Posted by Elmo P. Shagnasty on May 14, 2011, 7:41 pm
What's that I said, Bob?
Posted by bwilson4web on May 14, 2011, 9:53 pm
Well I'm so glad you asked. We were discussing Prius tire tuning ...
matching the right tire side for optimum performance. To summarize:
o too small - the speedometer indicates higher than true which results
in slow speeds that reduce drag and improve actual MPG. However, it
also leads Prius drivers to being slower than other traffic and
inflates the indicated MPG. It is also associated with 'skitterish'
handling as it needs more driver attention.
o just right - the speedometer is right on as is the indicated MPG.
Slightly larger diameter leads to more straight-line stability.
o oversized - the speedometer indicates lower than true speed which
can result in more drag and a loss of true MPG. Also, the indicated
MPG is lower. However, straight-line stability and handling is
Thanks for the question, I needed a reason to summarize the thread,
Posted by Neo on May 15, 2011, 11:59 am
Did you ever measure how the tire
sizes might effect the braking distance?
While I would suspect that the 195/70R14
would provide better traction which
would translate to better stability and
braking performance - how much better
might one expect?
It sounds like that these bigger tires
are fooling the ECU (which is still thinking
the prius is using the smaller tire size) to
allowing for a slightly higher velocity at
which the Prius changes from all electric
to electric and gas. If I recall correctly
that hybrid transition speed where the Prius
goes from all electric mode to running both
the electric motor and the ICE was to protect
the ICE from damage. While extending the
top speed for a Super Highway Mode Cycle
sound great - I am wondering if there are
any risk . While I suspect that Toyota has
probably engineered enough tolerance to
allow for your slight increase in speed -
I would suspect that going signficantly
higher may damage the ICE.
2010 Toyota Prius III, Blue Ribbon/Dk Grey, OEM floormats
Yokohama Avid S33 (psi 48/44)
Scangauge II ( AVG, RPM, GPH, SoC)
no grill blocking
Dc/md/nova metro area hypermiler on training wheels
odeo 11800 miles, overall 58.7 mpg
last tank 4/19/2011, 598 miles of hilly urban/suburban, 9 gallons e10
87 oct regular, estimage FE=66 mpg
Posted by bwilson4web on May 15, 2011, 5:09 pm
. . .
Not yet since I'm trusting the ABS to moderate the braking force. My
understanding is the ABS releases braking force when the wheel begins
to stop, keeping it in the peak braking force region, and this is
independent of the tire diameter.
This is a hard problem as I don't have a good metric nor know of any,
the units of stability. Pretty much universal, the automotive
magazines and even TireRack use driver impressions. But two approaches
come to mind:
1) monitor the electric steering adjustment voltages over a straight-
2) monitor the horizontal accelerations over a straight-line course
We still have a problem with absence of a known impulse that would
require a steering adjustment. Perhaps a vertical air foil that
oscillates between +5 and -5 angle of attack. Each transition would
move a force from one side to the other, a well defined force
application, and then the vehicle and steering response could be
I thought about this before the experiment and came up with these
acceleration - the amount of torque is engine and transmission limited
so I'm expecting slower, maximum acceleration. But I don't do hard
accelerations except when benchmarking the car. I just don't have
frequent requirements for maximum acceleration and the NHW11 at 13
seconds to reach 60 mph, is more like a truck than a sports car.
hill climb - the torque needed to climb even an 8% grade at 55 mph is
well within the capabilities of the existing car. I've done maximum
speed hill climbs and topped at 85 mph so I don't see a risk.
regenerative braking - will have a maximum reverse torque and the
larger diameter tire may lead to more mechanical braking. However, I
drive so as to minimize regenerative braking so no impact is expected.
What we've done is made the last gear, the tires, taller.
I've only been testing these tires on our 2003 Prius. We have a 2010
Prius but while it remains on warranty, I'm keeping the new Prius
stock with one exception. I've started a 5K, 15K, 30K, and 60k,
transaxle oil change and test plan. I want to measure transaxle wear
over time and make sure I know the change interval based upon metrics.
Posted by Elmo P. Shagnasty on May 15, 2011, 5:50 pm
Ummmm....no. They use instrumented data to measure braking performance.
It's a simple thing to do, really--and yet you can't/won't do it?