I don't drive to work very often, but I'm about to move location and will be doing a commute of about 35 miles in total a day.
DP rarely drives to work and has a much shorter route.
His car has been playing up and he is getting it repaired. It's over 5 years old and mine is over 10.
When I am established in new office (and confirmed car is best way to travel etc.), I intend to replace my dinky banger with a nice, larger new car, which I will then use for my commute.
DP today offered the cheaper solution
that I buy him a new car (he won't use dinky banger, even though it's actually in fantastic nick and has never failed an MOT, no rust etc) and take his old one. AIBU to think he has a bloody cheek 
We do have shared finances, but for various reasons car expenses tended to come off personal rather than joint account, with the person with the smaller car buying diesel if we went on longer trips and chipping in for the odd big repair bill. We split things evenly as a starting point but don't count the pennies (are lucky enough not to need to particularly), but we don't prioritise cars given we have been living so close to work.
DP has had 2 high end & one middle of the road saloon in the time I've had dinky banger. Dinky banger isn't even a banger, but he just doesn't want to drive it (embarrassed I think) and it's not the ideal car for 175 miles minimum per week, especially in winter.