• My short commute is only 4 miles/day. I figure I will do that 4 days/week = 16 miles/week = 720 miles/year (assumes 45 weeks' worth of bike commuting)
• I do at least one grocery trip per week, which is another 4 miles x 45 = 180 miles/yr
• 180 + 720 = 900 mi/yr
• My car gets 25 mpg around town.
• 900 mi/yr ÷ 25 mi/gal = 36 gallons of gas
• 36 gallons of gas x $3/gallon = $108 yr saved just in gas.
• ↑ Savings would be a LOT more for longer commutes.

What if we eliminate a typical $600/mo car payment too?

...then about $100/mo for insuring the car? Maintenance on the car vs. that of the bike.

Car upkeep would be near zero for those first 5 years on a new car with the big payment, but beaters start to cost that money again; depends on how good your beater is.

By all means, do your own numbers, but be realistic. A nice bike or eBike is easy to justify vs. a car, even a paid-for beater car.