TIRE/TYRE SPEED RATINGS
by GoMoG

What They REALLY Mean

SPEED RATINGS REFER TO MORE THAN JUST SPEED!!!! Contrary to their name, speed ratings are also about ride comfort, wear and cornering ability. Typically, the higher the speed rating, the better the grip and stopping power, but the lower the tread life. You can always increase the speed rating of the tires on your vehicle for improved performance, but you can never decrease it without reducing the vehicle top speed to that of the lower speed rating selected, while enduring less comfort and handling ability.

And though I can assure every one of you that any trad Morgan can be tweaked and tuned by YOU to have the most sublime suspension and confort you have ever experienced, the most significant suspension component in a Morgan suspension is the tyres you put on it. Tyre sellers and manufacturers will tell you simply to match the speed rating you have, but it is NOT their hobby, it is yours! The difference, for any Morgan, between a cheaper rating and a better one is very significant. Any mogger with a rating lower than "H" is missing a LOT OF FUN and added safety. I use a minimum of a "V" rating and prefer "W". Yes, better tyres will not last as long, but there is no extra cost that is expensive considering the added joy you get. When the Factory delivered me a car with "H" rated tyres, I swapped them out three days after and was thankful I did so. (I gave them to my daughter's boyfriend of the day who I didn't like!)

The reasons behind this fact should be obvious. The quality of the rubber improves with the higher the rating. To make a tyre last a long time, additives are used. These long life additives make the rubber harder, less road compliant, less comfortable. They cut into your fun. When I see moggers with cheaper tyres I feel bad for them. Every mogger I convince to upgrade their rubber has the same reaction. They are angry at themselves that they took so long to do so.

History of Speed Ratings

Thank Germany’s famous Autobahns for tyre speed ratings. They became logical in that environment and we should all be happy for it.

Tyre speed ratings range from A (the lowest) to Y (the highest). But the chart is not completely in alphabetical order. For example, H is between U and V, with the common perception that H stood for “high performance” at one time. As manufacturers continue to add speed to their vehicles, tyre speed ratings evolve to match the speeds. For example, Z was the highest rated speed at 149+ until W & Y were used to match the higher speeds of exotic sports cars.

Mixing Speed Ratings

Don’t. But if tyres of different speed ratings are mounted on a vehicle, the lower speed-rated tyres should be placed on the front axle regardless of which axle is driven. This is to prevent a potential oversteer condition. Vehicle handling will also be affected, and the vehicle’s speed capacity is now limited to the lowest speed-rated tyre. For best performance, it is recommended that the same size, type of tyre and age be used on all four wheel positions.

Load Index

The load index (99) is the tyre size's assigned numerical value used to compare relative load carrying capabilities PER TYRE!!! The higher the tyre's load index number, the greater its load carrying capacity and the less compliant it will be.

97 = 1,609 pounds
98 = 1,653 pounds
99 = 1,709 pounds

A tyre with a higher load index than that of the Original Equipment tyre indicates an increase in load capacity. A tyre with a load index equal to that of the Original Equipment tyre indicates an equivalent load capacity. A tyre with a lower load index than the Original Equipment tyre indicates the tyre does not equal the load capacity of the original. Typically, the load indexes of the tyres used on passenger cars and light trucks range from 70 to 110. With a hyper-light Morgan, I go for the lowest load index I can find subject to the desired Speed Rating.