Colons are seen by search engines as what they are. You say something, a word, then a colon, and then comes an explanation or enumeration.
In your example, you did it right, perhaps you should move the colon to where they belong, right next to the last letter of the brand, so it reads: GENERAL ALTIMAX ARCTIC: 225/45R17 91Q
The idea you mentioned, building titles for users, not for engines, is the way to go. However, there are some tweaks you can make to make it easier for both.
As in your example, the title could become: GENERAL TIRE: ALTIMAX ARCTIC 225/45R17 91Q - YOURSITENAME (personally I would put the colon next to the brand, and then comes the rest of the product name + you end up with " - YOURSITENAME" to help build YOUR brand.
Hope that helps!