Hi keepitpg,
let's look at how just getting up to 65 MPH can dilute MPG. Hypothetical door to door trip with no stopping.
0 to 65 MPH requires 0.550 MJ or 738 HP-sec. Until I have better data, assume Fit takes 15 HP to run this speed and can do it at 43 MPG steady.
A 10 mile trip with hard braking at the end would take 0.02 gal to get up to speed and 0.233 gal to make the trip = 0.253 gal => 39.6 MPG. Yuk.
Now let's try and coast. Assume 7.5 HP and 30 MPH average. Convert kinetic energy to distance x force. 738 HP-sec at 7.5 HP is 98 seconds, or about 0.82 miles. Actual coastdown won't be so good so lets use 65 seconds (decelerate 1 MPH/sec). That works out to 0.59 mi, I think, maybe too optimistic, but let's do the math. Gas is only burned for 9.4 miles, so 0.02 + 0.219 gal = 0.239 gal. That is 41.9 MPG. Worth the effort.
Now let's compare to a shorter trip, 5 miles. I get 0.02 + 0.116 gal = 0.136 gal => 36.8 MPG, 40.9 with coasting.
A pattern emerges here if we look at a long trip, say 2 hours with a rest stop (how I drive to Oregon from CA). 0.02 + 3.029 gal = 3.049 gal => 42.6 MPG for 130 miles. Compare with 42.8 for the coasting case.
So a table looks like
trip distance MPG(stop) MPG(coast)
2.5 miles 32.1 39.0
5 miles 36.8 40.9
10 miles 39.6 41.9
20 miles 41.2 42.4
130 miles 42.6 42.8
Conclusions: (1) Even though the Fit can get superb gas mileage, the actual trip or tank MPG will suffer if your trips are short enough. (2) Coast to a stop if possible. (3) Expect climbing hills to cause the same dilution of MPG, coast baby coast.
Next time: AC on, idling at the stoplight, hills
