My stock '93 SSEi has 140,000 miles. I've had it since 122,000 miles, and it has always yielded anywhere from 15 to 20 MPG on 91 octane, with mostly city driving. The average is closer to 16 MPG. It once got 26 MPG on a long trip, but that was a one-time exception. My wife drives this car and is very gentle on the gas pedal.

Once about 6 weeks ago it only got 12.6 MPG and it still ran fine and had no problem symptoms or error codes, so I replaced the O2 sensor with a new Bosch unit (I'm still looking to buy a couple of Delco O2 sensors). Since replacing the O2 sensor I've tanked-up twice - once it got 16.8 MPG and the second time it got 16.0 MPG.

Is this normal for a '93 supercharged engine with this mileage? It has relatively new plugs and wires, rebuilt GM SRTA transmission, new oil and filter, new catalytic converter, new tires and brakes, new TPS sensor, a low-mileage supercharger, and more...I've read some posts claiming 30 MPG or more, but realistically, what'* normal?

My other Bonneville, a '92 SSE */c, gets at least 18 or 19 MPG in mostly city driving, but it'* engine was rebuilt around 40,000 miles ago.

Thanks.

First of all you shouldn't have gone with the bosch. Some people around here have had a bad time with them. And it seems a lot of people have been getting bad gas mileage. My 93 SSE is getting anywhere from 15 to 17 miles a gallon. Which really sucks with the rising gas prices.

I've been getting 16 lately. its the cold.

yeah, dump the Bosch.
I put a Bosch in my dad'* 1989 Lesabre and it lasted all of 120 miles. My brother had a little better luck (he put his in the same night in his 1988 Olds 98 Touring Sedan) and it lasted him about 400 miles!!! He switched in a AC Delco unit and has gone 3000 miles incident free and gets even better mileage than when the Bosch was good.
Can't wait to get a job so I can afford to put a Delco o2 sensor in and put more than $5 of gas in at a time and having it last me about 25 miles (yeah, that'* right, I'm only getting about 8-10 mpg at 60 mph). :(

Do yourself a favor and clean the throttle body, too. There'* some pics around here somewhere of mine at 70k miles. Noticeable difference after cleaning. Mileage and power.

What do you use to clean your TB...just gas and a toothbrush???

No...hahah, its easier then that. You can purchase TB spray cleaner from Autozone (US) or NAPA.

When I had my oil changed last they did a fuel system conditioning. You should have seen the black crap that came out of the TB, the wire screen was nasty, but now its nice and clean and clear.

What I was also kinda asking is about the possible relationship with the mileage on the engine. Does any of you have a supercharged Bonnie with miles this high or higher that gets noticeably better gas mileage?

I didn't mention it, but I cleaned the throttle body not too long ago.

My only tentative plan is to install a GM o2 sensor.

I wonder if a sharp mechanic would be able to use a scan tool to see if anything looks unusual. Unfortunately, I don't own a scan tool or know anyone who does... :(

Yes...My '93 SSEi stills gets 20 city 30 hiway..running 93 octane ALWAYS and WITH my foot in the throttle body!!...

My 93 gets 29 highway with the cruise on 70. I have a few mods, some help, some don't. But I only have 80k miles.

I average 350 to almost 400 gallons per tank depending on where or how I drive. Long freeway trips are much better.