so from what i can tell...the average city for an ss is around 17. highway 24. thats to be expected with a stronger engine. obviously if you made some major changes to your engine it will change the mpg which is why i asked to share all the info that would make the difference.
I had an 83 and an 88 tbird with 302's in both of them, and my gas mileage was around the same. ranged between 18-20. Dealers make a big stink about better fuel mileage these days....but yet my old tbird with a way stronger engine got around the same gas mileage as my moms newer 4cyl suzuki. Its tempting to go after another one. I miss those cars.....

 

With the newer cars it is all in the Computers, I had wondered about changing out the gears to get the thing to have lower RPM's to make it get better mileage but that would not let the computer work properly, same what if you changed stuff in the exhaust system.
I know they can make them get good gas mileage I had an 1989 Olds 98 that got 35 mpg even when I drove it at 85 on the Interstate. Not much town driving but that wasn't bad either.
When you have an SS ( Super Sport) it is supposed to be a hot car and they build it that way. However forget the mileage.
I used to have a friend that liked the Impala I had and went and bought one just because of mine. I didn't have an SS, they bought an SS and then kinda was upset it didn't get very good mileage like mine did?
Then they put some new tires on the thing and had some issues with the tires. "asked me if I knew when you put new tires on a car you should get an alignment" I said yes I knew that!
So then it was my fault that they didn't get that done. They should consult the tire store, they do alignments. Go figure? I only rode in this car once and when I did I could hear the tires making noise.
You hear tire noise on a smooth surface you should get the balance checked out, tire pressure, alignment also. As expensive as the stuff is you can't always wait.
Anyway keep a fresh air filter, use a synthetic oil , clean injectors often with fuel tank injector cleaner Lucas Oil will increase your mileage a bit. Also some of the additives that are available will help some to reduce engine friction. Could possibly do a bit better for the car.
Driving style is a thing to look at also. So if you are at the light and are the 1st one off when it changes, and race to the next one and are on the brakes when you get there. You might not get very good mileage. I try not to be the first on in the intersection after the light changes. I have seen a few of the folks get hit doing that.

 

If I set cruise at 55mph going down the highway, I can get into the mid-upper 30's. Running through town, I get 20-25.

If I drive regularly(heavy footed), I get more like 18-23.

 

Well, a few things in check for min is that I have bad tires, plus I haven't done a tune up yet since I got her. When I first got her, before all the burn outs, my city used to be somewhere around 18-19 and that's before I got a tune. I'm hp tuned with way better shift points and higher Rpms. I got tuned by joe here in St. Louis who tunes vipers and corvettes and other exotic cars. I think he's the best viper tuner around or atleast I've heard...but yeah, after I get a new set of much thicker tires and tune up. I should be back around 18-19 maybe more, since I don't have dod anymore

 

22city/30 hwy usually

 

Here's a surprise, with no 3rd gear Ol YellaJAC gets a little over 20 on the back roads and between 26 and 28 on the highway. I want the 31 mpg that the sticker and consumer reports said it got right off the factory floor. But I average about 23 mpg. I drive 30 miles a day for work. That's about 65% back roads and 35% high way on my 3.8 n/a.

 

That's actually not a surprise. Speed and RPM have more to do with that than what "gear" you're in.

I don't have 4th on my Cutlass Supreme and I get better mileage driving it constantly at 45mph than driving it on the interstate at 60. So my RPM at 45 is 2000, but at 60 its 3000. I get 17 mixed because of that, and I feel it driving interstate only... BUT if I drive around local roads at 45 I'm averaging around 18 or 19... maybe even 20. I know it's not much but I do notice I conserve more gas this way than driving 60mph from my house to a place 20 miles away because the needle drops as soon as I get there.

 

Just averaged 19 MPG city in my beast

 

I get mid 20's consistently with 85% HWY. Generally I'm 26.3ish MPG, occasionally it will surprise me with a 28 and occasionally it will dip to 24.5 or so. But I go tank after tank getting 26.X.

If I do an all HWY road trip with average speeds of 80-90, I turn in about 29-29.5 MPG. I've had it well about 30MPG before, but I have to keep it under 80MPH to get in the 30's.

 

I get 27-29 mpg, mixed rural and town daily commute. I got 30 mpg once but can't keep my foot out of it.