Tires: TIRES, rubber compounds, inflation pressure

Question
HI,WE RUNNING A FLEET OF 80 TRACTOR UNITS AND 93 THREE AXLE TRAILERS.WE ARE CURRENTLY RUNNING ON 12R 22.5.HOW CAN YOU COMPARE 315/80/22.5 TO THE 12R IN TERMS OF IMPROVED MILEAGE,REDUCED FUEL CONSUMPTION,ETC

Answer
Kepha,

This is a question that should be asked of your tire suppliers as wear / fuel economy, etc. are only loosely tied to tire size and are more tied to the properties that the tire manufacturer designs into the product.

But a 12R22.5 Load Range G has a load carrying capacity of 6610# at 105 psi, with a width of 11.8" on a 9.00" rim, and an overall diameter of 42.7" (highway tread)

By contrast, a 315/80R22.5 Load Range G has a load carrying capacity of 6940# at 95 psi, with a width of 12.3" on a 9.00" rim and an overall diameter of 42.4" (highway tread).

Everything else being equal (which they usually aren't), the fuel economy of a 315/80R22.5 will be 1)  worse because the tire is wider, 2) better because the  load carrying capacity is higher for the same inflation pressure, 3)  worse because it is smaller in diameter.

Of these, #2 is probably the most significant, so I would expect overall the 315/80R22.5 to be better - all other things being equal.

But like I said, the way the tire is built - what rubber compounds are used - is going to have more of an effect than the size itself.