RPD's verses MPG; 99 Sierra

[caktus]

Haddie Yall's! First time posting at AF. I'll try to stay within the Forum guidlines. If I happen to stray it's just my ignorance. <img>

I have a 99 Siearra 6cyl, AT. The eighth figure in the VIN is W (hope I got that right.) It's at 150,000 mi, kept up as far as maintneance and runs good.

I am confused regarding relationship between RPM's, traveling speed and MPG.

For nearly 10 years The vehical has been getting 20MPG. I figure this is probably about right on as I always top-off with the same "cheap" 87 oct gas, from the same pump, at the same gas station. Also I usually do not use the Tow/Haul mode, and weightload and distribution are generally about the same. Rarely do I stray from these habits.

At Operating temp (about 190f) at 55MPH it does 2000RPMs, 70MPR 2200RPMs.

Leately I have been using a big brand name 87 oct gas and otherwise following the same fueling habits. With this expensive gas at 55 it does 1800RPMs, and at 70 2000RPMs.

In both cases at 45 it turns 1100RPMs.

I'm not an auto techie, but I figured the only thing that should change is the amount of gas it takes to create the same RPM's. Assuming this is correct, what gives?<img> As mentioned I'm no auto tech, so please keep it simple for me.

Thanks.

Charlie

[maxwedge]

Welcome to AF. Impossible for fuel quality to change engine rpms, which is a function of tire size, speed and gearing.

[caktus]

Originally Posted by maxwedge

Welcome to AF. Impossible for fuel quality to change engine rpms, which is a function of tire size, speed and gearing.

Hi Max. Thank you for responding. With correct tire pressure and driving habits unchanged would the symptoms I described mean there may have been a mechanical problem prior to the fuel brand change? About two fill-ups prior to changing fuel brands I had the GMC dealer change the fuel filter on schedule and the filter did contain some filth. To me the filth looked like very small, perhapd even microsopic peices of rubber, but no visible rust.



Thanks.

Charlie

[maxwedge]

Normal, the filter was doing it's job. What actually is your question? Is the mileage radically different or the issue with the percieved rpm change?

[caktus]

Originally Posted by maxwedge

Normal, the filter was doing it's job. What actually is your question? Is the mileage radically different or the issue with the perceived rpm change?

Just curious about the lower RPMs appearantly resulting in higher MPG following change from GOGAS to Shell brand gasoline, i.e. 10% fewer RPMs resulting in approx 10% higher MPG. Sounds impossible to me too. Perhaps my driving habits have changed and I just haven't realized it.

As far as mentioning the fuel filter, just trying to provide any information that I thought may be pertinent.

Charlie