Well, without controlling for weather or traffic, I have found out. Made the drive from south Georgia to South Florida twice in two weeks. When setting the cruise at 80 mph my calculated gas mileage was 23.5 mpg. The computer reads 24.5, but is always 0.8 mpg optimistic. The second time I set the cruise for 72 mph, and the car measured 28 mpg on the highway, and my calculated number was 27.2.
So the numbers are real, as advertised by Hyundai, but I am disappointed that my 1998 Lexus GS 400 with a V8 engine would average 24.5 mpg at 80 mph on flat land.
I wonder if the increased HP and Torque actually helps mileage at the higher speeds, because the engine is not working as hard.
I am always going to set my cruise at 10 mph over the speed limit. I never get a ticket.
When this car is worn out, though, I will take into consideration this disappointment. For a V6 the mileage is not what I expected from technology 12 years newer than the V8 Lexus.
By the old methods of rating, which were more optimistic than current, the Lexus was rated 17 mpg city, and 23 mpg highway, and always exceeded the number on the highway, and was just accurate in the city.
Still, I love this car. Some of my angst is caused by my getting out of a V8 Phaeton that weighed over 5K lbs and did 20 mpg at 80 mph. I got rid of the car because it was always needing work, even with less than 60K miles, and because I had to drive a long way to have the work done, and the dealers still would not fix everything correctly. It was a complicated car, took up too much of my time to get it fixed, but I used the excuse of being ecologically responsible to justify getting the V6 Hyundai....that has a local dealer (albeit service is marginal, though well intended). I did a monetary hit on the Phaeton, but saying so is discounting my time which I obviously valued enough to get out from under the big car. Now, my Hyundai is much more fun to drive on a daily basis, gets noticed more by a lot, has almost as much interior space except for back seat leg room, and lacks the absurdity quotient requirement for attention to proprietary details the Phaeton required. Oh, and the key fobs for all Phaetons would work only if within 15 feet of the car. I had started to hate the car, but I didn't imagine it could be as bad as it was wheh I bought it just 7 months before I dumped it. The quality of the Phaeton interior was mesmerizing. I am just not ready to lose faith in my decision making regarding cars, but I am realizing the V8 may have been a better car because the retention of resale value may have paid the meager cost difference from fuel savings...and the fuel savings from the using the V6 seem to be about nil.
For what it's worth.
I have spent thounds of dollars more on cars and they didn't even come close to making me as happy as this car. 
