2011 Tahoe bad towing mileage + premium gas = better mpg (temporarily)

johnparkercom · October 20, 2014

Hi, & thanks for reading. I have a 2011 Tahoe 5.3L (4x4 with 3.42 rear end) I bought used (30k miles currently) & use mostly for towing. I have towed the exact same trailer & weight for 15 years so have a basis for comparison.

Previously towed same rig with a 2004 Suburban & got ±11mpg on the highway. Hadn't before monitored mileage since getting the Tahoe (figuring it was similar to the Suburban) but intuitively it felt like it was progressively getting lower so I started monitoring. Turns out I was getting ±8mpg hwy when towing & 10mpg around town when NOT towing.

Took it to the Chevy dealer - no codes but they reprogrammed the ECU, they said something like "the alcohol % was reading high" (?) That reprogramming did little/nothing long-term. Decided yesterday to try Premium gas (why not) for a highway round-trip, towing, of about 500 miles. Fueled up from a 1/4 tank to full with 91 octane & proceeded to get 9.8MPG towing on the first 1/2 of the trip, It also seemed noticeably more powerful, with much less transmission hunting on hills. (PS; this was an exact trip I had taken recently getting ±8mpg towing) - However on the return trip mileage quickly dropped back to 8 - so 9.8mpg first 1/2 of the trip & power was lower (made noticeable by a lot more transmission hunting to maintain speed) & I returned to ±8mpg 2nd half of the trip. I filled up with 91-93 octane the 2 times I refueled. Net mileage for the whole 500 mile trip was 8.6mpg - despite 9.8mpg the 1st half of the trip.

Is it possible the computer, at first, ran the engine leaner on the higher octane then re-compensated back to a richer mixture - hampering mileage? I realize it's a difference of ±2mpg - but when that 2mpg is 8mpg vs 10mpg - that's a LOT. I've heard O2 sensors can be "lazy" & hamper mileage without throwing codes/CEL - & at this point I'm open to trying exploring anything. Looking for suggestions. THANKS !!! john

desrtrat · October 20, 2014

regear to 3.73's will help..i don.t understand why in a vehicle that size GM puts in 3.42 gears..

Stevens03 · October 20, 2014

If you have the 6-speed transmission, the 3.42's shouldn't be that bad for towing and you should be seeing better mileage than that. If you have the 4-speed, then there lies your issue. Still, as suggested 3.73's would be the better gear choice. In this case, a lower gear will help your towing and non-towing mileage. Also in a non-tuned vehicle (example Blackbear), using high octane fuel wouldn't change a thing. Air temps, wind, humidity, altitude, terrain all play a big part when towing and how it reflects on mileage

What is the dry weight of your trailer?

SouthernSilveradoGuy85 · October 20, 2014

regear to 3.73's will help..i don.t understand why in a vehicle that size GM puts in 3.42 gears..

3.42 isn't bad, a pretty good combo of daily driving and work ratio. The ratio I don't think GM should have used is what's in my '14, 3.08s. 3.23 is also pretty high for a full size truck, but still better than 3.08. I plan on regearing mine to 3.73 like my old 98.

Doug_Scott · October 21, 2014

Depending on the quality of your regular grade fuel it is possible to get better mileage from premium, if, the ECM can put more timing in before retarding for pinging. With crap regular fuel(by than I mean the actual fuel you got, not the entire regular fuel product) you may have been getting pinging when towing that the ECM would retard the timing to correct. That will give you crap mileage. If you put a high quality premium fuel in, maybe the ECM could run more timing, giving you more power, and better mileage.

I am a firm believer in not putting in higher octane fuel than the engine was designed to run on. It will not make your truck any faster(other than for the loss in weight due to money gone from wallet), unless other changes are made to take advantage of it.

I know from experience that running Sunoco 94 back in 2000 in my GTP got me 10% less mileage than I was getting from 93. The 94 had 10% methanol, and the 93 had no methanol.

The percentage of methanol in the fuel has an impact as well. You need to run more tanks of fuel to get proper mileage results. The test you just performed could very well have been impacted by you subconsciously changing your driving habits without even noticing. Keep in mind that we are now in the change fuel composition time of the year.

fondupot · October 20, 2014

2 mpg is not a lot. A variety of factors could net that Mpg. wind...temp...tire pressure...gas quality. etc.

johnparkercom · October 21, 2014

Thanks for all the replies - some responses to questions/comments;

TRAILER: 6x10' enclosed cargo trailer 3000# total weight.

Tahoe transmission: SIX speed

10mpg vs 8mpg is a 25% improvement. I'd like to have the 2mpg back. I tow this trailer 10,000-12,000 miles a year & that's over $1000 more gas used from that 2mpg loss. Seems worth trying to improve.

City driving with no trailer is about 10-11 mpg.

Zebrafive · October 20, 2014

Is your 5.3 flex fuel allowing E85 use? Maybe that is the high alcohol % they were referring to. The ECU has to "know" E85 is being used and change engine calibration.

Have you checked non towing mileage? How does it compare to the Suburban?

If the dealer can't improve the mileage, might be time to try a shop that can tune it.