I average 18-19 50/50 HWY/CITY.

I don't you can get 21+ if your GS4 was your DD to work everyday. Long trips shouldn't be accounted for because it's not average.

 

Thus makes me cry, I get 13mpg when I drive like I usually do, step on it and cruise 10 over speed limit, 17mpg driving like a grandma, I got 223k On chassis, rebuilt motor at 180k. I use standard castrol gtx 10-30 with a quart if Lucas oil stabilizer and tires should be around 36-40 psi. It's like there is no hope, this car is just that much of a gas hog

 

I'm lucky to get 16 out of mine.
gte swapped 99 gs

 

I just recently took a trip that was about 80-90% highway, and I got 23.5mpg average with additional weight of people and luggage. I usually get 17-18mpg in average driving, which is probably 50/50 city/highway. I do have a little bit of a lead foot with this thing though and I am usually toting people around

Stock gs430 on 245/45-17 tires.