Tire Comp lets you compare the sizes of different tires, it will tell you what the variance is in the diameter of the new tire compared to the original. You must say speak the tire code in the format "width + ratio + height".
In a tire code like 255/55R15, the width is 255, the ratio is 55, and the height is 15.
For example you could say:
"Hey Google, ask Tire Comp to compare 192 + 65 + 13 with 255 + 55 + 15"
However when you are typing to Google Assistant it's probably easier to separate the numbers with spaces, so it will work when you type like this:
"Ask Tire Comp to compare 192 65 13 with 255 55 15"
When speaking the command all in one, the new tire comes first and the original tire comes second, so in these examples, 192 65 13 is the new tire and 255 55 15 is the original tire.
If you say "ask Tire Comp to compare" it will prompt you for the new tire and the original tire separately.
If you say "talk to Tire Comp", it will say hello and tell you the different commands that are available.