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Analyzing Engine Performance

LML: 
1K views 9 replies 5 participants last post by  rmoore0852 
#1 ·
So I think.. I've decided that my truck needs a trip to the dealership. At first I thought the combination of winter fuel, non completed regens, and slightly heavier tires was the cause but something else must be a cause of a large drop in fuel mileage.

Mileage is down all around city and highway.. It seems like it struggles to increase in average mpg but drops significant average mpg with very little stop and go

I just made a 766 mile round trip that i've now driven 4 times. First 3 times I've driven it my average fuel mileage was 21/22 mpg. I drive it the same way every time and ambient air temps are always ~high 30's low 40's

I just drove it this past weekend and I'm sitting on 16.5 mpg.. Complete crap.. Everyone says to do hand calculating and I do.. I find it hard to believe that the extra 32lbs in rubber would kill 5 mpg. Fuel and air filter are brand new. Is it realistic to think the dealer will do anything? There are no check engine lights. I hate thinking that I should need to program this thing
 
#2 ·
Im betting that in addition to heavier tires, that they are bigger than stock. Going from a stock size highway tire to an oversized off-road tread tire will easily cause that kind of mpg change. It's not so much the weight increase as much as it is the changed final drive ratio caused by larger tires. You didn't say what tires you changed to so I am just assuming larger and more aggressive tread.
 
#3 ·
It is a larger, but not a significant difference. I went from 265/70r18 to 275/70r18.. 1/2" difference.. over the coarse of this road trip its a total of 12.2 miles difference of actual travel vs. truck recorded miles
 
#4 ·
different tire rollng resistance coupled with winter fuel and/or a difference in the wind on the road could easily result in the difference your seeing.
 
#5 · (Edited)
i'll re-iterate...

every time i've made this trip, its been at the same time of year when winter diesel is only available and i'll average 21/22 mpg

the heavier tires are teh only different factor this go around. 32 extra.lbs shouldn't kill off 5 mpgs which is why i'm wondering if its possible the engine is underperforming for some other reason that isn't triggering a CEL
 
#7 ·
I lost mileage like that... But I think it's a bad wheel bearing and/or sticking front brakes... My hubs are warm to the touch, even at -20...

GM never greases caliper slides enough.
 
#8 ·
I wish that was the problem.. I pulled the rotors a week ago and turned them(they've been squealinq for the last 22K miles and the dealer did nothing). Lubed the slide pins while i was at it
 
#10 ·
The extra weight isn't an issue, it's the much worse rolling resistance of the more aggressive tread, as ktmrfs stated. My son has a new F-150. Factory tires gave him 20-22 mpg on the highway. The only change he made was going to a slightly larger, but more aggressive tread set of tires. He immediately dropped down to 15-17 mpg on the highway and it has remained there ever since.

Got any friends with the same size wheels as you that still have factory type tires on theirs? Maybe ask them to switch wheels for a week? You would throw a TPMS light but that would at least answer your questions.
 
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