315.70 tyres not rated at 7.5t

That Michelin chart is referring to an individual tyre’s rating. So if you managed somehow to impose 7.1 tonnes on your wheelbarrow’s single tyre then you would need it to have load index rating of 176.

This issue is nothing new it even goes back as far as the 900x20 and 1000x20 tubed tyres which dominated the market. If you look at section 7 of the HGV testers manual there are charts displaying the load rating of various tyres. Historically there were three different ply ratings available for both these sizes: 10,12,14 ply for a 900x20 and 12,14,16 ply for a 1000x20. As C&U regulations changed allowing higher gross and axle weights so became the need for higher rated tyres as front axle weights increased from the quite common 5 imperial tons to 6 tons and 16 ply tyres. The dead split 16ton 4x2 rigid offered zero tolerance for load position and was a sure fire stop to checkweigh at roadside. Some chassis offered the chance of a 6.5 ton front axle if tyre size went up to 1100x20 16 ply, however this was a noticeably larger tyre increasing overall height - very noticeably so on vehicles which had run on 900x 20 tyre only a few years before.

Move on to the low profile tyres which at least partially overcame this problem and once more various rated 275, 285, 295, 305 and 315 stroke 70 or 80 tyres became common. In recent years there have again been increases not only in front axle plated weights, but also in the weights of axle 2.