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Car Affordability
Car Affordability does the money and physics math of cars: what a trip costs, what a car is worth, and how it performs.
What this does
Car Affordability does the money and physics math of cars: what a trip costs, what a car is worth, and how it performs.
Most fuel math is one small division followed by one multiplication. You start with how far you drive and how much fuel the vehicle drinks per unit of distance, divide distance by efficiency, and you get the volume of fuel burned. Multiply that volume by the price at the pump and you have your trip or monthly cost. The only thing people consistently get wrong is mixing up units, so always check whether your economy figure is miles per gallon, liters per 100 km, or km per liter before you type anything in.
The formula, explained plainly
Comparing an EV to a petrol car honestly means looking past the fuel bill. Electricity per kilometer is usually cheaper than petrol per kilometer, but that is only one line of the budget. You also need to weigh the purchase price difference (or loan cost), insurance, tire wear (EVs are heavy and can chew tires faster), and the big one everyone ignores: depreciation. A fair comparison runs all of these over the same ownership period, typically five years, and the same annual distance, then divides the total by distance to get a true cost per kilometer.
Auto loan math is the same amortization you meet in mortgages, just shorter and with a higher rate. Every monthly payment is split: part pays the interest owed that month on the remaining balance, and the rest shrinks the balance. Early payments are mostly interest, later payments are mostly principal. That is why making the same extra payment in month six saves far more interest than making it in month forty, and why two loans with the same monthly payment but different terms can cost thousands apart in total interest.
Lease math answers a different question than loan math. A loan asks how fast you can buy the car; a lease asks what it costs to rent the newest years of the car's life. The lease payment covers the depreciation expected during the lease term plus interest (the "money factor") on the average outstanding balance. Because you hand the car back, the residual value guess matters enormously: an optimistic residual lowers your payment, and if you then buy the car at lease end you may overpay for it. Always compare lease versus buy on total cash out of pocket over the same horizon, not on monthly payment alone.
Tire math looks intimidating because of the cryptic sidewall codes, but a tire size like 205/55R16 is just three numbers: tread width in millimeters, the sidewall height as a percentage of that width (the aspect ratio), and the wheel diameter in inches. From those you can compute the tire's overall diameter and circumference, which is what tells you whether a new tire will throw off your speedometer. Anything beyond about a 3 percent change in overall diameter also changes your gearing, ground clearance, and braking feel, which is why upsizing needs the math checked first.
How to use it
- Enter annual income.
- Read the instant result and the breakdown below it.
- Adjust any input to compare scenarios.
Worked example
With annual income = 80000, the result is $28,000.00 car budget 20% down is $5,600.00. Payment under 8-10% of income is $666.67/mo.. Defaults use typical commuter figures.
Common mistakes
- Pricing EV charging at the home electricity rate when most of your charging will happen at public fast chargers, which can cost three to five times more per kWh.
- Leaving insurance out of total cost of ownership even though it can exceed fuel or charging costs for young drivers and luxury models.
- Forgetting the waste between fill-ups in fuel budgeting: gas you buy is not gas you burn, since some always sits in the tank as a reserve and idling burns fuel for zero distance.
- Assuming one mpg improvement is worth the same on every car, when going from 15 to 20 mpg saves more than twice as much fuel per year as going from 40 to 50 mpg at the same mileage.
- Mixing up MPG and L/100km direction: a bigger MPG number is better, but a bigger L/100km number is worse, so never rank vehicles by these two figures without converting to a common unit first.
- Using the advertised combined economy as gospel for a commute that is 90 percent stop-and-go traffic, when the city figure (often 20 to 30 percent worse) is the honest input for your driving.
- Comparing a lease payment to a loan payment as if they buy the same thing: the loan builds equity and ends, the lease is rent with a mileage cap and wear charges at return.
- Forgetting depreciation in EV versus petrol comparisons, which can be the largest single cost of ownership and sometimes flips which option is cheaper.
Limitations
- EV comparisons use today's electricity and fuel prices, which move independently, so a five-year projection can shift materially if either changes.
- Loan calculators assume a fixed rate and on-time payments; variable rates, late fees, and negative equity rollovers from a trade-in are not modeled.
- Lease estimates depend on the residual value the dealer sets, which is a negotiated guess, not a guaranteed future market price.
- Tire size math covers diameter and circumference only; it does not check wheel-arch clearance, load ratings, or whether your insurer accepts nonstandard fitments.
- Carbon footprint figures use average grid and fuel emissions factors, so they describe a typical driver, not your specific car, commute, or charging mix.
Expected accuracy
Fuel and cost results are as accurate as the economy figure and prices you enter; with real measured economy they are typically within a few percent. Loan amortization follows the standard fixed-payment formula exactly, so the math is precise to the cent given the inputs. EV comparisons are estimates by nature because electricity tariffs, battery degradation, and resale values vary widely.
Privacy
Everything you type stays on your device. The calculation runs in your browser with JavaScript; no input is sent to a server, stored in an account, or shared with anyone.
Sources and standards
- Fuel economy methodology follows the EPA test-cycle approach (city, highway, and combined ratings); loan and lease math uses the standard amortizing-loan and money-factor formulas used across the auto finance industry; tire sizing uses the industry-standard aspect-ratio and diameter convention printed on every sidewall.
Bottom line
Car Affordability turns a page of confusing auto math into one clear answer, whether that is a trip's fuel bill, the true cost of an EV, or the real size of a tire upgrade. Enter your own numbers, especially your real fuel economy and local prices, and the result will be honest enough to base a decision on.
Key insight
Fuel is usually less than half the true cost of a car. Depreciation, insurance, and maintenance dominate, which is why the cheapest car to buy is rarely the cheapest car to own.
Frequently asked questions
Why does the calculator ask for city versus highway mileage?
Engines are far less efficient in stop-and-go traffic than at steady highway speed, often 20 to 30 percent worse. Using the combined rating for an all-city commute underestimates your fuel bill noticeably. Enter the figure that matches most of your driving for the honest answer.
Is an EV really cheaper to run than a petrol car?
Per kilometer for energy, usually yes: electricity typically costs a third to a fifth of petrol per kilometer. But total ownership also includes purchase price, insurance, tires, and depreciation. Run the comparison over five years with your annual mileage before deciding, since low-mileage drivers sometimes do better with a cheap petrol car.
Should I compare lease and loan by monthly payment?
No, that is the classic trap. A lease payment looks lower because you are paying for depreciation plus interest and returning the car. Compare total cash paid over the same number of years, including the down payment, lease-end fees, and what a purchased car would be worth when sold.
What does the money factor on a lease mean?
The money factor is the lease's interest rate in disguise: multiply it by 2400 to get the approximate APR. A money factor of 0.00208 equals about 5 percent APR. Dealers quote it this way because a small decimal looks harmless next to a loan rate.
How much extra does a longer loan term really cost?
A lot. Stretching a loan from 48 to 72 months at the same rate can add 40 to 60 percent more interest paid while leaving you owing more than the car is worth for years. The calculator shows total interest explicitly so you can weigh a comfortable payment against the hidden cost.
Will bigger tires make my speedometer wrong?
Yes, if the overall diameter changes. The speedometer counts wheel rotations, so a taller tire travels farther per rotation and the speedometer under-reads. A 3 percent diameter increase means you are doing about 103 km/h when the dial shows 100. Stay within about 3 percent to keep readings and gearing safe.
What is the 3 percent rule for tire changes?
Tire professionals recommend keeping the new tire's overall diameter within 3 percent of the original. Beyond that, speedometer and odometer error grows, gearing and acceleration feel change, clearance issues can appear, and safety systems calibrated for the original rolling diameter may behave differently.
Why do small MPG improvements matter more on inefficient cars?
Fuel use is proportional to distance divided by economy, so the savings curve is steep at the low end. Improving from 15 to 20 mpg at 20,000 km a year saves roughly twice as much fuel as improving from 40 to 50 mpg. The calculator shows gallons or liters saved, which is the number that matters.
How do I estimate my car's depreciation?
A common rule of thumb is 15 to 25 percent per year for the first few years, slowing afterward, but brand, mileage, condition, and fuel prices shift this a lot. Use your car's actual purchase price and current market listings for similar models as anchors, and treat any depreciation figure as a range, not a promise.
What is included in total cost of ownership?
Depreciation, fuel or charging, insurance, maintenance and repairs, tires, taxes and registration, and financing interest, minus any resale or trade-in value at the end. Fuel is the most visible cost but often not the largest, which is why a fuel-only comparison can point you at the wrong car.
How do I calculate EV charging cost at home?
Multiply your battery's usable capacity (or the energy you add) by your electricity rate per kWh, then add roughly 10 percent for charging losses. A 60 kWh fill at 0.30 per kWh costs about 19.80. Compare that to the petrol cost of the same range to see the per-kilometer gap.
Why is my real fuel economy worse than the rating?
Official ratings come from standardized lab cycles with gentle acceleration, no air conditioning, and warm engines. Short trips with a cold engine, heavy traffic, high speeds, roof loads, and aggressive driving can push real consumption 15 to 35 percent above the rating. Track a few real fill-ups for your true figure.
Does a bigger down payment always help on a car loan?
It reduces the amount financed, so you pay less interest and avoid being underwater (owing more than the car is worth) in the early years. It does not change the interest rate itself. The sweet spot is enough down to stay ahead of depreciation without draining your emergency savings.