We recently took two identical trips to visit her side of the family, one in each car (one trip unexpectedly). OKC-ish to Springfield, IL and back. Obviously it's 100% impossible to get perfect scientific conditions, but both times were as identical as can be. Same weight (our cargo) in the cars, same calm-ish winds, nearly the same temperatures, etc. I used cruise control in both vehicles both ways set to the speed limit (I only took over on the extra large hills in MO, such as the Piney Rivers on 44). The speed limit for 95% of Oklahoma is 75, 95% of the rest is 70. I stopped at the exact same gas stations along the way for both trips. I used A/C for the entire trip both times. I hand calculated the mpgs using the tripometer and gallons used (filling on the slowest 'click' setting, stopping when the pump clicks off on it's own, no squeezing). I feel this is as good as it gets. I didn't record (because it's irrelevant) what the cars 'said' the mpgs were, however I remember the Elantra overcompensated 1-2 full mpgs, and the Civic was either on the money or overcompensated by a ~1/2mpg. For both vehicles, on the way there I had the ECO/ECON modes disabled. For the return trip I enabled these. This way I could directly compare both vehicles in both modes side by side. The results really surprised me. For the record: The 2016 Elantra 1.8L 6-speed auto is EPA rated 28/37 (31 average). The 2013 Civic 1.8L 5-speed auto is EPA rated 28/39 (32 average). My stop in-between was in St. Robert MO (farthest East you can go before paying IL gas prices... ). So we'll call home (closest gas station) A, St. Robert B, and Springfield C. The mileage from A<->B is 385.5 miles The mileage from B<->C is 241.9 miles The Elantra's numbers: A->B - 9.629 gallons - 40.04mpg (NON-ECO) B->C - 5.602 gallons - 43.18mpg (NON-ECO) C->B - 6.655 gallons - 36.35mpg (ECO) B->A - 9.829 gallons - 39.22mpg (ECO) The Civic's numbers: A->B - 9.835 gallons - 39.20mpg (NON-ECON) B->C - 5.931 gallons - 40.79mpg (NON-ECON) C->B - 6.810 gallons - 35.52mpg (ECON) B->A - 10.237 gallons - 37.66mpg (***NON-ECON***) ***After knowing the results from the Elantra's ECO mode, and after the first leg of the Civic's ECON mode, I decided it wasn't worth it for the data. It would clearly be worse. Regardless, I learned a few things. One, clearly the ECO/ECON modes are terrible for highway driving. Two, the EPA obviously played favorites with Honda. Three, I now know Hondas are overrated (literally) and I love my underrated Elantra even more. But I'll give credit where it's due, Honda's mpg calculator is pretty legit, at least for this model and year. Hopefully this wasn't too much of a novel and was enjoyable . I just felt this is data that someone would find interesting and/or could utilize, and may not be able to replicate themselves. It was definitely eye-opening for me.