My guess is that since the Prius is front wheel drive, the higher pressure
will help improve fuel economy. Lower pressure in the rear reduces some of
the ride harshness.
It is more common to see higher tire pressure specified for the rear tires,
especially for rear wheel drive vehicles, because that is where the cargo is
The front axle of a front-wheel drive car carries 55%-65% of the total car
weight, requiring more air pressure in the front tires.
Additionally, the front axle is carrying 80%-90% of the total car weight
under heavy braking, when the "weight-shift" causes the car to nosedive from
Um. I mis-read the pressure chart on my Prius and over-inflated
the tyres. Fortunately, not long afterwards I happened to visit
the dealer/garage (the same that gave out champagne to compensate
for a recall's "hassle") and happened to mention the pressure I'd
used... A strained look crossed the face of the dealer-bod, who
suggested that he take the car aside and adjust the pressure down
to what was _actually_ recommended on the chart. He did and the
mpg stayed much the same: currently getting around 57mpgUK. OTOH
I suspect tyre life and the safety of my ride improved. (FWIW, I
don't have the tyre pressure tables handy.)
(BTW, 57 mpgUK == 45.6 mpgUS.)
I read elsewhere about this use of 42 psi for mileage, but it seems to
me it would compromise safety by decreasing skid threshold in the event
of a rapid stop and/or rapid maneuver on wet roads?
Ron Sleeter wrote:
Maybe by one percent. Proper inflation is important to minimize the
possibility of bruise breaks from hitting pot holes and stress fractures
leading to a thrown tread. The 42/40 pressures recommended by many (I use
these pressures) are within my tire's rating.
I have never heard of the term "skid threshold" but if you are talking about
the point at which the tires lose traction with the road surface or
something like the coefficient of friction, then the higher tire pressure
doesn't compromise safety. There are so many variables that affect tire
performance and tire life that a change in 1 variable will be offset by
changes in others.
My wife and I have noticed that the difference between the 35 lbs that my
dealer uses and the 41 I put in at the next fill-up is about the difference
between 42.x and 47.x Avg.MPG. We have never had any problem with traction.
Here in Tidewater VA we run about 60/40 interstate/street. I-64 is concrete
mostly with longitudinal drain groove cutting. (don't know if that makes any
difference or not, but it makes some nifty 'whistling' at 60 when the
pavement is new and the grooves are just put down.)
"Ray O" <rokigawaATtristarassociatesDOTcom> wrote in message
Because more weight is in the front of the car than the rear, so
with a light load you want the pressure a bit higher at the heavy end
to keep the tire wear even.
I'll bet it calls for 35 and 35 if you have four or five passengers
and a full load in the trunk for a trip - and if not, that's where I'd
put it. If the sidewalls allow higher, I'd bump them both up to the
maximum pressure if you are carrying a maximum load.
Lots of cars do, some far more pronounced than that two-pound split.
Whenever all the weight is at one end, the tire pressures have to be
staggered to match.
My Chevrolet Corvair (rear engine aft of RWD axle) has a severe rear
weight bias and calls for 14 PSI front, 28 PSI rear. (With radials, I
ran it at 18/32.) The early VW Beetle and Porsche 911 series have the
same layout and a similar rear pressure higher stagger.
--<< Bruce >>--
something to do with weight distribution.
I tried running my Classic Prius with the same pressures front and
rear. I didn't do that for long - the car felt like it would fishtail
at a panic stop, whereas with a +2 or +3 psi difference in the front it
would stick those same panic stops. (Beware of those Dunkin Donut
shops in the morning commute!)
My cousin's Chevy Malibu is 29 psi front, 26 psi rear.
Fairly normal, certainly every car ive owned/driven has had higher
front pressures than rear. Its due to the weight distribution of the
car - the heavier front end requires more air to maintain the same
contact patch and sidewall deflection.
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