http://www.mixedpower.com/modules.php?name=Gas_Savings Using this calculator with using today's gas price not the price of gas when I bought the car this was my savings - noted my car before the HCH was a Jeep Cherokee 4x4 and my commute is 140 miles a day roundtrip.

Ouput Month Year 10 Years
Gallons of Gas Saved 146 1,763 17,630
Money Saved $310 $3,720 $37,201
C02 Saved (Pounds) 543 6,523 65,234

On edit: My Jeep got around 18 mpg according to EPA estimates so all of these numbers are using EPA estimates. But really when I drove my Jeep I drove 80mph home and so probably got less than 18mpg and now with my HCH I drive 60 miles per hour home and get better than the EPA average of 48mpg. But in the calculations above I used the EPA estimates. And the current price of gas where I live is 2.11