Short answer
A good day to buy a car or bike is one when you can inspect it, sort the insurance and pay without stretching, with the handover kept outside Rahu Kalam. Many families also skip their Chandrashtama days and like a small puja before the first drive. Find Good Times shows the traditional windows for your town on any date.
How ChiChakra reads this
ChiChakra treats a vehicle purchase as a daily-timing question. It marks Rahu Kalam, one eighth of daylight in a weekday-set slot (Monday's is the 2nd segment, Sunday's the 8th), and shows your Chandrashtama dates. A birth chart would add the 4th house, traditionally linked with vehicles, and the running Dasha.
Making the day work
Decide what you're timing: the payment, the collection or the first journey. Most people choose collection and the first drive.
On a 12-hour day Rahu Kalam lasts about 90 minutes, so it is easy to work around. Book the handover outside it, in good daylight.
Make sure the insurance is live before you drive away, and never skip the inspection because the date looks good.
Good to know
A good time is a personal observance; it says nothing about the vehicle's condition. Keep the budget realistic.