Accuate milage calculation.
1. fill the tank
2. reset trip odometer to 0 miles
3. drive and burn up at least 3/4 of the tank, more tank empty, better adverage.
4. refill the tank
5. divide miles driven on trip odometer by gallons used to refill tank.

Most accurate way to check gas milage.
I get about 19-20 mpg with my 05 6mt, worst was about 17 mpg. I drive only city and I do beat on it pretty good now and then. For regular around town driving alot of times I'll skip 4th and 5th gear and go straight to 6th. Car has plenty of power.
Just a few suggestions.

 

Actually on the highway with your windows open your gas miliage is worse that if you turn on your AC. (as it states in the manual)

When your talking about our wonderful gas tank you have to be careful when you say "I get only 240 miles on a tank" because that doesn't mean anything without saying how much your putting in. For whatever reason they made our cars with a 4-5 gallon reserve, so technically if you can stand driving on E for 100 miles you should be able to get between 340-400 miles before you would run completely dry on our 20 gallon tank. A common problem is the tank will not fill up completely either, but you will notice this from the gas gauge.

 

I now accept the fact that my G will never get great gas mileage. And I'm fine with that, cause everytime I open it up, it just puts a huge smile on my face

 

You know I've had issues with this before as well, but I changed my driving style a bit (04 coupe 6MT) and now i squeeze out 21~22 mpg. And thats with 85% city driving with around 25% spirited driving. its all about compromise