Genuine mileage testing question... in General E85 Discussions Posted March 30, 2012 · Report reply I doubt one or two tanks would cause buildup, plus future e85 fillings would clean it out. What I would worry about more is computer calibration. If I put in a tank of gas after running E85 for a while the gas mileage doesn't improve that drastically. The next tank will be better. The subsequent E85 filling will be worse then improve. Supposedly it takes the computer 50 miles or so to recalibrate after you reset it and I'd be its the same when switching back and forth. I usually get 15.6 or so on E85 in my Grand Marquis 17-18 on gas. If I run a tank of gas it only bumps it up to 16, then if I switch back its like 13. (Those are mostly city figures, highway would be slightly better). It takes a tank or two to balance back out. Consequently I just run E85 all the time.