<?php

$array = [
'215/75R17.5',
'235/75R17.5',
'7.01/75R17',
'8.25R16',
'7.00R16',
'11R22.5',
'7.50R16',
];

function customCompare($x, $y) {
    $x = explode('R', $x);
    $y = explode('R', $y);
    if ($x[1] != $y[1]) return $x[1] < $y[1] ? -1 : 1;
    return strnatcmp($x[0], $y[0]);
}

usort($array, 'customCompare');
print_r($array);