heres something i've figured out over the years with my fords.... if your engine runs more efficient and has better power on the higher octane fuels, you really aren't saving money on the lower octane fuels, the more timing advance you can get out of your motor, the better gas mileage/power you can pull out of it.... if you want to try and run a lower octane fuel, than try using lucas octane booster, not the standard but instead the new stuff thats not street legal, costs like $5 a bottle and treats 25 gallons, the mustang has a 16 gallon tank so you really aren't going to be saving much on the cheaper octane only a few cents difference... might as well pay the 20-30 cents more per gallon, definately dont drive it around constantly pinging either.....