my 99 tahoe gets an average of 11 mpg and i changed,plugs ac delco,cap & rotor, wires,oil & air filter.. i was told but a buddy to add sea foam to tank and some trew vaccuum to break booster? any other suggestions? Thanks

Do it, no harm, may help but don't get your hopes up.

is 11 mpg average?

Stop and start in cold weather depending on driving habits, could be.

would you add it like this? http://members.nuvox.net/~on.roz/cars/z28/seafoam.html

I would not add anything to the oil. Fuel tank, throttle body ok, thru the vacuum ok.

thanks