An incident I had a while back reminded me that my spare tire at the time didn’t match the four tires mounted on the car. Because I tend to buy all my tires used, I thought it might be a good idea to be prepared in case of another flat tire.
While a full-size spare is preferable to a donut spare, the donut spare has the benefits of light weight and small size. If I’m rallying, I take the full size spare. Otherwise, the donut strikes a balance between being prepared for a flat and being practical.
Let me start with a boring description of why I wanted to change this tire. The standard tires on my car are 205/50-R16. In case you don’t know how tire sizes are decoded, here’s a quick rundown:
The tires are 205mm wide
The sidewall is 50% of the tire width tall (ie. 205mm * 0.50 = 102.5mm)
The wheel is 16 inches tall. Yes, tire sizes use both millimeters and inches. Silly, I know.
To calculate diameter, we need to account for two sidewalls plus the wheel:
102.5mm sidewall * 2 sidewalls = 205mm = 8.1 inches. Add 16 inches of wheel and we get 24.1 inches in diameter.
If you want to skip the math, there are some easy tire size calculators on the interwebs that can tell you diameter and even compare different sizes (however, they generally won’t do spare tire sizes).
The spare tire included with my car is a T135/70-D16, which if you do the math works out to 23.4 inches in diameter. It is 0.7 inches shorter than the stock tires. The tires I use in the summer happen to be 205/55-R16(24.9in diameter) making that spare tire short by a full 1.5 inches!
On most cars, having one tire a little bit shorter than the others is really no big deal, especially if it is on a non-drive axle (the back in a FWD car, the front in a RWD car). Note that the spare should be installed this way - if you have a flat front tire in a FWD car, move one of the rear wheels to the front and put the spare on the back axle.
All-wheel-drive cars, however, are a case of their own. It can cause a lot of strain on the differentials to have a tire of a different size than the others - especially if those differentials are the expensive limited-slip variety as they are on my car.
Subaru warns their customers that a tire size difference of only ¼ inch is too much for their all-wheel-drive systems. They say that if one tire is damaged, or if one tire is worn prematurely, all tires must be replaced in order to keep the same diameter. This is the official word from Subaru, but they include a spare tire that is ¾ inch too small! I suppose if you are driving 5km to the nearest garage it might not make much difference, but I’d rather be able to drive all the way home to fix a tire myself.
In short, my spare tire is too small and my own peace of mind I decided to fix the situation.
While full-size car tires come in many different sizes, spare tires are much more limited. I calculated diameters of a few different available spare tire sizes until I came up with one that was much closer to my 205/55-R16 summer tire diameter and 205/50-R16 winter tire diameter - a T135/80-D16 tire, which at 24.5in diameter is squarely between the 24.1in of winter and the 24.9in of the summer tires. Not perfect, but much better than what was in my trunk already!
A trip to the junkyard scored me the T135/80-D16 tire I was seeking in the trunk of a Chrysler Intrepid. This was to be peeled off its wheel and installed on my spare tire wheel, so it will properly bolt to my car. Here’s the new tire, side-by-side with my original spare tire:
It doesn’t weigh noticeably more and still fits snugly in the spare wheel well under the floor in the back of the wagon. For all intents and purposes, there is no drawback - only the benefit of having a properly sized wheel what will be kinder to my car should I need to temporarily use it.
All that remains is to mount it on my wheel - stay tuned!
Total cost:
$13 - junkyard donut with T135/80-D16 tire
While a full-size spare is preferable to a donut spare, the donut spare has the benefits of light weight and small size. If I’m rallying, I take the full size spare. Otherwise, the donut strikes a balance between being prepared for a flat and being practical.
Let me start with a boring description of why I wanted to change this tire. The standard tires on my car are 205/50-R16. In case you don’t know how tire sizes are decoded, here’s a quick rundown:
The tires are 205mm wide
The sidewall is 50% of the tire width tall (ie. 205mm * 0.50 = 102.5mm)
The wheel is 16 inches tall. Yes, tire sizes use both millimeters and inches. Silly, I know.
To calculate diameter, we need to account for two sidewalls plus the wheel:
102.5mm sidewall * 2 sidewalls = 205mm = 8.1 inches. Add 16 inches of wheel and we get 24.1 inches in diameter.
If you want to skip the math, there are some easy tire size calculators on the interwebs that can tell you diameter and even compare different sizes (however, they generally won’t do spare tire sizes).
The spare tire included with my car is a T135/70-D16, which if you do the math works out to 23.4 inches in diameter. It is 0.7 inches shorter than the stock tires. The tires I use in the summer happen to be 205/55-R16(24.9in diameter) making that spare tire short by a full 1.5 inches!
On most cars, having one tire a little bit shorter than the others is really no big deal, especially if it is on a non-drive axle (the back in a FWD car, the front in a RWD car). Note that the spare should be installed this way - if you have a flat front tire in a FWD car, move one of the rear wheels to the front and put the spare on the back axle.
All-wheel-drive cars, however, are a case of their own. It can cause a lot of strain on the differentials to have a tire of a different size than the others - especially if those differentials are the expensive limited-slip variety as they are on my car.
Subaru warns their customers that a tire size difference of only ¼ inch is too much for their all-wheel-drive systems. They say that if one tire is damaged, or if one tire is worn prematurely, all tires must be replaced in order to keep the same diameter. This is the official word from Subaru, but they include a spare tire that is ¾ inch too small! I suppose if you are driving 5km to the nearest garage it might not make much difference, but I’d rather be able to drive all the way home to fix a tire myself.
In short, my spare tire is too small and my own peace of mind I decided to fix the situation.
While full-size car tires come in many different sizes, spare tires are much more limited. I calculated diameters of a few different available spare tire sizes until I came up with one that was much closer to my 205/55-R16 summer tire diameter and 205/50-R16 winter tire diameter - a T135/80-D16 tire, which at 24.5in diameter is squarely between the 24.1in of winter and the 24.9in of the summer tires. Not perfect, but much better than what was in my trunk already!
A trip to the junkyard scored me the T135/80-D16 tire I was seeking in the trunk of a Chrysler Intrepid. This was to be peeled off its wheel and installed on my spare tire wheel, so it will properly bolt to my car. Here’s the new tire, side-by-side with my original spare tire:
It doesn’t weigh noticeably more and still fits snugly in the spare wheel well under the floor in the back of the wagon. For all intents and purposes, there is no drawback - only the benefit of having a properly sized wheel what will be kinder to my car should I need to temporarily use it.
All that remains is to mount it on my wheel - stay tuned!
Total cost:
$13 - junkyard donut with T135/80-D16 tire
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