NCF: tires

Assuming wheel mount patterns allow a swap and using Michelin Premier A/S

• 195/65R15 - 832 rev/mile, +1.4% true mph over indicated, true MPG low over indicated
• 215/45R17 - 844 rev/mile, -1.0% true mph under indicated, true MPG high over indicated

To compare 15" and 17" tire performance, the distance and fuel consumed have to be true. The problem is the brake controller counts the four wheel sensors and sends a distance and speed pulse to the odometer/tripmeter/speedometer. In the Gen-3 TechStream, there is no adjustment and we know the brake controller value is off by ~6%.

The only true indicator in every Prius is dividing the indicated distance by indicated miles per gallon to get true gallons. The division cancels out the brake controller error. So if 17" wheel give worse mileage, the test needs:

1. GPS - speed and distance
2. Non-trivial distance - +10 miles
3. Calculated true fuel = indicated distance / indicated MPG

To compare the two, the speed of the 15" and 17" test cars needs to be GPS set and held constant. But holding all of the other variables constant for a single car with tire change would be a challenge.

A better test would be something like the old Edmunds 'Smackdown' with a 15" Prius and 17" Prius to settle the question. Normalize the cars, fuel and then drive from San Diego to Las Vegas via country roads; next day tour the town, and; third day return via Interstate. San Francisco to Reno would work too.

Now if we could just find two NASA folks at say Ames or JPL with similar level, 15" and 17" equipped Prius, and the curiosity to run the test.

Bob Wilson