[Nine Ball]

Guys, pay attention to the aspect ratio on those tire sizes. The second number in the size.

305/35-20 and 255/35-20

That 35 is a percentage number, known as "Aspect Ratio". It means the sidewall thickness is 35% of the tire width. On a wider tire, that makes the sidewall taller, too. From your example:

305mm x .35 = 106.75mm sidewall height
255mm x .35 = 89.25mm sidewall height

Total Tire Height is calculated by:

(2 x sidewall) x (0.039 to convert to inches) + wheel diameter inches

Your example:

(2 x 106.75) x (0.039) + 20 = 28.32 inches tall tire
(2 x 89.25) x (0.039) + 20 = 26.96 inches tall tire

Big difference. These two need to be within 0.5 inches of each other.

The correct sized front tire would have been 255/40-20, which is 28.0 inches tall.

Tony