Very low mpg, help plz

[BH2000S]

Thanks for starting this thread. I'm also a Boxster S newby and have put about 1000 miles on my 2000 that has 54k on the ODO and 25k on the crate engine Porsche put in it. My computer was telling me over 21 MPG but that seemed high. I checked on the last tank and it was below 16 MPG (I also figured out how to reset the MPG on the computer and it is now reading about 15.8 MPG). Everything checks out, fresh oil change, correct tire pressure, clean K&N air filter. I do mostly city driving using crap Los Angeles 91 octane gas, mostly Shell. I did run a bottle of Techroline through the tank recently. I try to shift at higher RPM's but that usually puts me way over the speed limit and on the brakes so that I don't rear end someone. Going to keep watching MPG closely, maybe fresh plugs too. Car seems to run great.