My car has 277000 and started missing a
few months ago. With regular fuel it has
been pinging on and off so I started using
premium gas and the pinging had stopped
for the most part, but the engine still
misses on town and on the highway and
idling. I had a tuneup but it still misses.
Are you sure they are misfires? Detonation/pinging is different than misfires. Misfires are caused by poor spark, or not enough fuel. Check the injectors, maybe one or some are clogged/worn. at 277K miles your engine is on borrowed time and is need of a rebuild soon.