Tire load index

[mprichar87]

So apparently you're not supposed to run tires with a load index smaller than your OEM tires. 13 Boss 302 came with 100 front / 103 rear. I was looking at going to Bridgestone RE71Rs (since apparently they're the bees knees) on my new 18x10 Apex wheels. These tires have a load index on 95 and my tire shop says that's a show stopper. Anyone have knowledge on the subject? Dos/donts? Horror stories?

Thanks for the info!


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[blacksheep-1]

No tire shop will knowingly put those tires on that car because of liability issues, should you (and I emphasize the word "YOU") choose to do so you will have to have the tires installed off the car and will have to come up with an appropriate line of...of...of creative thinking that will justify the install on your wheels. whether that amount of difference will actually be detrimental depends on what you do with the car, my recommendation would be to find a tire that meets the standard.

 

[k98dave]

The tire load index lets you know the load–carrying capacity of a tire. In other words, it’s the amount of weight your tire can support safely.

For example, if a tire has a load index of 92, it can support 1,389 pounds at maximum air pressure. Multiply that by four tires (4 x 1,389 = 5,556 pounds) to get your car’s maximum load–carrying capacity.

- See more at: http://www.tiresplus.com/shop-for-tires/tire-buying-guide/tire-load-index-chart/#sthash.GWrxATfr.dpuf

Hard to figure, if you multply X 4 as shown that gives a max load for the 103 index tires on the rear of the boss our max load would be 7716 lbs. Don't see how this would ever come close to being reached.

Looking at listing for 285's on Tire Rack and most are load rated at 99, only the Mich Sport Cup was 103.


Load Index From Tire Rack

# Lbs Kilo Lbs Kilo
71 761 34 91 1356 615
72 783 355 92 1389 630
73 805 365 93 1433 650
74 827 375 94 1477 670
75 853 387 95 1521 690
76 882 400 96 1565 710
77 908 412 97 1609 730
78 937 425 98 1653 750
79 963 437 99 1709 775
80 992 450 100 1764 800
81 1019 462 101 1819 825
82 1047 475 102 1874 850
83 1074 487 103 1929 875
84 1102 500 104 1984 900
85 1135 515 105 2039 925
86 1168 530 106 2094 950
87 1201 545 107 2149 975
88 1235 560 108 2205 1000
89 1279 580 109 2271 1030
90 1323 600 110 2337 1060

 

[13Boss#3328]

This is what I think I've learned about the mustang Gross Axle weight rating (GAWR) and the tire load index, thus far.

Tire load index tells you the maximum load of weight the tire can carry safely.

The Boss 302 Front GAWR is 2220 and the tire we have is 255/40-19 with 100 XL load. They recommend cold pressure of 35 psi.

The max weight of 100 load index (XL load, not SL) is 1764 Ib. At 35 psi the tire load is 1564 Ib.

So there is around 15.4 % safety margin Ford uses with the stock tires accomodating for the dynamic axle weight change due to weight transfer. If you calculate the 1564 and 1764 is around 15.4%.

Hypothetically, Let us say I want to downsize to 245/35-18. The max load is 1430. So the cold tire pressure should already be put so high to accomodate for what Ford wanted to be at OEM tires.

1430+1430= 2860 Ib which is still good for the front GAWR, but that is STATIC condition. What is going to happen if you shift more weight on those tires during braking and body roll? Will you have enough room to take more weight?

We know that the front stock tires will take together up to 3524 Ib at max load.

What puzzles me sometimes is seeing cars as heavy as the mustang, however the manufacturer will use lower load index e.g. Some Lexus cars and Audi...etc.

I need experts opinions please. Thanks!

 

[k98dave]

The rating info refers to max vehicle load at "max tire pressure" I'm curious if this is meaning "sustained" loads?

Whats more confusing is look at the ratings for full R rubber for example the Hoosier R7, the numbers are very low!

http://www.tirerack.com/tires/tires.jsp?tireMake=Hoosier&tireModel=R7
Click on Specs and scroll to 19" sizes

 

[mprichar87]

Talked to the people at Tire Rack. Looks like if the tires per axle are higher than the GAWR on the door jamb sticker you're good to go. It may be the case that the Boss came with such high rated tires just for the simple fact the were the right size & performance level for what they wanted to sell with the car. The base GT comes with tires rated at 97 and I doubt it weighs much less, if any, than the Boss does.

Front GAWR: 2200LB
Rear GAWR: 2210LB

In theory, based on this info, you could use tires rated at 84 (1102 lbs) and 85 (1135 lbs) for the F & R axle (respectively). The engineer in me wants a more margin than that, but it looks like 95 (1521 lbs) rated tires are fine for the Boss.