Our reader bought his car second hand with 195/65 R15 steel wheels. He then changed the steel wheels for 205/55 R16 alloys. But he wants to know if he can still use one of the car’s original wheels as an emergency spare.
Can you use a smaller wheel as a spare?
The short answer is yes. If you think about it, space saver spare wheels frequently have a smaller diameter than regular wheels. For emergencies, using a smaller wheel to get you home or to a tyre repairer is fine.
Is there any speed/distance restriction?
Space saver spares are only designed to travel at 50mph but they aren’t restricted to any specific distance. However, tyre makers/retailers suggest drivers use them for no more than 50 miles. This is because they have less grip than regular tyres.
If you’re using a regular tyre, we’d suggest you adopt a similar tactic. When it’s mounted on the car, the smaller wheel might feel slightly different to having four identically sized wheels and tyres.
Won’t a smaller wheel upset a car’s balance?
That depends on the wheel. Some cars can get away with radically different tyre and wheel sizes between axle. For example the Ford hot rod in our main picture. But let’s look at the different overall sizes between the two types of wheel our reader is suggesting using. The 195/65 R15 tyre has a tread width of 195mm. The aspect ratio (65%) tells us the sidewall is 126.75mm. Meanwhile the 205/55 R16 has a tread width of 205mm. And the aspect ratio of 55% tells us the side wall is 112.75mm.
Counter intuitively, the larger 16” wheel actually has a smaller total diameter and therefore radius than the smaller 15” wheel.
Some quick maths shows us that the total diameter of the 15” wheel plus tyre is 634.5mm. The total diameter of the 16” wheel plus tyre is 631.9mm. That makes the radius of the 15” wheel 317.25mm; the radius of the 16” wheel 315.95mm.
What is the permitted difference?
The difference in rolling radius between the two sizes of wheel is 1.3mm. You’re unlikely to feel that when you’re driving the car. But depending on the tread levels and type of tyre there may be differing amounts of grip between the spare and the other three tyres.
The difference between the reader’s existing and proposed emergency spare tyre is less than half a per cent. The tyre industry permits a difference of 2.5 and 3% between the existing wheel/tyre combination and spare. The 0.4% difference between our reader’s wheel/tyres should present no problem at all.
What about legality?
Even if a tyre is a spare, it must still have at least 1.6mm of tread on it. And as with a regular tyre it must not have any lumps, cuts or other damage that could indicate it might have suffered structural damage and be likely to experience a sudden deflation or blow out.
And we recommend that if you’re using a regular tyre as a spare, you keep an eye on how well inflated it is, checking it when you examine your other tyres.