Ihave a 2004 Mh with a 8.1 chevy engine. My Question is the manual says 87 octane. I live in Montana@ 5000 ft and they have 85.5 octane as regular gas. Since my manual calls for 87 as a minimum should I use a higher grade or is the 85.5 OK at this altitude? I know that the computer will retard the ignition to accommodate the lower gas but would I get better performance and mileage if I use the 88.5 gas? Any Info Would be appreciated. MB.

I have a 2006 Chevy 6.0 gasser and the manual specifically encourages the use of higher grade gas for heavy loads, especially if the engine begins to knock. On my last trip to the White Mountains of Arizona I encountered many long, steep grades during which I was very glad I used the 91 octane formula. The engin ran comfortably, pulled our 25foot 5'er well and I actually got nearly 12mpg. (AMAZING) I was very pleased with its performance. About town I run 87 octane and get about 14mpg. On the open road, no load, I get just less than 16. For a 2500HD I think that is pretty good.

I would try the higher octane, for at least on tank under load, but watch your oil pressure, temp, and transmission temp carefully. If you get better performance without adverse affects you've learned something. If not, you've also learned something.

Good luck.

PS: As a minimum I'd likely run the 88 octane, rather than the 85 in any case. Running too low an octane is worse the over running it a bit.


Senior Member

I think there is a relationship between height and octane, where the higher you are, the less octane you need for the same usage. That is why they sell 85.5 instead of 87 where you are.

I'd check with the manufacturer. Trying a tank full of the 88.5 is also a good idea, just to see what it does for you.
Re: Gasoline

I agree. For the ten cents difference I don't think it is worth listening to engine knock. I use the 89 octane in mine bacause I spend a lot of time driving through the mountains of WV, V, & NC. I know they aren't as high as the Rockies but I am also pulling a pick-up and have two women on board that seem to think they need to empty the closet when packing for a trip. I have to admit though, the big GM engine pulls my 37ft motorhome and attachments very well. I checked the mileage once and it was over 11 mpg. Didn't want to check it again when gas went over $3 a gallon. I heard this loud sucking sound coming out of my bank account. LOL