Speed Calculations

Many High-performance vehicles are capable of higher speeds than their speedometer can register. If the vehicle is equipped with a tachometer, its speed can estimated with the following formula:   overall tire diameter x achieved engine rpm = speed differential ratio x top gear ratio x 336 EXAMPLE: A 1989 Camaro fitted with 235/60HR15 tires, a 3.73 differential ratio, .73 overdrive fifth gear ratio, and achieving 4400 engine rpm in fifth gear. 26.10 x 4400 3.73 x .73 x 336 = 114840 914.8944 = 125.5 mph WARNING: Don’t perform calculations while driving.

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Tire Measurements

Light truck metric and P-metric passenger car tire measurements systems are the same. LT 235 / 75 R 15 / C LT Light Truck Designation 235 Section Width in Millimeters 75 Aspect Ratio R Radial Construction 15 Rim Diameter in Inches C Load Range LIGHT TRUCK FLOTATION Light truck high flotation tires evolved in the mid-seventies as lower aspect ratio tires became more popular on light trucks. The combination of lower aspect ratios and high flotation yielded better traction on sand and soft soil found in watery off-road situations. 31 x 10.50 R 15 LT / C 31 Tire Diameter in Inches 10.50 Section Width in Inches R Radial Construction 15 Rim Diameter in Inches LT Light Truck Designation C Load Range LIGHT TRUCK NUMERIC This older system is still widely used, mostly on commercial vehicles. 7.50 R 16 LT / D 7.50 Section Width in Inches R Radial Construction 16 Rim Diameter in Inches LT Light Truck Designation D Load Range

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Speed-Rating System

The speed of a tire indicates the speed category (or range of speeds) at which the tire can carry a load under specified service conditions. The speed-rating system used today was developed in Europe in response to the need to control the safe performance of tires at standardized speeds. A letter from A to Z symbolizes a tire’s certified speed rating, ranging from 5km/hr (3 miles per hour) to over 240 km/hr (149 mph). This rating system, listed below, describes the top speed for which a tire is certified. It does not indicate the total performance capability of a tire. When this speed-rating system was originally developed, the Unlimited V category of over 210 km/hr (130 miles per hour) was the top speed rating a tire could achieve. As manufactures make more tires that fit into this category, it was necessary to better regulate performance at standardized speeds to ensure safety. The Limited V category of 240 km/hr (149 miles per hour) was then created, and the Z speed rating was added as the top speed rating that a tire could achieve. Always consult the manufacturer for the maximum speed of Unlimited V and Z tires. Speed rating is identified as a part of the tire’s sizing or service description. For example, as part of the tire’s sizing it appears as follows:  OLD SYSTEM 205 / 60 H R 15 205 Section Width Millimeters 60 Aspect Ratio H Speed Rating R Radial Construction 15 Rim Diameter in Inches

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Speed Symbol

Speed Symbol  Speed (km/hr)  Speed (mph)  205A1 Section Width Millimeters5 3 60A2 Aspect Ratio10 6 A3 15 9 A4 20 12 A5 25 16 A6 30 19 A7 35 22 A8 40 25 B 50 31 C 60 37 D 65 40 E 70 43 E 70 43 F 80 50 G 90 56 J 100 62 K 110 68 L 120 74 M 130 81 N 140 87 P 150 94 Q 160 100 R 170 106 S 180 112 T 190 118 U 200 124 H 210 130 V 240 149 W 270 168 V Over 210 Over 130 Z Over 240 Over 149

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Service Descriptions

In another attempt to standardize tire designations, S, T, H and Limited V tires incorporate the speed symbol and load index as the tire’s service description. For example:  NEW SYSTEM 205 / 60 R 15 89 H 205 Section Width Millimeters 60 Aspect Ratio R Radial Construction 15 Rim Diameter in Inches 89 Load Index H Speed Rating The load index is an assigned number ranging from 0 to 279 that corresponds with the load-carrying capacity of a tire. Most passenger car tire load indexes range from 75 to 100 - although some T/A passenger tires carry more. In the above sample, the load index of 89 corresponds to a load-carrying capacity of 580 kg (1279 lbs.) at maximum inflation. The speed symbol of the tire is the second part of the service description. (See the previous section for speed categories.) The service description system is used only with tires that have Unlimited V and Z rated tires, consult the vehicle manufacturer for maximum speed. NOTE: For the time being, the speed symbol may appear in two places as the transition from the old to the new system occurs. The speed symbol may be located in the tire size between the aspect ratio and construction symbol as shown in the “Speed Rating System” section and/or as part of the service description shown.

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Performance Formulas

You can calculate nominal dimensions by using the following calculation. Individuals tires may vary slightly from these dimensions.  235   /   60   R   H   15  TO CONVERT TIRE SECTION WIDTH IN MILLIMETERS TO INCHES: Section width in millimeters ÷ 25.4 = Section width in inches EXAMPLE:  235   ÷   25.4   =   9.25" nominal section width  TO CALCULATE TIRE SECTION HEIGHT: Section width x aspect ratio percentage=section height EXAMPLE:  9.25   x   60%   =   5.55" nominal section height  TO CALCULATE NOMINAL OVERALL TIRE DIAMETER: (section height x 2) + wheel diameter = overall tire diameter EXAMPLE:  (5.55 x 2)   +   15   =   26.1" nominal overall diameter  TO ESTIMATE TIRE REVOLUTIONS PER MILE: 20,800 ÷ overall tire diameter = tire revolutions per mile   *This section width is correct, assuming the tire is mounted on a wheel width that is 70% of the section width as specified by the Tire and Rim Association for measurement. In this case the rim would be 6.48” wide. Since a tire’s section width changes 0.2” for every 0.5” change in rim width, this change must be added or subtracted from the cross section width when alternate rim widths are being considered. The tire’s section of 9.25” would be virtually unchanged when mounted on a 6.5” wheel, and would further increase to 9.85” when mounted on an 8.0” wide wheel.

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Standard Wheel Terminology

PILOT Nominal diameter of center hole that fits over hub or axle. RIM DIAMETER Nominal diameter measured from bead seat. REVERSIBLE Wheel can be mounted with either side of the disc face to the hub face. NON-REVERSIBLE Wheel will mount with one side of the disc face only. CHAMFER Breaks in the sharp edges of mounting bolt holes to assist in assembly. SAE Society of Automotive Engineers. When a wheel conforms to published SAE specifications, it is assigned a number. RIM WIDTH Nominal width measured between rim flanges. BEAD SEAT Position where tire rests on inside of rim and seals. CENTER LINE One half of rim width. CAPACITY Load rating given to a wheel or hub package, based on single wheel application. NOTE-dual, tandem, or walking beam loading NOT included. Capacities are based on applicable SAE standards. INSET/OUTSET Inset is positive offset, in which the center line of the wheel is inboard from the mounting face of the disc. Outset in negative offset, in which the center line of the wheel is located outboard from the mounting face of the disc. BACKSPACING Also called rear spacing. Distance from extreme back edge of rim, not bead seat, to the mounting pad. To measure back spacing, lay a straight edge across back edge of rim and measure down to the mounting pad. Nearly all custom wheels use this measurement method to indicate the location of the mounting pad in relation to the rim. BOLT CIRCLE Also called bolt pattern. The diameter of an imaginary circle drawn through the center of each lug hole or stud.

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