So I'm taking a roadtrip (actually in the middle of it) from Detroit,MI to Big Sky, MT as I have an internship this summer. So I was getting 19mpg from MI to MN but once we got into SD and the 75mph freeways my milage dropped to about 15 seems pretty huge the only thing different besides speed was the fuel was 89 octane instead of 87 (89 was cheaper in MN and SD). Also when I fueled up in Wyoming there regular unleaded is 85oct but i put in 87 as per manual recommendations, has anyone used 85 octane regualrly or should i just keep on using 87.

Thanks guys

 

I have never even seen 85 octane... hrmm....

Hey I have a question for you off topic, do you have a Facebook account? If so... what's the name I would search for you under?

 

From what I understand, the ECU can compensate for octane ratings of 87+. Below that you can get uncontrolled detonation which can damage the engine. As for worse mileage, you probably filled up with 10% ethanol. I am from the great plains and they have the stuff everywhere. It is 89 octane and almost all of the time is cheaper than regular unleaded. The problem with ethanol is that mileage is 10-15% lower even though the octane is higher. Don't know if that helps, but have a great trip!

 

The faster these truck go they eat up much more gas. At 70 I easly see 20 mpg at 75/80 is drops to about 17. It's amazing what 5 mph can do to something with horrable arodynamics.

 

I figured it was the speed as it happened in South Dakota where speed jumped to 75mph and I know for a fact it was not the 10% ethanol as we have that in MI and that is all she ever drinks!

But what do you guys think about the lower grade in Montana its 85.5 octane I've heard this is due to the elevation and that fuel detonates easier in higher altitudes, if this is true that would be great as the "regular" 85.5 is 10 cents cheaper than the "plus" 87.

I guess I could I stop in at the local dodge dealer and ask them!!!

 

I have noticed a 10% increase in fuel mileage in both my vehicles since the change to E10 fuel !

 

If I remember correctly, octane ratings change with elevation somehow, as in 87 becoming 85 at a higher altitude, or something like that. I would have to do a little more research to be sure.