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6.5 fuel economy?

7K views 54 replies 21 participants last post by  crankme69 
#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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#5 ·
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.
 
#6 ·
(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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#8 ·
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.
 
#24 ·
I got 20 in mine frequently a few years ago.

Roof Rack - Removed;
Air Dam Modification Added (larger than stock;)
...

do you have a source for the air dam you like?
 
#9 ·
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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#11 ·
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.
 
#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!
 
#12 · (Edited)
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...)
 
#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.
 
#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)
 
#16 ·
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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#18 ·
thx old6500td for the math!
 
#22 ·
:gr_grin::gr_grin::gr_grin:
Yes this brick runs true, i invested in a lifetime alignment and keep steering parts new. has good batteries. No front air dam, but good suggestion.
And yeah the VSSB isnt a simple adjustment from what i can see, but sounds possible. Is that that only way to calibrate drivetrain to new gearing?
And great tip on the GPS HUD. Seems like a great tool.
 
#19 ·
All 6.5 diesels have a VSSB, gassers got rid of them around 1995.
 
#20 ·
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.
 
#21 ·
My K1500 Burb with 3.42 gets around 18 combined mpg...

When my daughter drives it, she gets around 22... I drive too fast.
 
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#25 ·
Same gearing in my burb, driving it nice I can get 22 easy. My all time best was 26MPG. Driving her a bit harder 16-18.

Lately I've been driving her nice, easier on the wallet.
 
#23 ·
I was curious how much difference there is between a Suburban and pickup. EPA doesn't have the diesel listed but just comparing gas versions a 97 Burb is 12-17 combined 14. Same pickup 12-18 combined 14 so pretty close.
 
#26 · (Edited)
Depends on the specific models (reg cab short bed, ext cab, crew cab, short bed, long bed)

Either way theres about a 1000-2000lb difference between the two (flying brick - I like that) burb and p/u

The civilian k5 / 2dr blazer / 4dr Tahoe sits in the middle... heaviest is the burb, lightest is the p/u, blazer sits in between.

:thumb:

If anyone is near a cat scale.. grab the weights WITH a full tank. Between the burb and different model trucks. Cat scale ticket should be about 12 bucks... but legally it must be within 5 lbs.. so the weight is accurate. I'm kinda curious myself...
 
#27 ·
My 89 K5 350 Blazer weighs about the same than the truck in sig. With the top off it weighed in around 5600. Top adds around 250#
My truck with me in it, wet, and the usual weight I carry, weighs 6200.
 
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#29 ·
My 92 Reg cab long box 2500 4x4 weighs in with the junk I carry all the time just shy of 6000lb. With the 4:10 gears, I get a combined country/small city driving of 16.5 to 17mpg. In the winter it drops off a mile or two.
 
#30 ·
Got a speedo/odometer app. Based on about 15 miles of driving today, my instrument cluster - both speedo and odometer - read slower and less miles than GPS measures. Speedo avgs about 5-7MPH slower than GPS. Odometer needs a correction of 1.13X to match GPS.


So over a full tank I'm guessing this will bring my avg MPG up to over 15, as opposed to the 13 I'm tracking now.


I also have injectors on the way, so maybe I can get up to the fabled 18MPG! thx for all the input everyone. I was surprise by how off the odometer is compared to GPS.
 
#31 ·
OT: I'm curious what is going on w my instrument cluster. Do they just get tired? I've checked and cleaned the ground points...but my voltmeter also reads low...wondering if i need to replace it or just deal w it...
 
#32 ·
Is the entire cluster acting up or just a gauge?
There are some links to gauge fixes in the FAQ
 
#35 ·
Hey my apologies everyone, I didn't see the FAQ ***The Official MPG Thread*** which is pretty good!


I do appreciate all the patient answers though, esp considering it's been covered before!
 
#36 · (Edited)
:hehe:

Theres a bazillion posts on pmd relocation, bad this and that... an endless rabbit hole of the same ole song and dance. No need for apologies.

Besides... what fun would that be if we all stuck to the same old boring post with 500 friggin pages... exempting a few... like the "what have you done to your (variable) today?" Or ? I think maybe one or two more....the funny CL ads and rides we all want... :thumb:
 
#37 ·
UPDATES:

new injectors didn't really bump it up much. with 1.13X conversion on odo readings, and the handful of upgrades, I'm now at 15MPG city/17MPG hwy.


I guess for a K2500, that's prob the best I can hope for?


I put in Pensacola Diesel new injectors, so maybe I could have gone Bosch or better and gotten better performance?
 
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