Changing Tire Size and Load Ratings

[Calypso]

I recently attempted to buy tires to replace the standard Goodyear Territory MT's on my '23 Heritage 2 door. I am looking to reduce the weight and size of the tire/wheel combo, and had settled on 255/75-17 (standard on non-Sasquatch Big Bend). I do not intend to perform other than forest service road wheeling in my Bronco, using it primarily for everyday driving/commuting.

The Territory is the 315/70-17 Sasquatch standard tire. C2 load range, 113 load index. Max load capacity of 2,535 lbs per tire @ max 35 psi cold inflation pressure.

The 255/75-17 I'm considering is SL load range, 115 load index. Max load capacity of 2,679 lbs per tire @ max 35 psi cold inflation pressure.

Heritage rims are 17 x 8.5", appropriate for the 255/75 (typical max width 9" for P-tires. 8.5" for LT)

The sales rep is telling me that they will only install a tire with a C2 load range or greater (D, E etc) because that is the OEM specification for the Heritage, and in consideration of the intended load capacity of the vehicle. I pointed out that the 255/75 is standard on the Big Bend model, even when equipped with the optional rear locker, electronic transfer case and tow package. As I see it, the essential difference in the weight of the vehicle would be that added by the front locker, a minimal increase compared to the overall vehicle weight plus payload.

I also pointed out that the capacity of the SL tire was greater than that of the OEM tire, and the rep countered that the load indexes are not comparable because the tires are of different load ranges. I have never seen any discussion of load index that implies the 'apples-to-apples only' limitation of load range relative to load index.

I offer my questions to the forum:

Am I uneducated about tire ratings, or correct in thinking that weight is weight, and it doesn't care what tires are supporting it?
Is it appropriate to only compare load index without consideration of the load range?
Are there other safety or liability considerations in selecting a non-OEM tire size for replacements?

I am here to learn. Thanks in advance for your replies!

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[kodiakisland]

If the SL is a P-metric tire and the load rating of the vehicle is based on a LT tire, then they are not apples to apples comparison, if that is what you are asking. Reduce the load rating of the P-metric by 10% to equal the LT load rating.

P-Metric or hard metric tires on Light Trucks
When a P-metric or metric tire is installed on a light truck (SUV, pickup, minivan), the load capacity of the tire is reduced by a factor of 1.101 as prescribed by the Federal Motor Vehicle Safety Standards (FMVSS). For example, 305/50R20 has a maximum load capacity of 3086 lbs. If this tire is fitted to a light truck, then the actual allowable load capacity for the tire is 2805 lbs. (3086 lbs. divided by 1.1). If you replace the original tires with the exact same type (P-metric, hard metric, LT-metric, or flotation), size designations, and ply as the tires that were originally installed, just follow the vehicle’s tire information placard for proper inflation pressures. If, however, you apply a ‘Plus zero’ or plus-1, etc., fitment to a light truck, you must discount the replacement tire’s load by the 1.10 factor and ensure that the replacement tire has sufficient load capacity by inflation to support the load of the originally installed tires.

application_of_load_inflation_tables_20200723.pdf (toyotires.com)

In your example, the tire you want would be rated below what is recommended based on GVWR. That's not to say you could not get away with a tire rated lower if you are always well below GVWR, but I understand why a tire shop wouldn't want to do that. A lot of smaller tire shops don't care, so you might try somewhere else if that's what you really want to do, but again stay below GVWR if that's the route you take.

 

[Calypso]

OK! That explains why the particular 255/75 would not qualify as a replacement after de-rating.

I'll need to go to a 117 load index (2833 lbs) to exceed the OEM load rating.

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