I was just wondering what the norm was as far as miles per gallon. I get pretty good mileage on the highway (20-25) but horrible mileage in the city (10-15). Another question I had was what kind of gas do you use? I know the manual says over 89 grade but does this really make a difference. Thanks.

i get about the same mpg as you. i use 87 and have no problems.

I always use the highest grade I can get. It runs so much more smoothly and slightly more responsive. I get between 26 and 28 on the highway and between 18 and 21 in the city. It runs like crap whenever I used anything under 89 and my check engine light will come on after a half tank or so. Maybe that's why you get poor millage. You're engine is all gummed up. The cadillac engines are designed to run with higher octanes which burn cleaner.

On a continuous trip, I can get almost 30 mpg, 28-29 is typical. :) However, around town, I get about half of that.:eek: I always used premium. I do think you get better mileage because when you use a lower octane, the car's controller has to retard the timing to avoid detonation and damage to the engine.

I guess using high octane gas does affect the mileage. I am probably going to start using the good stuff and see how it works. I also heard if you add pure acetone to your gas it's supposed to boost your gas mileage a lot. Has anyone heard of this and know if it works?

When i use high octane i noticed about a 12-18mpg increase on my tank.

yea, but that stuff is like in the ball park of seven bucks isn't it? You're only getting maybe an extra 18 miles you said. That's less than a gallon of gas.

Just to add: when i use the 87 i don't drive it that hard, but i do notice a little increase in HP and do run it harder. When i get my ScanGuage i'll post some results for all three and in hard and grandma modes.

If you look at the math of mpg costs and saving, there are a number of factors to consider. For instance, if you can increase your mpg by 5mpg from 15 to 20, the benefiet is far greater than if you increase from 30 to 35. The Catera falls in the middle of this curve - around 20mpg is past where you get a large $$$ benefiet from smaller gains in mpg; but still before the curve greatly flattens out.

The other factor that matters when you talk about switching to a higher grade of gas to increase mpg is the price of gas. The more expensive the gas, the greater the $$$ benefiet of increasing your mpg. This is because, first, the cost difference between grades becomes less significant when the price per gallon increases. And second, you are saving more $$$ for every gallon of fuel you save. So increased price per gallon shifts the curve to the right.

I've checked on my 98 Cat, and I get an increase of about 2-3 mpg - I do a mix of city/highway driving. I get about 22 or 23 mpg This increase is definitely worth the extra $0.20 per gallon. And I've also appreciate the increased performance.

This chart assumes $2 per gallon - so at $2.80 or $3.00 per gallon like it is now, the curve would shift to the right.

Probably way more info than you wanted - and you can guess which character I fall closer to - "MAC" or "PC" guy.
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Yeah I figured if I only have to spend $0.20 per gallon to have it run better, its worth it. After switching to the 93 grade gas I noticed my engine runs much more smoothly. I think I'm going to get my fuel filter changed soon and see if that does anything to the gas mileage.