Monday, August 1, 2011

Truck Tire Load Ranges

Read the small print on tires lately?  Thought not.  The only thing it has in common with a good spy thriller is the indecipherable code.  But all that verbiage molded into the sidewall has meanings. and most of it is important.  Let's discuss one aspect of the mini-novel in rubber: Tire Load Rating. 

The Basic Math - If the load rating of the tire is too low, it can overheat or even self-destruct.  The basic formula for approximate load capacity of each tire uses the Gross Vehicle Weight (GVW), divided by four (dual rear wheels are slightly more complicated).  If the gross weight of the truck is 6,000lbs, then each tire must support at least 1,500 pounds.  However, that's only for an empty truck.  No driver, no payload.  Unless that vehicle is going to be part of a static display in a truck museum, it's going to need tires with a higher load rating. 

The Real World.  As a real world example, let's look at a type of tire often used for application such as a 2011 F-350 Super Duty Chassis equipped with dual rear wheels and a landscape dump body: LT245/75R17E.  That's a light truck tire, 245/75 size and aspect ratio for a 17 inch wheel.  The 'E' is for Load Range E (LRE)  The only problem is that information molded into the tire wall probably doesn't read exactly the same as the catalog specification.  The tire description can include other numbers, and the load range index is likely located elsewhere on the tire.  Confused yet?

The six tires on this particular F-350 have large sidewall markings from BFGoodrich; LT245/75R17 121/1188 M+S.  Those other digits are material for a future article, covering sipes (sounds like a device for milking reindeer, but isn't) to section widths.  Today the topic is tire load range.  Look at a line of the text near the wheel rim and you'll notice that the missing inform is spelled out: LOAD RANGE E.  The "E" designation also indicates a 10ply rating and on our BFGoodrich tire example, a maximum inflation of 80 psi.  It is important to check with the tire manufacturer as tires of the same ply rating may require different inflation pressures.  A ply rating is the equivalent strength compared to outdated bias ply tires.  Also note that any tire pressure listed on a sidewall is the maximum allowed.  The normal tire pressure is shown on the vehicle's Safety Compliance Certification Label, located on the left front door lock facing or on the door latch pillar.