The 20/4/10 Rule
A widely used guideline for car affordability: put at least 20% down, finance for no more than 4 years (48 months), and keep total monthly vehicle costs -- loan payment plus insurance -- at or below 10% of your gross monthly income. On $6,000 gross monthly income, total vehicle costs should stay at or below $600 per month.
True Cost of Vehicle Ownership
Monthly loan payment and insurance are only part of vehicle cost. Fuel, routine maintenance, registration, and unexpected repairs add significant ongoing costs. A useful planning target is $0.50 to $0.75 per mile in total cost of ownership. For a driver covering 12,000 miles per year, that implies $6,000 to $9,000 in total annual vehicle costs ($500 to $750 per month).
Longer Loan Terms Increase Total Cost
72-month and 84-month loans offer lower monthly payments but increase total interest paid, create extended periods of negative equity (owing more than the vehicle is worth), and extend the risk period during which mechanical problems may emerge.
A Worked Example
On $6,000 gross monthly income, the 10% cap is $600/mo for loan payment plus insurance. If insurance runs $150/mo, that leaves $450/mo for the loan payment. At 7% over 60 months, $450/mo supports a loan of roughly $22,800 -- a useful ceiling to know before you start shopping, well before you fall in love with a specific vehicle.
Common Car Affordability Mistakes
Forgetting insurance in the affordability math. As shown above, insurance can take a meaningful bite out of the 10% budget before the loan payment is even considered.
Basing the budget on the monthly payment a dealer offers. Work backward from your own affordability cap, not forward from whatever payment the dealer proposes for a given vehicle.
Frequently Asked Questions
Should I include gas and maintenance in my budget cap? The 20/4/10 rule covers loan payment and insurance; fuel and maintenance are additional costs worth budgeting separately using the per-mile estimate above.
Is leasing a way to afford a more expensive car? Leasing often lowers the monthly payment on a given vehicle, but you build no equity and face mileage and condition limits -- it changes the cost structure, not the total value of car you can sustainably afford.