Hybrid Car – More Fun with Less Gas

Optimal Tire Pressure - Page 5

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Posted by Bob & Holly Wilson on December 25, 2007, 8:41 pm

I found this converter page:

So at 1 bar, your running ~14.5 psi relative to the outside air?
bar - psi
1 - 14.5
2 - 29
3 - 43.5
4 - 58

Use just the radial tire coefficient of drag.

There are non-linear effects that occur between 65-75 mph that can run
my NH11 Prius mileage from a high of 53 MPG at 65 mph down to 39 MPG at
75 mph. Apparently tires also make a contribution along with a control
law. This fall off in mileage does not follow the V**2, aerodynamic drag

The practical effect is to identify another performance limitation along
with the control law that limits MG1 to 6,500 rpm. Larger diameter tires
and wheels, say 10% larger, would still be subject to radial tire drag
at higher speeds. However, they would shift the MG1 speed management
laws up 10%. If this brings 50 mph through 75 mph, the results would be
most excellent.

Today, I'm using Sumitomo P175/65R14, T4, 919 rev/mi. It may be possible
to fit P205/70R14, 831 rev/mi. and cut the vehicle rpm by 10% at any
given speed. Thus an indicated 65 mph would be an actual 71 mph. This
could defer the MG1 rpm control law but still be subject to the
increased tire drag.

Bob Wilson  

Posted by Marc Gerges on December 26, 2007, 5:05 pm

Yep, seems like it.

Obviously there's a bunch of development going in them, and I'd expect
tires built for low resistance to score differently to tires built with
other characteristics in focus.

The tires my car came with are V rated, that's 240 km/h, around 150 mph.
I presume that's not because they'd expect me to drive that fast, but
because they wanted the stiffest possible tire construction for
minimizing rolling resistance.


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