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Prius Tire Tuning

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Posted by bwilson4web on May 13, 2011, 8:36 pm
One of the interesting aspects of a Prius is the effect of different
tire sizes for performance tuning. Recently someone asked 'what is the
largest tire' and 'what are the effects on mileage' so this is what I

* * *
I have experience with Sumitomo:

175/65R14 (919 revs/mile) - what Toyota service centers sell as
replacement tires. I got 50,000 miles from first set and only an
unrepairable puncture led to their retirement.GPS and mile markers all
confirmed the actual was a little generous and the actual speed a
little lower. But this was taken as the 'stock tire' from Toyota. This
goes a long way to explaining another reason why Prius may be
perceived as being slow ... their speedometers are showing a faster
than true speed.

175/70R14 (886 revs/mile) - this first pair tested revealed a slight
stability improvement on a 750 mile trip to Madison WI and back. I
drove up with the smaller 175/65R14 on front and 175/70R14 on the
back. At Madison, I swapped front and back so the larger 175/70R14 was
on the front and that was when I noticed the slightly improved,
straight-line stability. GPS and mile markers show the speed and MPG
are less than 1% from true. I really think these are the best tire for
the car.

195/70R14 (849 revs/mile) - a pair are now my front tires and the
175/70R14 are the rear. Again, my perception is more stability and
certainly it is not so flighty. However, it indicates ~6% slower speed
than actual (47 mph indicated is 50 mph true) and lower MPG. However,
when I correct for the true MPG, there has been no loss. Best of all,
the true hybrid transition speed has gone from 42 mph to just under 45
mph. This means I keep up with traffic and the engine continues to
shutdown when it can. <grins>

Here is an image of the spreadsheet showing what I was looking for:
(GPS and mile marker testing showed the 175/70TR14 brings the
indicated and measured MPG to within 1% of true)

I wanted to find the largest possible tire that could fit on an NHW11
Prius. Of the candidates, the 195/70TR14 was the largest so I bought
one and mounted it to see if it would fit in the rear wheel well ...
it did. I then drove around looking for speed bumps and dips to make
sure it didn't rub ... it worked fine. So I ordered a second one and
put both on the front.

* * *
Now I have to be careful and use my GPS speedometer or always subtract
6% from the indicated to know my true speed. I also have to adjust my
mileage records by 6%. But so far, there has not been a loss of
vehicle MPG other than I catch myself driving a little faster now.

The Prius has a threshold speed, 42 mph for the NHW11, that above that
speed, the engine runs all the time. Below that speed, the engine can
shutdown. With the larger tires, I can travel 40 mph with traffic and
the engine will shutdown whenever it can giving better mileage at this
speed. But above 45 mph true, drag rules and the car only gets 52 MPG
at 65 mph which still beats the snot out of gassers and diesels:


Bob Wilson

Posted by Elmo P. Shagnasty on May 14, 2011, 3:12 pm
In article

The Prius is a gasser too, Bob--or have you forgotten that already?

Posted by bwilson4web on May 14, 2011, 4:58 pm

No Elmo, this is a Prius technical site, not a dating service.

Bob Wilson

Posted by Elmo P. Shagnasty on May 14, 2011, 6:00 pm
 In article

hehehehe  So Bob *does* lie and make things up in order to try to make
himself look better to the ignorant others in the world.

Posted by bwilson4web on May 14, 2011, 7:21 pm

<SIGH>You really should head over to the alt. sex sites. They will
welcome you with open flys.

Bob Wilson

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