In my heavy 1998 Bonneville SSE I saw 34.3 mpg on the freeway last weekend. Typically with a 70% hwy/30% cty I see 29-31. It also depends on the average throttle % used.

My sister with a 1997 GP GT 4dr (bare bones GT with no leather, no HUD, standard 4 speakers, no digi climate control, no moonroof, etc.. -well she's lucky to see 27-28. The best she ever got in the 30k shes had it was 29 on her 850 mile trip to/from Milwaukee. Looks like her 3.29's are really killing her mpg's.
So far I've managed to gut her airbox and drop a K&N drop-in in it without her knowing, as well as a 180 *t-stat drilled 2x 1/16", TR55 plugs (gapped at stock .060"). I've also put a new pcv valve, fuel filter, plug wires (mid-grades from NAPA), and cleaned the TB, but I am going to see if I can get her to drive my SSE for a day while I sneak it in to Alex Exhaust and slap a new high flow Magnaflow/Carsound 2.5" cat in as well as delete the u-bend and also put a 3" DP in. Hopefully with the exhaust work she'll see another 2-3 mpg average.
As soon as I get my mani's ported (or S&S headers depending on what I here for reviews) and find how much my SSE benefits I might throw some ported mani's on and possibly do a light port job on the TB.
Synopsis:
1998 SSE 170,xxx miles (L36 + 3.05's) with 3" DP, Magnaflow/Carsound 2.5" high flow cat, 3" Magnaflow Wide Open, 180* T-stat drilled 4x 1/16", NGK TR55's gapped at .057", TPS mod, removed MAF screen, Belden Max wires, Thrasher CA box with K&N RE-0810 9" cone
25-26 City/34 HWY
1997 GP GT 145,xxx miles (L36 + 3.29's) with gutted stock airbox, K&N drop-in, NGK TR55's gapped at .060", mid-grade 7mm wires, 180* t-stat drilled 2x 1/16"
22-24 city/ 27-28 HWY
1993 Buick Park Avenue Ultra 92,xxx miles (Series I L67 + 2.97's) NGK TR55's gapped .060", removed MAF screen, fresh M62, ricer CAI (no box)

, 180* t-stat drilled 2x 1/16"
20-22 City/27-29 HWY