I just tested my car again (it has 3800 miles). I got 21mpg on the highway cruising at 70mph. I reset the dic in town and got 14mpg. I will admit I am more agressive in town because I just love to hear the exhaust rumble. So that may explain some of it. I know my parent have a 1999 sedan deville and it gets 12 city and 18 highway but that car is a tank. I will be driving it more in the next couple of weeks. The engine in my work car (99 GTP) failed so I will have to drive the GXP for work until I get the other car fixed. I will be driving lots of highway miles.

I've got a 94 Eldorado and the northstar seems to be about the same specs as the gxp northstar. 270hp 300fp. I get 14mpg around town and 24 on the hw. My old 97 Aurora with the smaller northstar got about 25 local and 31 hw.

WOW, that seems considerably higher than my average! I get about 20mpg on my 4.0L daily driving, which is a 50/50 mix of city & highway. I'm always looking for ways to increase it, of course... did you have any engine mods on your Aurora? I haven't been on any long highway-only trips since I bought my '99, although I've heard stories of 30+ mpg in those cases.

That was on a very long trip, I usually got in the high 20'* on the hw. The car was stock other than a hole in the oil pan and the AC going out. I recommend against those two mods.

In long straight lines the 4.0L in my car does 33-36.9 MPG but city is another story :? I'm still having a hard time figuring out why GXP'* are getting such lousy mileage...or is it only the 2 or 3 repeat posters in this thread.

I think more studies need to happen.

With so many people (including myself) not getting the greatest mileage, has anyone done an actual "manual" test for mileage instead of of depending entirely on the DIC? Actually writing down the mileage after a fill-up and then seeing exactly how much goes in after the next fill-up. I'm going to try that myself and compare the DIC and see what happens. I'm not saying the DIC is wrong, I just want to verify it.

I do this on every tank of gas in every vehicle I own. I have a log book for every car. DIC'* are notorious for being wrong. My explorer was 3 to 5 MPG worse then the DIC everytime.

I bought mine with the hopes of improving the mileage I was getting form my 03 Silverado 1500. I am happy to report the 04 GXP get better mileage! I drive approximately 80 miles to and from work each day 70 of which are on the wild interstate roads of central Florida. My truck gets about 17mpg driving at 70mph, my GXP gets around 18mpg driving at 80mph. LOL! So I was successful at increasing my mpg by buying a new vehicle, however the GXP is so much fun to get on that it may well be a wash! And with the increase in my average speed I thank god for my RMR C450 Detector/scrambler!!! So in the end I get an additional mpg and I get to and from work 20 minutes faster! Big boys and their toys!!!

I've been doing some "scientific" experiments with my mileage and here is what I have discovered:

Highway driving at 65-70mph and my gas mileage is 22.2mpg

Highway driving at 80+mph and my gas mileage is 18.7mpg

City driving (stop and go) my mileage is 17.5mpg

It would seem that the 2004 Bonneville GXP meets predicted mileage based on the above driving conditions. Further experiments will include "aggressive teen road rage" and "stop and go racing house-wife" to see if I can further impact my findings.

Sorry for the ill attempt at humor... I think I am having a bad reaction to the turkey and stuffing! Hope everyone had a great Thanksgiving.

Something for anyone on the left coast to remember is we have gas that is weaker than what the rest of the nation gets so we can save our air. Any of my family that has driven out of state has seen an economy improvement on the first tank of non-california gas they ran.