Andy, if I were you, I wouldn't try and do it by numbers...
You seem to have calced your tin box at ~26mpg and a bike at ~46mpg, the latter might be a bit optimistic if you start having any fun. Not sure how bad Outbacks are but if I were averaging 26mpg I'd be getting shot of that vehicle regardless of commuting.
On the tyres, I have to agree with TJ (gulp :-)). You're more likely to need 3-5 rears and 3-4 fronts on that mileage. Yes they get squared off on the motorway but only if you 'pootle' about on a geriatric BMW 🙂 (that was a feeble attempt at a joke at TJ's expense and not an invitation to all out internet ping pong).
I'd be shocked, nay stunned if you were to save any money by commuting on a boik and remember, a bike carries the mileage less well than a car so its residual may be impacted more.
