MPG Difference - 17 inch tires versus 18 inch tires

[hybridbear]

I looked back through my trip logs today and found the following data for the city route trip to my parents' house.

On Saturday I had my best ever Regen miles for a trip home from their house with 1.0 Regen miles. Without that trip the Avg Regen mi is 0.8. I consciously tried to wait longer and brake closer to the 35 kW max as often as possible.

As you can see in the table above, the black FFH did consume more SOC from the battery on average, leading to higher MPG. The black FFH consumed an average of 31.64 more wH from the HVB on these trips (3.14 - 0.88) x 1.4 / 1000. This amounts to 0.003 extra gallons of gas, assuming 33% efficiency of the ICE 31.64 / 33705 / 0.33. Since the black FFH consumed an average of 0.193 gallons if we add the extra gas we get 0.196 gallons instead. The white FFH is consuming an average of 0.219 gallons of gas. The adjusted black FFH MPG is 59.7 MPG. This leaves a difference of approximately 10.5% between the two cars.

Some of the difference could be "break in" related since the white FFH had 1000-4500 miles on the odometer when I gathered this data and the black FFH had 18,000-19,000 miles on the odometer at the time of this data collection. But most of it I attribute to rolling resistance.

I don't have warm weather data from the black FFH with SOC values to compare, but I will continue gathering data from the white FFH to update this data in the future.

Edited May 19, 2014 by hybridbear

[inco]

_{Bottom line - Size Matters! :)}

[hybridbear]

Larryh makes a great point here

 

The average gas consumed per trip by the Black FFH, was 11.7/59.7 = 0.196. For the White FFH, it was 11.7/53.4 = 0.219.

The Black FFH consumed gas during the 11.7 - 9.2 = 2.5 non EV Miles. The White FFH, consumed gas during the 11.7 - 8.9 = 2.8 non EV miles. The average gas consumption/mile while the ICE was on was 0.196/2.5 = 0.0784 gallons/mile for the White FFH and 0.219/2.8 = 0.0782 gallons/miles. If they had the same EV miles, the mileage would be identical for the White and Black FFHs. The total discrepancy is explained by the difference in EV miles. Now the question is why is there a difference in EV miles?

 

This seems to be the consequence of the 18 inch tires and their increased rolling resistance. Less regen due to increased rolling resistance equals less Regen miles which equals less EV miles and less MPG. The ICE is no less efficient in the white FFH, it's just the tires.

[larryh]

The following link shows how tire size affects mileage: http://www.caranddriver.com/features/effects-of-upsized-wheels-and-tires-tested.

For this particular car, increasing tire size from 17" to 18" decreased mileage by 4%. The 18" tire had better grip than the 17" tire--so it was not the best comparison since the tires were not the same--same model of tire, but different wear ratings.

 

And how rolling resistance impacts mileage: http://www.tirerack.com/tires/tests/testDisplay.jsp?ttid=121.

The difference in MPG between the best (Michelin Energy Saver A/S) and worst tire was 7.4%.

Edited May 20, 2014 by larryh

[acdii]

Tip, the more miles on the tires the better the MPG gets and the worse the traction gets. I took a measurement of my tires with 23K on them. I can see the top of Lincolns head.