When your foot is off the gas pedal coasting in gear, there is no fuel being used until about 1000-1200 rpm until the ecu turns on the injectors again. This way its safe when going down hill and not using gas. The oil is still circulating and no friction. Keep it below 3500 on acceleration and you will save gas. Here is some info on the motor. Honda L-Series SOHC i-VTEC Engines for the new GD Honda Fit/Jazz

The Truly Amazing Honda Fit/Jazz

 

When my car only had a short ram intake and muffler shop fabricated axle back pipe I was averaging 38MPG at 85 to 90MPH. With the KWSC base kit, same axle back pipe and lighter wheels and tires going the same speeds I lost 1.5 MPG and lost another 2 MPG with the full tilt boogie high boost kit with header 2.5" pipe into a Vibrant resonator into a Aero Turbine 4" inside diameter muffler and outlet... I totally agree with Sol Bros about the throttle position... The electronic throttle controller set in it's slowest responding EC 5 position and monitoring the % of throttle opening on the readout on the ECT has been working well for me when I need to conserve fuel. I do pretty good just under 60 or just over 80 but between those speeds it doesn't do as well. That is with the high boost kit.

 

How could you possibly be "pretty sure" about that?

 

I have a 2007 AT and I average between 40-45 (45+ on nicer days) on the highway. When i have to be in DC, I average 32-38mpg. I log and record almost every trip (from when the car is turned on to when the car is turned off) on my scanguage.
I dont use cruise control, more than 80% of my driving is highway and my average highway speeds are between 65 and 70. I dont find it necessary to press the gas down a lot when accelerating on the highway. Even in the city, it does not need much throttle to get moving. The car is very light and even with the little throttle pressure I give I am passing other cars beside me at a stop light and still getting 30+ mpg.

 

I've had ScanGauge for past 5 months and on my 2010 Honda Fit Sport M/T the best highway speed for best possible fuel economy is around 60MPH.

 

Why would keeping your gears in neutral be illegal when going down the hill??? Please explain

 

depends on the area.

if the hill is steep enough for long enough... and the driver rides the brakes all the way, the brakes can over heat the fluid (boil it) and ... no more brakes.

Now you have a runaway car.

At this point, you have to rely on the ebrakes and swerving in hopes of slowing the car down. foot brakes are harder to control than hand brakes, since it's not easily adjustable for intensity, once its down... it's down until you fully release it. in either case, ebrakes can fail too. and swerving is a tricky move for the inexperienced (like most of us).

making it illegal is one way to prevent some folks from getting in that situation.

 

And similarly you may need to GO instead of cruise, and being in neutral delays that. Let's say there's a fire truck behind you and you get flustered and keep stepping on the gas and the car doesn't go... in the time it takes you to realize you're not in gear, the fire's gotten bigger and bigger...

 

45mph had me getting 51mpg...but who wants to drive 45mph... 65mph only had me getting 34mpg. M/T with 16" HPF tires aired up to 40psi