JB.
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...to match the load capacity of a P-metric tire.*
*A Standard Load tire (SL), usually P-metric (or metric, euro-metric -- a nearly identical twin). This is the tire found on nearly every new car, SUV, and Light Truck. On the sidewall it looks like P265/70/17 (or 265/70/17) as opposed to LT265/70/17.
How much more pressure?
This continues to come up and is often a source of confusion.
Not surprising though, considering that both of the following statements are true:
LTs carry less load than P-metric.
LTs carry more load than P-metric.
Throw Load Range into the discussion and things get nice and tangled. Describing the relationships in words doesn't paint a clear picture so I thought I'd try to make some -- pictures, that is.
The charts are based on tire size 265/70/17 and, in the 2nd chart, how it relates to the 5th gen 4Runner because that proved easiest, but the application to other generations is obvious and not too dissimilar. The curves should be reasonably accurate and more than good enough to illustrate the point. P-metric tires get load capacity de-rated 9% when used on an SUV and the curves reflect that. The tables used for reference come from this.
Hopefully it's easy to see how an LT can carry both less and more load than P-metric. It's all about air pressure and, for this tire size, it takes about 12psi added to the LT to match the load capacity of the P-metric tire. Hopefully it's also clear how pretty much the only distinction that matters is between P and LT. Load Range doesn't meaningfully affect the choice of air pressure for LT -- unless very high pressures are desired -- since the the longer load E curve is overlapped by the shorter D and C curves.There may, however, be implications for speed capability in the selection of load range. See Baldwin here.
Extending the line for LT down past where it ends (because load tables stop there) in this chart might seem to imply that making a pressure adjustment when switching to LT isn't important since it appears there would still be load reserve at 32psi even if no additional pressure was added, but that doesn't mean there's no benefit to adding more pressure (leaving it at 32 would be the equivalent of around 20psi in the stock tire). For one thing, the curve bends more as it goes down the chart towards zero psi, so calculating it as a straight line overestimates load capacity. Secondly, having plenty of load reserve is desirable since tires change over time -- and they don't get stronger. Lower pressures result in more heat which speeds the process. The cumulative effects of dynamic loading, speed, heat, age, hard use -- not to mention the inevitably lower speed rating in an LT -- are all good reasons the load reserve established by the factory tire at recommended pressure should be given due consideration. If nothing else, the level where the tpms triggers should be a good clue to that. Most of this is covered in the doc already linked. There is also the option to Ask a tire engineer - Toyota 4Runner Forum - Largest 4Runner Forum. The horizontal lines on the P-metric tires represent the ability to add pressure for special conditions (like high speed) but the extra psi assigns no additional load capacity. The language in load tables indicates that extra pressure for LTs at high speed is recommended, as well, and at lower speed thresholds than P-metric. The capability of a tire in both load and speed necessarily overlap each other.
Tire size makes a difference. More air equals more load capacity, whether it's a bigger tire or more pressure in a given tire. The grey curve extends the full distance, so it would be Load E
*A Standard Load tire (SL), usually P-metric (or metric, euro-metric -- a nearly identical twin). This is the tire found on nearly every new car, SUV, and Light Truck. On the sidewall it looks like P265/70/17 (or 265/70/17) as opposed to LT265/70/17.
How much more pressure?
- 10-15lbs, if an LT is switched for a P-metric tire with the same dimensions.
- LT load range (C,D,E) doesn't affect this.
- Tire size does.
This continues to come up and is often a source of confusion.
Not surprising though, considering that both of the following statements are true:
LTs carry less load than P-metric.
LTs carry more load than P-metric.
Throw Load Range into the discussion and things get nice and tangled. Describing the relationships in words doesn't paint a clear picture so I thought I'd try to make some -- pictures, that is.
The charts are based on tire size 265/70/17 and, in the 2nd chart, how it relates to the 5th gen 4Runner because that proved easiest, but the application to other generations is obvious and not too dissimilar. The curves should be reasonably accurate and more than good enough to illustrate the point. P-metric tires get load capacity de-rated 9% when used on an SUV and the curves reflect that. The tables used for reference come from this.
Hopefully it's easy to see how an LT can carry both less and more load than P-metric. It's all about air pressure and, for this tire size, it takes about 12psi added to the LT to match the load capacity of the P-metric tire. Hopefully it's also clear how pretty much the only distinction that matters is between P and LT. Load Range doesn't meaningfully affect the choice of air pressure for LT -- unless very high pressures are desired -- since the the longer load E curve is overlapped by the shorter D and C curves.There may, however, be implications for speed capability in the selection of load range. See Baldwin here.
Extending the line for LT down past where it ends (because load tables stop there) in this chart might seem to imply that making a pressure adjustment when switching to LT isn't important since it appears there would still be load reserve at 32psi even if no additional pressure was added, but that doesn't mean there's no benefit to adding more pressure (leaving it at 32 would be the equivalent of around 20psi in the stock tire). For one thing, the curve bends more as it goes down the chart towards zero psi, so calculating it as a straight line overestimates load capacity. Secondly, having plenty of load reserve is desirable since tires change over time -- and they don't get stronger. Lower pressures result in more heat which speeds the process. The cumulative effects of dynamic loading, speed, heat, age, hard use -- not to mention the inevitably lower speed rating in an LT -- are all good reasons the load reserve established by the factory tire at recommended pressure should be given due consideration. If nothing else, the level where the tpms triggers should be a good clue to that. Most of this is covered in the doc already linked. There is also the option to Ask a tire engineer - Toyota 4Runner Forum - Largest 4Runner Forum. The horizontal lines on the P-metric tires represent the ability to add pressure for special conditions (like high speed) but the extra psi assigns no additional load capacity. The language in load tables indicates that extra pressure for LTs at high speed is recommended, as well, and at lower speed thresholds than P-metric. The capability of a tire in both load and speed necessarily overlap each other.
Tire size makes a difference. More air equals more load capacity, whether it's a bigger tire or more pressure in a given tire. The grey curve extends the full distance, so it would be Load E
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