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With my 2000 SLE, around town I usually get 18.5ish mpg running 87 octane 10% ethanol blend gas. On the interstate I average 27 or so if it's not windy. This is generally driving pretty reasonably, no drag car launches from stop lights or anything.

My mom has a 2000 Bonneville SE, she gets over 20 in town and in the 33 to 35 range regularly on the interstate running the same 10% ethanol blend gas. What could be causing that much of a difference in two almost identical cars? They are both in the 90K mile range and in very good shape mechanically.

For one, her SE probably has the 2.87 gears, and yours for sure has the 3.05 gears. That makes a difference. Your driving habits compared to hers are probably different. To know for sure, trade cars for a week, and compare mileage. There are other factors, where you drive, etc. It all adds up. My daily drive involves 2-lane highways, stopping, slowing through towns, which all conspire to about 22-23 mpg max (plus a heavy right foot, and love of boost) . If I had the same distance drive one one road with no stops or slowing, I could probably get 25. This is in an SSEi, but you get what I mean. Where you drive is as important as how you drive as far as it's impact on your fuel economy.

Yeah that is crazy that cars in that good of shape could be so different.
I know its not really your question but:
I have a 1997 SE with 275,00 miles and I regularly get 20-22 in the city and 30+ highway
I find it amazing that a car with that many miles can run so good and get such good gas mileage

My overall average is ~ 27 MPG combined. That includes stop and go stuff running the kids, errands, and driving to work. I commute 82 miles a day, primarily interstate, over the Allegheny Mountains with the cruise set at 75. On long trips with flatter terrain, I can get ~ 31-33 MPG. Starts and heavy acceleration are going to kill your MPGs. Extraneous stuff in your trunk won't help either. Reasonable driving habits and a well maintained vehicle will yield your best mileage.

Mark

My car varies a lot between city and hwy I get about 18-20 city and anywhere between 27-34 hwy with about 24 combined I'm not sure what makes my hwy vary so much but I am gonna find out

Im usually a caring driver, but sometimes i can just burst into race mode. My City AVG is 17.5mpg and highway is usually about 29mpg

The Average on my 03 SLE in town is 21... On the road 28-30 is what I get...lol

City average for me is about 18
Highway average is around 26-28.

My combined at the moment is 22.6 mpg.

For me city is around 18 and highway is around 27.

"Town" mileage is impossible to compare, as we all drive differently, in different conditions. But for highway mileage you should still be able to pull 30mpg @ 70
If i keep to 55-60, I can pull in 35mpg.

Same here Ryan... I have pulled 36 Mpg crusing at 60 Mph... I do 28 at roughly 80 Mph.. 30 at 70 Mph..

I, in the SSEI, get 16-20 in town, sometimes delivering pizza. 25-35 Hwy.

My wife gets 19-23 in town and 28-35 hwy in the SLE.

In my 2000 Bonneville SSEi I use the 91 Octane premium fuel and get around 19 in town and 28 on the freeway. Pretty good for a supercharged V6.

Has one had ANY maintanance done that the other hasnt? Plugs, wires, fuel filter? And Im sure your mom has very different driving habits from you.

My computer says my average is 21. Even on the freeway, though it doesnt stay over 30 for long. My commute is only 15 miles so on cold days its hardly up to temp by the time I het home.

Mine was high teens for city and mid 20's for highway, but I had a bad LIM and it seems to have done other internal damage to the engine as it spun a bearing. So will get some new numbers in the next couple of months after I get it back on the road with the L32.

Jon

in my GXP i get around 330 km out of a full tank in the city..

over 550 on the highway..

I usually get 19-21 city and as high as 32 mpg on the highway