Rico;
You didn't by chance happen to change the alternator belt recently. If the belt was thinner and fit deeper into the pulley, it would read at a higher rpm. Vice-versa, if the belt was wider, it would rider further out from the center spinning the alternater slower. I realise that you just had your fuel tank flushed out , due to the gasoline fill up, but I would hate to see you chasing ghosts. I recently had this problem concerning my belt. That is why I mention it. Reading what happenned with the fuel issue, personally I wouldn't be that worried. Seeing as you never ran the engine on the gasoline at all, I really don't see how any of it would get into the fuel filter(let alone the engine) unless it wasn't all drained out. Also, I wouldn't be surprised if your owners manual mentions running some gasoline during the (cold) winter months, to prevent fuel gelling. Best of luck!