Manual Transmission cars ALWAYS get at leat 15% better gas mileage than an automatic trans.
This used to be the case before electronically controlled transmissions with lockup torque converters. It is no longer ALWAYS true, and sometimes automatics get BETTER mileage.

Other things that lower a car's mpg are cruise control, air conditioning and the engine going past 3000 rpms.
A/C and high rpm's - true - as long as you can comfortably drive the car with the A/C off and the windows closed. Cruise? Only if you are very skilled and don't mind aggravating other drivers with constant speed increases and decreases - and the concentration doesn't tire you out. I have a 5 speed in my truck and ALWAYS use the cruise out on the highways. If I didn't, I would get tired a LOT quicker.