I don't own one but here's what I've read. A flex fuel vehicle can run on regular unleaded gas or E85 (85% ethanol and 15% gas) or anything in between (10% ethanol for example). The more ethanol in the mixture, the greater the effect on mileage.
30% is the worst case scenario, the best I've read is an 18% reduction and all of those were in the E85 category. I haven't seen anything that says E10 reduces mileage - which likely means that driving habits & maintenance (e.g. tire pressure) are as influental as 10% ethanol in terms of mileage.
I had a 1999 Ranger XLT that was flex fuel and it spark knocked like a [email protected]#$. I was so disappointed because I had purchased it new and otherwise loved the truck. I don't think I kept it for a year.
epsylum - you're probably right. I was pulling from memory - given the state of my memory, it is very different than citing a credible source.
00blkgt, that's very unusual. Ethanol octane is MUCH higher than gas, and octane usually works to reduce knock. I don't doubt you, but it makes me wonder if something was wrong with the sensor / oxygen regulator (that makes an engine flex fuel).