I seem to get no better than 23 mpg (mostly highway driving at about 70+
but also a lot of stop and go). Using 87 octane fuel. Does this sound about right for an '04 DeVille with the 275 HP Northstar?

Thanks in advance for any improvement help

My 03 Deville gets 23 - 27.9 running 75 - 80 down the highway with a/c on. Around town I average 18 - 20 Of course, if you're pulling up a lot of hills and such, your mileage will be worse.

I get 28 with my wifes

Never more than 12, 13 in town, 22 highway. 300 HP DTS.

28-31 HW, and 15-17 city.

That's with 34,000 miles, K&N AF and Bosche platinium spark plugs. (regular plugs) Everything else is factory still.

Guess I can't complain.

I seem to get no better than 23 mpg (mostly highway driving at about 70+
but also a lot of stop and go). Using 87 octane fuel. Does this sound about right for an '04 DeVille with the 275 HP Northstar?

Thanks in advance for any improvement help
Sounds about right. Just about the same as I get with the same type of driving.