*Rule Map–>New Consumer Tire Label–> RATING FORMULAS*

From focus group studies, NHTSA learned that 0-100 rating scales are preferred by US consumers.* Related issue for comment*: graphics and other elements of label design format.

**Proposed Fuel Efficiency Rating Calculation**:

*RFE = (RRFmax – RRF) * 100/(RRFmax – RRFmin) *, where

*RFE* is the particular tire model’s fuel efficiency rating; *RRFmax* is the highest rolling resistance rating on the scale, or 25 lbf ; *RRF* is the rolling resistance rating of the particular tire model, determined in accordance with the specified test procedure and metric; and *RRFmin* is the lowest rolling resistance rating on the scale, or 5 lbf.

NHTSA set *RRFmax* and *RRFmin* based on two sets of tire testing data: its own study and a study done by the California Energy Commission. It picked an *RRFmax* value slightly higher than the worst performing tire studied (22.8 lbf) to cover possible untested models. It picked an *RRFmin* value slightly lower than the best performing tire studied (7.5 lbf) to allow for manufacturing improvements over time. Filling in these values, the formula is R*FE=(25-RRF)*100/(25-5) = (25-RRF)*5*

NHTSA calculates that this rating formula yields the following consumer rule of thumb: Every 5 points on the scale translates to a fuel saving of 3 gallons/10,000 miles per 4 replacement tires. *Related issue for comment*: use of RRF as the rolling resistance metric.

**Proposed Safety/Wet Traction Calculation**:

*RTC = ((µAPA + µAPC)(1-[(µAPA-µAPC)/(µAPA+µAPC)]2)-0.6)*(100/2.0)*, where

*µAPA* = (Measured Candidate Tire Average Peak Coefficient of Friction for Asphalt + 0.75) – (Measured Standard Tire Average Peak Coefficient of Friction for Asphalt) and * µAPC = *(Measured Candidate Tire Average Peak Coefficient of Friction for Concrete + 0.60) – (Measured Standard Tire Average Peak Coefficient of Friction for Concrete)

The formula produces a single rating for both asphalt and concrete, normalized to a 0 to 100 scale. It continues the current traction rating philosophy of UTQGS, which penalizes a tires rating if either the asphalt or concrete coefficients are in a lower relative category than the other. *Related issue for comment:* use of adjusted peak coefficient of friction testing from UTQGS

The constants are based on NHTSA test data for the adjustment of the average peak coefficient of friction for asphalt (.75) and concrete (.60) pavements. The agency particularly seeks comment on the use or change of these constants.

Data from NHTSA’s test facility shows an actual range of 0.7 to 2.3 for Adjusted Peak Coefficient of Friction. To allow room in the rating scale for anticipated technical improvements in traction, NHTSA proposes to expand the range so that 0.6 = 0 on the new rating scale, and 2.3 = 100. The agency particularly seeks comment on this derivation of the 0-100 scale, which would place the current actual range between 5 and 85 on the new scale, including how to expand the rating scale to allow for future potential traction improvements.

NHTSA anticipates new data once the skid pad surfaces on its San Angelo Test Facility are repaved. It plans to update the formula with these data in the final rule, “to bring the rating scale into a range that can be expected for state of the art tires.”

NHTSA does not anticipate a consumer rule of thumb to convert differences on the traction rating scale to a quantifiable percentage or dollar increase in safety, because of variations in handling characteristics of vehicles, force of brake application, and load.

**Proposed Durability Rating Calculation**:

*RTW* = *TWUTQGS* /10, where

*RTW* is the particular tire model’s tire wear rating*, andTWUTQGS* is the existing tread wear grade as determined for the UTQGS

This formula simply converts the UTQGS range of up to 800 onto the planned 0-100 scale. NHTSA considers consumer meaning to be “straightforward, as the tread wear rating is a relative rating compared to a control tire, which would be rated 10 on our scale. A tire rated 20 should last twice as long as a tire rated 10.”

*Related issue for comment*: Manufacturers could decide whether to rate tires by actual testing or, instead, by extrapolation from data collected on other models and sizes.

Tags: rating formulas