Alright I've had my 03 for about 8mo and I'm wondering what is considered good city/highway MPG for these things? Right now with about 50/50 city highway driving I am averaging 19-20mpg...seems low to me, or am I just nuts?

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2003 Pontiac Bonneville SSEI
3.8L V6 SC, 180*T (Drilled),
Custom PCM w/Perf Shift,
SLP Ceramic Coated Headers,
Thrasher Cold Air Intake,
GXP Tails, Interior Door handles, Trim pieces etc.
40th Aniv Grand Prix GTP Wheels (Very Shiny & Very Annoying )

Depending on what level of stop and go you consider city, I've seen as low as 15-16 in Chicago traffic jams. I think a more moderate city average is about 17-19. I usually get about 20-24 in my mixed driving. Straight highway is high 20s/low 30s depending on speed. Since you have an SSEi your fuel economy should be just a tad lower so I think what you're seeing isn't bad at all.

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-Car Guy Carl
"Penelope" - 2003 SLE - 247k Miles
"Sydney" - 2000 Honda S2000 - 98k miles

So I'm just going to post, which may or may not apply to you, in hopes that it helps you.

A couple months ago my fuel pressure regulator went bad. Thing is, it was bad for a long time and I didnt know it.

When it got so bad that the car would no longer start I replaced it.

I swear my mileage went up 5 to 7 mpg.

I agree with nos4blood70's assessment. Your mpgs look good.

When the cars were new they supposedly achieved 16 mpg city and 25 mpg highway.
Everyone knows the fuel ratings were unachievable for most, since no one drives in ideal conditions without anyone or anything in the car besides the driver....except those testing mpg for automakers.
The SSEi was not designed as an economy car.

For optimal mpg, your tires must be in good condition, properly inflated, the suspension tight and all 4 wheels aligned, plus the plugs, wires and filters in good shape, belts, cooling system and the sensors all working properly--as well as the fuel system delivering a fine fuel mist at the right pressure--and the engine to have good compression.

Of course, checking all the maintenance items and looking at live data on a scan tool to be sure there are no codes and the fuel trims are good wouldn't be a bad idea.
Here's the list. Pay attention to the brands that are listed, particularly those shown as best avoided
care-and-feeding/topic40156.html

sorry for the delayed reply, been busy with work and working on the Landy...this makes me feel better then, I owned a w-body car a long time ago that always got close to 29-31 on the highway so I was thinking my numbers were low, but I forget there is a pretty good weight difference between H-body and W-body cars. Thank you for all the input, and for the link MKMike...I will check it out later...now back to the shop to do more work on the Landy.

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2003 Pontiac Bonneville SSEI
3.8L V6 SC, 180*T (Drilled),
Custom PCM w/Perf Shift,
SLP Ceramic Coated Headers,
Thrasher Cold Air Intake,
GXP Tails, Interior Door handles, Trim pieces etc.
40th Aniv Grand Prix GTP Wheels (Very Shiny & Very Annoying )

I DD a 2003 Bonneville SSEi and I'm averaging 25 mpg overall for the last few months. My driving is about 2/3rd highway and 1/3rd city. I have gotten low 20s here and there and haven;t quite managed to hit 30 mpg yet.

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2003 Pontiac Bonneville SSEi (L67) - Galaxy Silver Metallic
1996 Chevrolet Camaro (L36) - Sebring Silver

2001 SSei, cold air intake, 1.9 rockers, 3.2 pulley, Speed Daddy headers, hi-flow cat, GXP suspension, and I average about 24mpg with probably more highway than in-town driving. Only problem is driving slow enough. Unless I set cruise control, I can't tell the difference between 70 (usual county highway speed) and 95 if it's daylight so I can't see the HUD, and the highway patrol is really unsympathetic.