Tahoe poor mileage - help please

[Boost]

A co-worker of mine who is a Jaguar tech has 22 inch aftermarket wheels on his otherwise stock new style Tahoe. Says the speedo is mostly correct, but even manually calculated only gets around 9 mpg and about 250 miles for a tank of gas. No codes or apparent problems, and the dealer says normal operation. I will check on it today but shouldn't it get at least in the low teens and over 300 miles for a 26 gallon tank? And are there any known issues that would severely affect mileage? Lastly, what can I check and correct in the tune to make a good difference. There is potential to make yet another EFILive / GM fan here :) Of course I need to fix the tire size and obvious little things but just need as much info as possible. Thanks guys, and God bless you in this new year!!!
Promising so far, already 3-4 tunes lined up for little Torque South! :anitoof:

[Taz]

If you get a chance, please post the tunes - both the ECM and TCM. There are too many variables to make any kind of meaningful comment without reviewing the tunes.


Regards,
Taz

[Boost]

Here are the stock E38 ECM and T43 TCM tunes. There were no history or pending codes in ECM, and the only modification is 305/40/22 tires. It has 3.08 final drive. I noticed in Speedo Calc. that there is around a 3-4 % difference because of this, but I also suggested to him to fuel from a quality gas station in the monrning (no cheating meters, more dense fuel) and pay little attention to the Range and Avg. Economy readings (estimated and possibly inaccurate).

None the less, he states that he gets exceptionally poor economy and just over 200 miles from a tank of fuel. Since the tunes are bone stock, I know I can tweak several things here and there in both controllers to help a little, and of course correct for the tires. He agreed so I need suggestions and known issues / good mods on these. I know they wouldn't sell as well as they do if they didn't get MPG in at least the low teens around town. Here are the files, thanks!!!

[Boost]

Also, a side note: I was able to pull what appears the be a good read from the E38 with the key OFF (!) While it was unintentional and I don't plan on making a habit out of it, was it because the modules were still awake? Interesting.

Different topic: Could at least some of the issue be due to the 87 octane he has been using? Theoratically, if the timing is on the retarded side long term, does that not cause a less efficient combustion and also less momentum to get the massive truck moving all the time? He tried a tank of premium but no short improvement, maybe the Scalar is lazy? More importantly, on the E38 what is the best PID to Log so I can see what Spark and Octane Scalar are doing to evaluate this idea? Thanks a bunch!!!

[VEGASROBBI]

I compared your 2010 tune to one of my 2009 5.3/6l80 tunes. I was surprised how much difference there was in the fuel and timing tables. In your tune the virtual fuel table is much fatter(richer) in most cells, both normal and AFM. Also, your timing table is far more advanced, 8-10 degrees in some cells. That surprised me because I find adding just a couple degrees to the stock 5.3 causes knock(false or real). DFCO is similar though. What are the LTFT's at?

:unsure:
I just realized your Tahoe has a camshaft phaser, this may explain the fuel and timing differences. I also noted your compression is at 9.83 vs 10.5 of the earlier 5.3's, seems backwards. Some manufacturers are running over 11:1 with VVT (Land Rover), but they require premium fuel. Could be GM feels when the cam is advanced cylinder pressures were too high to run regular fuel at 10.5.

[Boost]

Thanks for looking, great info! So this '10 has a camshaft phaser whereas the '09 did not? Then I guess your '09 stock tune would not be compatible to see if mileage improves? I will log the trims and post. Could anyone chime in and verify these differences between '09 - '10 and possible relation to fuel economy? Could they be attempting to experiment with further reducing emissions? This guy usually runs regular fuel. I have a gut feeling that if he ran premium all the time there may be some savings overall but switching one tank didn't give sufficient time for the tune to adjust. Can I speed up / de-sensitize the scalar without hurting it when the cam is advanced? I appreciate any suggestions and info! :)