Load Ranges, Service Descriptions, and Upsizing.
I have addressed service descriptions before, but I want to combine this with load ranges today to dispel some of the misconceptions about these items.
First, what is a load range? Simply put the Load Range is synonymous with ply rating. Long ago, tires would have several plies of belting material, for example 6 belting plies gave us a 6 ply tire or C load range. Simple. Not so much anymore. You will see six plies in a tire, but what about a load range E or 10 ply tire? This tire might have actually had 10 plies back in the day of bias construction, but not anymore. Tires are made with less material these days and don’t require the additional belting material. Thus, a load range E 10 ply rated tire is just that rated at 10 plies. See what Toyo has to say in the following info-graphic.
Second, What is the service description? The service description is made of two components, the speed rating and the load index. The speed rating tells us how fast we can go on the tire and the load index tells us how much weight the tire carries. The service description looks like this – 123R. A tire with this description will have a max load per tire of 3417 lbs and a top speed of 106 MPH. When replacing tires, you can increase both*, but it’s recommended not to downgrade the speed rating or the load index. Otherwise, you lower the top speed the vehicle with a lower speed rating, and when you downgrade the load index, you take capacity away from the vehicle.
A rule of thumb in the industry has always been, if its bigger and has the same load range it will carry more weight and will work. Let’s take a look at how easy it is to unwittingly downgrade the load capacity of the truck based on this outdated thinking.
Example Vehicle: 2012 Ford F-250 Super Duty with 20 inch wheels
Original Tire: LT275/65R20 E 126S
Height: 34 inches tall
Replacement Tire: 35X12.50R20 E 121Q Open Country M/T
Height: 35 inches tall
So, why would I say this is tricky? Because when a person like myself sees the two sizes and load ranges, we would automatically think the 35 will work because it’s bigger. However, we see from examining the Load Ranges and Service Descriptions that this not the case.
Think it’s not a big deal and its close enough? Toyo, in an example like this, would not cover the tire for warranty since it has been miss-applied. So make sure you’re looking at not just the Load Range but also the Service Description when upsizing your vehicle and choose the right tire.
*Increasing the load index or ply rating of a tire does not increase the load capacity of the vehicle. It only increases the load capacity of the tire.