Your driving wheels, whether it’s a solid axle or independent, have a differential in the middle. The differential allows the wheels to turn at different speeds so you don’t skid through corners.
When you turn, more gears are turning in the differential than when you are driving in a straight line. When you put a compact spare on one side, that wheel has to turn faster than the other because it’s a smaller diameter. That makes the differential work as if you are constantly turning. More work causes more heat, and heat breaks down lubricants, and causes greater wear. If the differential has non metallic parts, such as clutches found in a Posi trac, excess heat will cause even more damage.
For that reason, manufacturers want to make sure you don’t do something that will overheat the differential.
请问：
Spare Tire P255/70R18 外径比 Ground Tires P275/55R20 多出4mm, 考虑到Ground Tires的磨损，实际差别应大于4mm. 即使有这样的外径误差，用Spare Tire应该没问题?
多谢！
Full Size Spare Tire外径、胎宽应该与Ground Tires尺寸完全一致。
难道略有误差的Spare Tire仍可称为Full Size?
有懂行的能解释一下吗？多谢！
而且厂家的eBrochure强调描述："Spare – Full-size, mounted under body"
我就不太明白：外径、胎宽不完全一致怎么可以称之为Full Size Spare?
即：如果是前驱车，Spare Tire最好装在后轮上；如果是后驱车，Spare Tire最好装在前轮上，对吧？
https://www.quora.com/Why-are-you-not-supposed-to-use-a-spare-tire-on-the-drive-wheels
Your driving wheels, whether it’s a solid axle or independent, have a differential in the middle. The differential allows the wheels to turn at different speeds so you don’t skid through corners.
When you turn, more gears are turning in the differential than when you are driving in a straight line. When you put a compact spare on one side, that wheel has to turn faster than the other because it’s a smaller diameter. That makes the differential work as if you are constantly turning. More work causes more heat, and heat breaks down lubricants, and causes greater wear. If the differential has non metallic parts, such as clutches found in a Posi trac, excess heat will cause even more damage.
For that reason, manufacturers want to make sure you don’t do something that will overheat the differential.
四驱车强调四胎直径一致，就是是为了减少差分器的负担。差分器长期过负，过度发热，差分油变质，长期或过度使用会造成差分器过度磨损。
但备胎暂时使用，不会有太大影响。我多年前曾用小一号的备胎把X-Drive的马车从Boston连夜开回NJ，当时也有点担心，不过现在看来也没感觉有何变化。
我的老款Highlander就被提供了这样的Full Size Spare: 备胎的尺寸、轮毂、胎压Sensor与其它四个完全一致，并且一起进行Tire Rotation. 遗憾的是，这样的Full Size Spare在市场中越来越少了...
而且， Spare Tire 的Speed Rating 是T 与Ground Tires H也不匹配。