Load rating Then we come to load and speed rating. Load rating, is easy, you find it on the placard, and any tyre you fit must meet or exceed that load rating, which is never a problem as any 4x4 tyre is going to be heavy-duty. A note about GVM upgrades, which involve an axle rerating, though: “Where a vehicle has its GVM re-rated, the tyre load capacity must be capable of carrying the revised GVM, both in total and across individual axles.” Speed rating Next is the speed rating, and here we have a problem. The favoured 4x4 tyre will be heavy-duty, light-truck construction with deep tread blocks and an open tread pattern. This does not make for a high-speed tyre, so it is entirely possible that such tyres may have a lower speed rating than the placard. Happily, VSB14 makes provision for this, saying that if a vehicle has “special features for off-road use” then it need only fit N, or 140km/h rated tyres, regardless of the placard. And just in case it wasn’t clear, VSB14 also says the tyres must be “rated by the tyre manufacturer as being suitable for road use”. Replacing tyres Ideally, you want to replace five tyres at a time, yes, including the spare, so it’s matched and doesn’t get too old. Especially with AWD 4X4s, which are those which are constant-4X4, you want to replace at least four tyres at a time. The reason is that you’ve got three differentials to consider: front axle, rear axle and centre. These differentials have two output shafts and complex gears, which don’t do anything until there is a difference in speed between the two shafts, such as when you’re turning. Another reason for shaft speed is tyre diameter differences, such as when you run a new tyre and an old tyre on the same axle. The difference in diameter might be 2-5% just due to worn tread blocks, and that then means the differential will be doing things it was never designed to do, and will fail early. So, you want to keep all four tyres with similar tread levels over the life of the car. For part-time 4WDs, those that run 2WD on the road, this is less important as there is no differential between front and rear axles, and when the car is in 4WD, there will be little enough traction such that a bit of tyre diameter difference doesn’t matter much. So there you have it So there you have it. You need to ensure the diameter increase and width increase are within limits, the load rating is at least that of the placard, and the speed rating is at least N. Read your state’s regulations, which probably refer to VSB 14, Section LS, then take that knowledge with you when shopping. Enjoy your new rubber! Credit: Robert Pepper - Unsealed 4X4.
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