"Extra load" tires

One more thing to keep in mind - The term "low rolling resistance" is not defined by any central agency so has different meanings to different people. I checked with a couple of tire outlets about this term and basically put they said that the compounds in the tires have a higher silica content (I think thats the correct component), and because of that higher silica content, have less resistance to rolling than non-silica compounds. I then was told by the same tire suppliers, that most , if not all newer tire models have this silica content. So in my opinion, if the tire can be inflated to the 44 # range then you will probably get a similar rolling resistance assuming tread design and size is about the same.

I think Pirelli has a 44psi summer tire available (XL rated) too.

PS -
Bridgstone RE92 replacement XL tires are @ 52$ per tire online versus lots more at Toyota.

(I am not advertising for the online tire supplier - so if you don't know who I mean, then just ask).

steve dickerson
'02 super white 10k plus miles... tires still good tread

[%sig%]

My theory: Low rolling resistance tires are nothing more than a tire with stiff side walls and a small contact are..no magic. This is all accomplished by having the 50 psi max tires as in the XL rated Re92's.

also here's calculation to help explain this...car weight 2700 lbs
+ 2 passengers...say 350 lbs + cargo say 50 lbs..= 3100 lbs.
if tires ARE at 50 psi (assume equal weight distribution for now)
then the average required area in contact to road is = 3100/50 = 62
sq.in. / 4 = 15.5 sq.in per tire..approx. equal to a 3 x 5 spot on
the ground..conclusion ..lower pressure requires more surface area in
contact..and 175 tires don't deform uniformally to accomodate a
larger surface contact area..thus cupped wear pattern. and why at 35
psi michelins need to be 185's.

of course low rolling resistance has a trade-off many cannot accept..harsh ride.

Doug

The Extra Load tires (load factor 84 - 1102 lb) are standard on the US and Canada Prius. It is my understanding that Prius in Europe and Japan have a lower load rating (load factor 81 - 1019 lb).

I looked at the label on the bottom of my driver door frame and noticed the following information.

GVWR (Gross Vehicle Weight Rating)
3615 LB

GAWR FR (Gross Axle Weight Rating - Front)
1970 LB

GAWR RR (Gross Axle Weigt Rating - Rear)
1685 LB

From this data I can see that the front axle is rated to support a maximum of 1970 LB. This means that even if I buy the load factor 81 (1019 LB) tire I will still exceed the weight limit of the front axle before I exceed the weight limit of the tires (assuming the tires are inflated to max pressure ... tires support less weight when pressure is below the max pressure).

So, why extra load tires in North America but not elsewhere? Good question. Some have suggested that it is because Americans tend to be overweight. I guess Toyota thinks we will overload our cars. I tend to agree. The curb weight of the Prius is about 2765 LB. The maximum weight the vehicle is designed to carry is 3615 LB. This gives the Prius a cargo capacity of 850 LBS. If you think about using the Prius to haul football players around it is easy to come up with a scenario that exceeds the vehicle weight limits. But if you have extra load tires you may not exceed the tire weight limits.

A tire can only support its maximum weight when it is filled to maximum cold pressure (the pressure will exceed this maximum when the tire heats up but as long as you don't exceed the maximum cold pressure it is OK). Why does the owners manual say to fill the tires to only 35/33 when the tires are rated at 50 psi? The tires will not support a 1102 LB load unless they are filled to 50 psi. If the Prius *required* extra load tires the owners manual would specify 50 psi. I think the extra load tires were chosen simply because they have a larger safety margin than standard tires and a larger safety margin equates to a lower failure rate.

By the way, you say you notice wear at 22,000 miles. This sounds about right. The OEM Bridgestone XL Potenza RE92 with a treadwear rating of 160 is a 25,000 mile tire. I plan to replace my tires with the Bridgestone XL Potenza RE922 with a treadwear rating of 260 that is advertised as a 40,000 mile tire.

For some reason Toyota decided to give the US Prius a low mileage tire (25,000 mi) with a higher safety margin instead of a regular tread life tire (40,000 mi) with a regular safety margin.