WORKED EXAMPLE – FORMULA METHOD

ONCE you know the offset of the factory wheel belonging to your chosen diff, you can use the formula below to work out the backspace you need for any wheel width, and meet the track allowance too. Factory offsets are available in the Tyre & Rim Association Standards Manual or the Njoy Universal Tyre

Fitment Guide. Of course, you could always just measure a factory wheel. Using the most popular differential in modified vehicles, the Ford nine-inch, as an example, we know the best available factory wheel from Ford to give us the maximum track and the best wheel look has -/ 1 4 in (-6mm) positive offset. All other factory wheels were worse in the sense that they had bigger backspaces and therefore larger positive offsets.

This means that the legal wheel is a +6.7mm negative offset for a nine-inch diff, because when you add the two half-inch (12.7mm per side) increases, you get the inch over track. So the calculation is: Offset + half rim width + 12.7mm = Backspace.

Consider first our legal Ford factory offset on a 10-inch (250mm) wheel, where we know the offset is -6mm but don’t know the backspace: -6 + 127 + 12.7 = 133.7mm = standard backspace = 5/ 1 4 in.

Now add 12.7mm (half an inch) to the wheel offset to increase track by one inch (2 x 1/2in) and we get : +6.7 + 127 + 12.7 = 146.4mm = max legal backspace = 5 3 / 4 in.