haha! vehix i had that problem.. i got some cheapo chrome stems from autozone and they didnt want to come off when i got my new good ones... using a sharp non serated knife i was able to cut through them with pressure (no sawing motion) and then use needle nose pliers to just tear them off, make sure to apply pressure only where there are threads under it though so you dont risk cutting into the rubber of your stem
costie - im pretty sure it hasnt done anything to your engine.. other things you might want to check is your differential and transmission fluids, if they are bad your gears wont turn as freely... also your driving style will greatly effect your gas mileage.. if i cruise on the highway at 65 i can get 18-20 mpg... at 80 i get 16... if you stomp on the gas in the city... your mileage is gonna go down too