were replaced. The last guy was a Natl Guard mechanic who worked on Humvees and says he went through the heads, so chances are they've been replaced at least 1x. But tbh his other work is a little off, he wasn't an ace w electrical and I've gone through and redone his mods, so not sure about brand or quality of work in head r&r



Do you all have tips on how to verify mileage w GPS? is there an app? or just phone map app? I'd like to do that.
If you use an Android device, try using "Speed Detector EVO".
It tracks speed and monitors fuel mileage and a little more.
 

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Discussion Starter #14
Great tips everyone. Thanks again. I guess I'll have to set the cruise at 60-65 and be content w that. lol


and injectors are on the way. :thumb:


good tip on the app thank you OKDually! not android but there's something out there I'm sure.
 

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Discussion Starter #15
looked into the VSSB. Great info here on the 90-91 thru 95 set-ups and how to reprogram. But from what I can tell, after 95, there's no VSSB.


Is anyone aware of any recalibration I might need to perform after having re-geared this truck? Between taller tires and 3.73 gears, just want to make sure I'm not missing something major affecting MPG


(aside from new injectors)
 

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The tyre and gearing changes are accounting for a ( approx) 13% variation in your odometer readouts...... instrument error and oem calibration could be accounting for some more.


At the very least you could divide your odo readouts by 7 and multiply by 8 for a much closer mileage/odo readout and mpg calculation.
 

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Does the flying brick (Suburban) have a front air dam installed? Does the truck run true? (No pulling to the side while driving) Are the batteries up to par? All fuel related questions.
 

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Discussion Starter #18
The tyre and gearing changes are accounting for a ( approx) 13% variation in your odometer readouts...... instrument error and oem calibration could be accounting for some more.


At the very least you could divide your odo readouts by 7 and multiply by 8 for a much closer mileage/odo readout and mpg calculation.

thx old6500td for the math!
 

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All 6.5 diesels have a VSSB, gassers got rid of them around 1995.
 

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VSSB adjustment could prove problematic, given (afaik) some physical repositioning/soldering of pins on the VSSB PCB may be required.....I'm happy to be corrected!

I have a vehicle with larger diameter tyres and speedo calibration issues and I installed a cheap ($50) GPS Speedo HUD. It does nothing for the odo, but works great for reading your speed down the road.

Once you know your combined error factor, you can easily calculate actual miles travelled etc as I previously mentioned.
 
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