A C172 pilot departed on four hour flight with full tanks. After landing fuel gages show thirty-four gallons were used. Reporter elects to add twenty five gallons for the return trip and runs dry eighteen miles short of destination. Dead stick landing at a handy airport for fuel purchase was made.

Narrative

I rented a Cessna 172SP to fly a four hour long trip. Maintenance informed me that this aircraft had long range tanks. On arrival I and had burned 34 gallons of fuel. It was close to what I had calculated which was about 36 gallons based on a 8.5 to 9 gallon an hour fuel burn in a worst case scenario. I put 25 gallons back in because I wanted to be safe. On the next leg it seemed as if I sprang a leak or was burning fuel faster. The gages showed over eight gallons and then they fluctuated between four and eight. I calculated the fuel over and over again and kept coming up with a number that would get me back to the airport and have an extra thirty minutes. Unfortunately; when I got directly over another airport; the engine cut off and then I put the mixture full rich and pumped the throttle then it came on and cut off again. I troubleshot and then it stayed below 1000 RPM and would cut off if I went to a higher setting. Based on the fact that I couldn't remain airborne nor enter a normal pattern; I declared an emergency and told them I have a few minutes of fuel on board. I was granted any runway to land and when I did. I taxied to the fuel station where I regained power back from my own engine. My mistake was that I should not have trusted the fuel gages and used a fuel stick to know how much fuel I really had when I got to my destination; because I might have actually burned more than the gages showed.