Sounds like a good idea mate but make sure that you research whether there is a market for this sort of thing in your area. I would try to plan my work in advance by working to appointments so that you can get the tyres as they are needed rather than having to hold large stocks.
Another thought has just occurred to me, could you go into partnership with your existing company where you work for yourself but agree to buy your tyres from them. That way they are getting a slice of the mobile tyre sales market as well as their regular work. You never know they may even help you with the setting up costs.