Better MPG with higher octane = something is wrong with engine???

[LB72004]

sorry if this is in the wrong place

ok, so a friend at work drives a 2008 chevy silverado 1500 4.8L crewcab

right now he clams that it gets 13 MPG max. even on the highway. it was supposed to get 14 city and 19 on the highway

he has taken it in the dealer several times. they have't found anything. says nothing is wrong. even loaded the latest updates from GM. still the MPG is 13 city and highway. as far as i know everything is stock.

recently he filled up on mid grade gas 93 octane. the MPGs went from 13 up to 16.75. this is all based on fill ups and odometer. none of this dash stuff. this is the third time he tried this and the results are the same.

now i know the engine was designed to run on low octane so what the deal here? i remember reading somewhere that if the engine gets better MPGs on higher octane fuel then something is wrong with the engine, Knock sensor?, baro or temp?

anyone have any ideas what to look for?

i know some of you are going to tell me that the slower burning higher octane gas will always get you better MPGs do to being able to advance the timing. what i am after is that the truck only gets 13 everywhere and the mid grade brings it back to where it is supposed to be.

anyone have any ideas what to look for? trying to help him out

Thanks Jason

[minytrker]

A 4.8 crew cab on 87 oct will run pretty shitty on the stock tune. It's probably got so much knock retard that it just runs on the low oct table. There ain't nothing wrong with the truck. The smaller engine in that big of truck doesn't have much going for it.

[johnv]

That has been my experience,
The LS PCMs with duel timing tables will make more power and get better MPG on higher oct fuel.

[LB72004]

interesting, i only have a V1 so i haven't been able to see whats been going on. so does 13 MPG sound right? even on the highway?

i kept telling him he should have got the 6.0L. could have gotten the same or better millage with twice the HP

[ScarabEpic22]

No, 13mpg sounds bad, even on the highway. My TBSS with the 6.0L LS2 and AWD gets 15.5-16 all stock at 75mph on the highway through a 3000ft pass from sea level.

My buddy has an 00 Silvy with the 4.8 and he got better mileage (I got ~17.5 and he got ~19-20) than I did when we drove the same 500mi round trip, his is a stick though. Now he wants me to tune his truck too!

And my 02 TB gets better mileage with 91/92 (all we have here) than with 87, but Ive also tuned it pretty well. Runs a little lean during light cruise to try and squeeze a little more MPG and right on 12.5-12.6 at WOT. When it was bone stock I never tried running it on regular, normally Id say Id flash it back to stock and test but the stock tune is so crappy I cant bring myself to do that!

Have him split the cost with you to upgrade to a V2 and use one of the licenses to tune his truck!

[LB72004]

Have him split the cost with you to upgrade to a V2 and use one of the licenses to tune his truck!

i would love this option, unfortunately he doesn't have much money (its worse for him as he has to fill up often) so it may be all up to me to upgrade. right now i dont have much need as it works fine on my LB7 Duramax/5sp Allison

[Texas Edition]

If you get him to split the cost with you and you tune his truck, then you could increase his gas mileage which would mean he wouldnt be making as many trips to the gas station to fill up. It might cost him more up front, but it will pay for itself after a few months and then after it pays for itself, then he would be saving money. I'd talk to him about it. It would be worth it to him.

There could be other things that are causing him getting poor gas mileage. 13mpg city and highway is terrible for a 4.8l engine. My 5800+lb 2008 GMC Sierra Crew Cab 2wd 5.3l gets 20mg on the highway with no wind on 87 octane. Right now I am running mid grade octane to see if my computer can advance the timing enough to make my truck run better performance and gas mileage wise. Im more in for fuel economy than the non noticeable performance gains

[nonnieselman]

Yikes thats crazy!!
I got better than that towing a 5900# tahoe on a 1800# trailer with my turbo 6.0/80E and 3.42s in my 1500 !

Ive seen quite a few 4.8s that are just dogs, tune them and they get better mileage and a lil more peppier if you set the part throttle shifts a lil higher so its not bogging.

[joecar]

When fuel mileage goes down, a possible cause may be the front NBO2 sensors are worn out.

[LB72004]

When fuel mileage goes down, a possible cause may be the front NBO2 sensors are worn out.

will this throw a code? not having O2 sensors in my duramax my knowledge of them for this truck is limited.

he did get it checked out buy the dealer not long ago and they said nothing is wrong with it and everything is operating as it should. they also flashed it with the latest GM update and it now has i little more low end grunt but no change in the mileage.

anyone know if there is anything we can check like "tricking" the baro sensor so the ecm advances the timing to see if it is a ecm/timing thing? it would suck to have to replace the NBO2 sensors and still see no change. if i only had a V2...

thanks Jason