Summary of Research on Load Index
1. The essence of science is description and replicability – we should be able to accurately describe phenomena – including the “formula” for describing the process of determining the “acceptable” (safe?) load for a passenger tire. Any recommendations by “experts” should include an explanation that allows “others” to verify facts. An “opinion” of an expert is just that (so an art not a science), unless it can be verified.
2. Based on the internet research I have done in the last few hours (contained below) it is actually quite simple to determine an appropriate (“safe”) load for a Base MINI Cooper. Here is the formula:
Weight of MINI (without passengers) + weight of cargo including passengers divided by 4 (wheels) = maximal load at 36 PSI: this value should be looked up in an appropriate chart to find the load index value.
Calculations for a 2012 Base MINI Cooper (manual):
2535 lbs. + 195 lb. driver + 150 lb. passenger + 50 lb. cargo (estimate) = 2930 lbs.;
2930 divided by 4 = 732.5 lbs.
732.5 lbs. = load index of 74 (rounding UP to 739 using ISO ETRTO Table at 36 PSI)
*Iike to inflate my tire pressure to 33 lbs. so the load index needs to be shifted to compensate – to a load of 77 (rounding up to 750 lbs.).
The load index on my desired tires (TOYO Proxes T1-R 195/4515) is 78 – or 827 lbs. per tire (3,308 lbs vs. MINI weight plus cargo of 2,930 lbs. – so a margin of error of 378 lbs. – i.e., I could carry an additional cargo of 377 lbs. safely) – provided I stay under 210 KPH (130 MPH) which should be no problem as the Base Cooper’s maximum speed is 126 MPH – or I stay under the speed rating of the tire (which is associated with the load index for safe operation – V = 140 MPH).
3. At this point we can examine the load index of the OE 175/65R15 tires that come with the Base Cooper – 84 -or 981 lbs. per tire – 3,924 lbs. total – so a margin for error of 994 lbs. using our example weights above. This margin appears excessive, but if the MINI was an automatic (77 lbs. heavier) and the cargo included four 200 lb. passengers (plus two 50 lb. suitcases) for a total of an extra 532 lbs., the margin of error is reduced to 412 lbs. seems in line with the U.S. preoccupation with liability/lawsuits/recalls, etc. and therefore not unreasonable. TireRack refers to manufacturers making adjustments due to the fact that vehicles may be “occasionally overloaded” (but not in my case as I am going to do a rear seat delete – so no extra 400 lbs. of passengers!):
“P-metric tires used on passenger cars and station wagons are rated to carry 100% of the load indicated on the tire’s sidewall (or listed for the tire in industry load/inflation charts). However, if the same P-metric tires are used on light trucks, (pickup trucks and sport utility vehicles for example), their carrying capacity is reduced to 91% of the load indicated on the tire’s sidewall. This reduction in load results in causing light truck vehicle manufacturers to select proportionately larger P-metric sized tires for their vehicles to help offset the forces and loads resulting from a light truck’s higher center of gravity and increased possibility of being occasionally “overloaded.””
In my opinion, every Tire Tech person should be able to explain this and also explain the formula and give practical examples (as I have done here) of the process of determining load index for a particular vehicle.
Not much use:
“Load Index (from http://www.tireteam.com/info_load_index )
Load index indicates the amount of weight a tire can safely support. The higher the index number, the greater the load-carrying capabilities. In order to calculate your vehicle’s maximum carrying capacity, multiply your tire’s load index grade by four. Average load index scores for passenger cars and light trucks range from 75 to 105.”
Better (from http://www.tire-information-world.com/load-rating.html ):
A tire’s load rating, sometimes signified by the load index code is the amount of weight that that tire is designed to carry within a maximum speed…
The maximum speed which the tire is designed to operate at it’s speed rating and the two are closely linked together –each affecting the other. (Read the linked article to learn how, if you’re not certain.)
Usually you even see the two indexes noted together when they’re shown on the tire sidewall or written in technical descriptions of specific tires as in
In the above example the three characters at the very end are what interest us. The load rating of that tire is shown by the code 89 which, if you’ll look in the table below is 1,279 pounds or 580 kilograms. That is the maximum load, or weight which the tire is designed to be used with when operated at a speed no greater than the “S” speed rating indicates (112 mph).
If your needs are greater than wither of these indexes or ratings state, then you should be shopping for tires with ratings that at least meet what you expect to be using, otherwise you’re inviting a problem to happen and, as most problems tend to do, likely at the most inconvenient time.
The load rating you need in a tire is calculated by adding up the total weight that will be carried on each axle and dividing by the number of wheels on that axle. On a passenger vehicle such as a car or truck that’s usually 2, but, as you’ve noted some vehicles will have 4 wheels across (and a very few specialty vehicles have only 1).
When you know the maximum weight that each tire on an axle is expected to carry you can then consult the table below to learn the load index you need. In general terms, there is no harm in using a load rating which is greater because you are left with a larger margin of safety, but if you go far beyond what you normally expect to use on a regular basis, you may end up at least paying more for your tires than you need to since a greater load rating tends to be more costly than a tire with a lower rating.
TIRE LOAD INDEX
The load index indicates the load-carrying capacity of a tire. In other words, it tells you how much weight your tire can support. 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. Never install tires with a lower load carrying capacity than the tires originally installed on your car in the factory.”
Falken indicates a more refined calculation of load using the actual tire pressure run on the tires (36 psi being the pressure used to calcluate posted load indexes – i.e., if you use 33 you should recalculate the load index): http://www.falkentire.com
Their chart only starts at 80 – at 80 the difference between 33 and 36 psi is about 75, so 4 X 75 = 300 – so in our MINI example if you actually inflate tires to only 33 lbs. then load capacity should be reduced by 300 lbs. (total) or 75 per tire.
Although we should not buy tires based solely on load index ratings, this link explains how to simply figure out the load carrying capacity you need: http://www.ehow.com/how_2383883_buy-tires-based-load-ratings.html
This Pirelli link affirms that the load index is STATIC – in other words, refers to the car at rest – unless speeds exceed 210 KPH: http://www.pirelli.com/tyre/ww/en/car/genericPage/all_about_tyres
THE LOAD INDEX
The Load Index is a numerical code associated with the maximum load a tyre can carry (except for loads at speeds above 210 Km/h) at a speed indicated its Speed Symbol under service conditions specified by the tyre manufacturer.