[CapriRacer]

1) In addition to the Green Seal report being quite old (2003), the report also does not account for the different tire sizes. The rolling resistance of otherwise identical tires varies according to tire size - as does rolling resistance coefficient.

The tire size vs rolling resistance (and rolling resistance coefficient) of tires has not been explored enough to be able to predict the RR (or RRC) from a single, but different, tire size.

2) Speed Ratings: There is a test that a speed rated tire has to pass. This is not a "test the tire, then assign the rating" sort of thing. It's a "this tire has this speed rating, so it passed the test" sort of thing.

Among the testing conditions specified are the rated load and the rated inflation pressure. The rated load is the one listed on the sidewall, BUT the rated inflation pressure is 35 or 36 psi for Standard Load (SL) tires and 41 psi for Extra Load (XL) tires. In other words, the rated pressure has nothing to do with what pressure appears on the sidewall. What is written on the sidewall is a MAXIMUM.

However, the load on the tires on a given vehicle and the inflation pressure used for that vehicle are not the same as the test conditions. Therefore the actual speed capability of a tire used in a particular situation may not be the same as the speed rating.

This is an area that is not well explored. While it is known that it is the standing wave that limits the speed capability of a tire (and not the heat being generated), the exact effect of inflation pressure vs load is not well understood - too many variables - except to say that both lower inflation pressures and higher loads reduce the speed capability of a tire.

We also know that there is an upper limit to inflation pressure - the burst pressure. Clearly the burst pressure is dependent on the strength of the materials in the tire's casing. Put another way, the stronger the casing, the higher the burst pressure.

- BUT -

Increased speed also means increased stresses, and therefore a stronger casing will be needed for higher speed ratings. The exact nature of the "stronger casing" is a trade secret for each tire manufacturer - but it is common knowledge that a circumferential overlay on top of the belt greatly enhances the speed capability of a tire (meaning that the sidewall plays very little role in speed ratings)

BTW, the maximum pressure listed on the sidewall of a tire is not a reliable indicator of burst pressure - not to mention that tires degrade over time as does the actual burst pressure. (It has been known for very old tires to burst at normal inflation pressure.)

In addition, overlays of the belt package also decrease a tire footrpint's sensitivity to inflation pressure. In other words, a tire with a cap ply maintains a more uniform pressure distribution over a range of inflation pressures.