I never knew my 1999 Civic HX was a 50+ mpg hwy car! I just did a long road trip, Houston to Chicago and back! I set several records, but also had really bad luck!
1st record - Houston to past Little Rock, 502.3 mi with 190 mi A/C on, 50.1697 mpg, half tank mark at 370 miles! Drafted a semi for 55 miles.
2nd record - North Memphis to Chicago, no A/C 508.6 mi at 50.252 mpg, 358 mi at the half tank mark! No hypermiling.
Total cost in gas Houston to Chicago $94.04 [had a bad top off tank of 26.58 mpg 2.76 gal - bad pump].
3rd Final record - Chicago to North Memphis, 486.6 mi 51.151 mpg with 56.6 mi of A/C, 393 to 400 miles at the half tank mark! I could have broke 52 mpg with no A/C! No hypermiling, except for coasting in neutral down a long slope.
All above driving mostly at 70 mph.
Then incredible bad luck after my record 51.151 mpg tanks! - Filled up the next 2 tanks with Shelll and Citgo and my half tank mark for hwy driving were at 270 and 240 miles - suddenly really bad mpg [all A/C]! And my last fillup was real bad too - 30.97 mpg! I'm wondering why the sudden bad mpg!? Maybe they gave me E85, and I'm trying to get rid of it! No check engine light or codes, my synthetic oil now has 8000 miles, and I recently did a valve adjustment and PCV Valve change around 102,000 miles. Now I have 111,000+ miles.
I have had really bad luck with E10 and they don't have to tell you that it is E10 either. Wal-mart gas is like that. they say it is enriched with ethanol. my wife used to get the stuff and talked about how her mileage was sucking. it isn't worth the nickel cheaper a gallon because of the worse mileage.
I don't know if this is your issue or not but it may be.
Be the change you wish to see in the world
I use E10 and I have been getting 65mpg. But I drive much slower than 70mph and hypermile extensively and over inflate my tires and drive a VX. Maybe I'll do a tank or two on non E10 gas sometime and compare.