[DoktorDreem]

Part one of Fit physics from the Doc,

We are in this discussion because we want the best MPG from our Fit or understand the best we can do. For any car the MPG will be a product of

the engine and transmission: efficiency vs load and RPM
the car: rolling resistance, weight
the road: hills vs flat, curves, traffic, speed limit
the driver: acceleration rate, coasting, speed, AC on, idling vs stop, length of trip

We don't have hybrids so the game is to get the most out of a gallon of gas and do it comfortably and safely using only acceleration, cruising and coasting. Our hybrid friends get to exchange the energy of the gas and kinetic energy of the car with electrical charge.

A gallon of 87 octane contains nominally (depends who you ask) 124,000 BTUs of energy. Since we don't want to know how much steam we can generate, first convert the energy from thermal to equivalent kinetic energy, the get from point A to point B kind of answer. For these discussions I prefer the metric system.

Kinetic energy of a system is one half the mass times the speed squared. Useful units here are Joules and Horsepower (HP).

1 HP = 746 Watts
1 W = 1 Joule/second
1 kWh (kilowatt-hour) = 3413 BTUs = 3.6 megaJoules (MJ)

Therefore our 124,000 BTU = 36.33 kWh = 130.8 MJ = 48.7 HP-hours or
175,378 HP-seconds

For the sake of discussion if we have a car that takes a mere 15 HP to cruise at 55 MPH on a level road, and assume the engine is 100% efficient at converting thermal energy to kinetic energy (not possible), one gallon would last us 48.7 HP-h/15 HP = 3.247 hours of driving time. At 55 MPH that is an incredible 178.6 MPG. A car that is 30% efficient would get an impressive 53.6 MPG. But 15 HP is a rather small number for a decent size car.

How efficient is our Fit? I don't know. We will just have to measure it with some simple science experiments. We do know:

how much the car weights (per Honda 2009 AT is 2604 lbs empty)
how much we and our passengers weigh (typical 400 lbs for me and mine)
how much gas weighs (6 lbs per gallon)
how fast we are driving (assume our speedo is accurate)
how to count seconds (stop watch or "one-one-thousand")

The experiment is to count how long it takes to coast from, say 70 to 65 MPH, and calculate how much energy was lost to wind, tires and engine drag. I invite you to send me your data for AT and MT, in gear or in neutral, for a range of speeds. I recommend a 5 MPH difference as the coasting time varies drastically at higher speeds Yes, jadr09fit, the AT is towable and coasts very nicely. Next time I'll go through the equations for calculating your coastdown HP. I'll want to get three things for a level road before we take on hills and other factors:

wind resistance
tire rolling resistance
engine drag

Cheers.