Hi folks,

I recorded a trip from Huntsville AL to Knoxville TN using the ICE air flow for fuel consumption and MG2 rpm to calculate speed. Two cruise control speeds were used, 65 miles per hour and 70 miles per hour and the data generated this chart:

Several things stand out:

• cruise control variance +/- 2 miles per hour
• @65 miles per hour, ICE idle (backside of hills) gives +200 MPG
• @70 miles per hour, ICE idle disappears except below 70 miles per hour

Early on I found highway milage dropped off around 70 miles per hour and at 75 miles per hour, it was pretty low, 39 MPG. But this data shows the actual boundary is fairly sharp and suggests the mechanism.

The 2001-03 Prius, model NHW11, has a speed limit on MG1 rpm, 6,500 rpm. With an ICE idle speed of 1,200 rpm, that comes out right at 70 miles per hour. Sure enough, below 70 miles per hour, the ICE can idle with very low fuel flow. At 70 and above, the ICE has to run and burn fuel:
http://eahart.com/prius/psd/

The best strategy is to set the cruise control to the highest speed that still allows ICE idle and stays under 70 miles per hour. The optimum country speed is 68 miles per hour, 110 kph, as anything faster and MPG will take a hit.

There are a scattering of points up at 500 MPG but they are relatively sparse. I'll continue to look at the data to see if I can figure out what is going on. In the meanwhile, 68 miles per hour is my new, favorite cross country speed.

Bob Wilson