NHW11 tire study

[bwilson4web]

Hi,

I need to combine tables and photos so I've started a report on changing tire sizes for the NHW11 (2003) Prius:

NHW11 tire size study - PriusChat Forums

This multi-year study of the Sumitomo T4s in three different sizes:


Enjoy,
Bob Wilson

[bwilson4web]

First phase with the 195/70R14 on the rear and 175/70R14 on the front.

I've run my first set of tests using a nuvi GPS as the true speedometer and trip distance. The indicated trip meter distance and MPG are used to calculate the fuel burn. The larger tires required a correction of ~6.5%. The indicated mph and distance were too high and the indicated MPG too low by 6.5%. The GPS corrected data:

mph - MPG
18.0 85.7
25.0 79.5
39.5 73.0
40.0 71.1
50.3 62.7
65.0 48.8
65.0 50.6
74.6 40.7
(Details)

So I plotted this data against an earlier drag vs. efficiency model:


One curious aspect is this chart appears to show the hybrid charge-discharge efficiency impact below 42 mph. At 55 mph, we see what appears to be a peak efficiency point where the ICE runs in an efficient power area while powering the car.

My next test is to swap front and rear tires putting the lager diameter 195/70R14 on the front and repeat. The expected impacts:

1. lower transaxle mechanical losses - running about 6% slower, there should be less 'stirring' and other friction losses.
   
2. extending hybrid mode speed to ~45 mph - this may result in a net loss of efficiency by more charge/discharge losses. Whether these are mitigated by low transaxle efficiency is the question.

As for handling, I felt a very slight, 'squeezy' at high speed, 75 mph. It was very subtle and could have been road conditions, waving the surface from the recent hot weather. This is not a metric but a subjective impression.

Bob Wilson