Where the rubber meets the road
From the October 2020 print edition
Drivers of fleet vehicles rarely think of the tires their cars run on until there is a problem such as a flat. However, all drivers should be trained on basic functions and how to inspect their tires because they are an important part of the maintenance team. Fleet managers should consider that tires are one of the top vehicle costs and educate themselves on the variety of manufacturers, models and types of tires.
Functions of tires
Tires have several important functions beyond the obvious of providing traction so a vehicle can move down the road and change direction. Tires are an important part of a vehicle’s suspension system and knowing how they support loads is essential for drivers of pickups and trucks. Tires also help to dampen shocks from uneven road surfaces, potholes, hazards and so on. Finally, tires also work with a car’s brakes to stop a vehicle quickly and safely. Brand new brakes will struggle to stop your car if the tires are bald.
Picking the correct tire requires you to be able to match the tire to the type of vehicle and its service. To do this you need to know how to decipher tire jargon, codes and sidewall information. Consider a tire that has the markings LT255/40 R17 XL 94W M&S. These codes mean the following:
- LT denotes the tire type. In this case Light Truck. P would stand for Passenger, T for spare, and ST for Special Trailer.
- 255 denotes the section width of the tread in millimeters from sidewall to sidewall.
- 40 refers to the height of the tire in terms of the aspect ratio, meaning percentage of sidewall height to section width.
- R denotes the tire’s construction as a radial, which applies to most tires these days.
- 17 refers to the rim diameter in inches
- XL means the tire has been reinforced to provide a higher load capacity.
- 94 indicates the load index, which can be found online. A tire with 94 it can support 670kgs.
- W is the maximum speed rating of the tire, 270kph in this case.
- M&S stands for mud and snow, meaning this is an all-season tire. Note this is not the same as a winter tire specially constructed for ice, snow and low temperatures. These tires will have a winter tire symbol (mountain with snowflake inside).
Tires also have a serial number, known as a DOT number (for the United States Department of Transportation). This number provides the manufacturer and plant the tire came from, as well as the date of production. The last four numbers indicate the week and year of manufacture. For example, 2018 would indicate that a tire was made in the 21st week of 2018.
All of this information is valuable for fleet managers to use when specifying new vehicles and especially when replacing tires on vehicles already in the fleet. For instance, many pickups today come standard with passenger (P) rather than light truck (LT) tires. Passenger tires provide a smoother and quieter ride but are not the right choice for pickups that will be used for hauling and towing heavy loads.
Tires have to be inspected and maintained to provide good performance. The primary focus for tire maintenance should be ensuring proper inflation. Under-inflated tires perform poorly, create more rolling resistance which impacts fuel economy and can lead to failure on the road.
All vehicles sold after 2007 are equipped with a tire pressure monitoring system (TPMS). This safety feature was mandated by governments because drivers do such a poor job of checking their tires and this was leading to accidents. The TPMS provides a visual warning when a tire falls below a recommended range – this range is set at 25 per cent by law but can be less depending on the car manufacturer.
The TPMS provides a valuable failsafe for monitoring tire pressure but should never replace regular air pressure checks. Tires that are underinflated by just five per cent begin to suffer from decreased fuel economy and increased internal temperatures. These issues become more pronounced as underinflation increases, so tires can be significantly underinflated without triggering a TPMS alert.
Drivers of fleet vehicles should be asked to check their tires at least on a monthly basis. The proper air pressure can be found on the driver-side door jamb of a vehicle. The maximum pressure that may be listed on a tire should never be used. Fleet managers should equip all of their vehicles with a quality digital or dial pressure gauge. Besides air pressure, monthly inspections of tires by drivers should include tread depth, any uneven wear, imbedded objects and sidewall cracks. Any issues in these areas should be immediately reported to the fleet manager.
Tires that have been properly maintained will provide service for tens of thousands of kilometres. However, all tires wear out and eventually have to be replaced. The primary reason tires have to be replaced is that the tread has worn down to an unsafe level. The legal limit of wear is 1.6mm or 2/32nds of an inch of tread depth. But most tire makers recommend tires be replaced before the legal limit of wear is reached to ensure better performance such as in wet weather. Many fleets will replace tires before the tread reaches 3.2mm or 4/32nds of an inch.
Tires can also wear out from shear age as the rubber and other compounds deteriorate. Michelin recommends replacement of tires after 10 years. Other manufactures have lower standards, particularly for severe duty such
as hauling heavy loads and running off road.
When buying new tires, fleet managers should start by considering if a passenger or light-truck tire is needed. Passenger tires are fine for most vehicles, including many SUV and pick-up trucks. These tires provide higher comfort ratings, lower noise levels and better fuel economy. Some pick-up and SUV’s require a more robust light-truck tire for hauling heavy loads and driving on rough terrain.
When replacing tires, fleet managers should do their homework about which tires and tire brands best fit their operations, including geographical area, weather conditions, service needs, driver habits and budget. Some tire manufacturers have better reputations for quality and longevity than others. This quality comes with a higher price, of course.
Fleet managers should also consider ratings for tire treadwear, traction and temperature. These ratings, which are most useful to compare models within – rather than between – manufactures, can be provided by your tire dealer. Also plan to have new valve stems installed when tires are replaced.