Tach vs. Hobbs
Most rental places charge for hobbs time, or clock time. HFC only charges for tach time, which is driven by engine rpm, and cheaper than hobbs.
Why is it cheaper?
Compare two imaginary airplanes: an HFC airplane and a rental airplane at your preferred location. Both are billed at $100 per hour. You fly each airplane for exactly one hour by the clock on the wall. When you pay for your hour at the rental place, you will pay $100 (or more if they have fuel surcharges, which are becoming more common). When you pay for your hour at HFC, you will pay $75 or $80, because tach hours accumulate slower than clock hours.
How much cheaper is Tach than hobbs?
Hobbs time increases as the clock moves forward. Tach time, on the other hand, moves faster or slower depending on engine RPM. So for example, while taxiing, tach time is only about 40% of Hobbs time. For pattern work, it's somewhere between 60 - 70%. For a long XC trip at 2500 RPM, tach time is close to or in some cases more than Hobbs. So the amount of money you save with tach time really depends on the type of flying you'll be doing.