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Discussion Starter · #1 ·
I was putting 85 in my tank then started putting 87 in. My MPG dropped by 2-3 MPG. I thought it would have gone up not down.

Can anyone explain this?
 

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If you are in Colorado (high altitude), then you can probably get away with lower octane. When I used to travel there, the pump numbers were typically about 2 below what sea level gas was. For me, regular = 87, for you, probably 85. If that is still true, then your 87 would be like our mid grade. It works because of the lower density of air at 6k feet. Climb up Pike's Peak (14k feet?) where the air is thin, and the octane requirements drop even more. The intake sensors do adjust mixture to compensate, but I don't think they are sophisticated enough to do the full job.

Hard to say why the mileage dropped with higher octane. Could be that you also hit the seasonal transition in fuel where they adjust the volatility by blending in other crap to cut evaporative emissions? For me, I usually see a small boost in mileage around April, but your fuel might be different. I'd say stay with what works best for your engine given the conditions.
 

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87 is prolly 85 with 10% ethanol added. gasoline with ethanol has less energy per gallon and will result in lower mileage.

i'd go back to the 85 if the van seemed to like it okay.
 

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Discussion Starter · #6 ·
I switched it hoping to improve my 17 mpg. Now that I went back to 85 I'm getting that again but my aunt has a 99 with 190,000 and it get 24mpg (in NJ).

I wonder if my 6th spark plug being old is doing this. It doesn't seem to want to come out. there is something "over" the plug though.
 

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87 is prolly 85 with 10% ethanol added. gasoline with ethanol has less energy per gallon and will result in lower mileage.

i'd go back to the 85 if the van seemed to like it okay.
It is rare to find 100% gas anymore. Therefore, both the 87 and 85 gas probably have 10% ethanol.
 
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