Auto & Vehicle Calculators
Car loans, affordability, fuel cost and MPG.
Four calculators for the loan, the budget and the running costs. The one thing they exist to show is that the purchase price is the smaller half: over five years, fuel, insurance, maintenance and depreciation routinely cost more than the interest.
All 4 auto & vehicle calculators
The sticker price is the smallest part
A car has two costs and most people budget for one of them. The purchase is visible and negotiable; the running cost is invisible and relentless. Over five years, fuel, insurance, maintenance and above all depreciation add up to more than the interest on the loan, and depreciation alone is usually the single largest line on the list.
| Cost | Nature | Covered by |
|---|---|---|
| Depreciation | Largest cost, invisible until you sell | Not a calculator; check real resale values |
| Loan interest | Fixed once signed | Car loan |
| Fuel | Scales with mileage | Fuel cost and MPG |
| Insurance | Varies sharply by model and driver | Quote it before you buy, not after |
| Maintenance | Rises steeply with age | Budget for it from year three |
The 20/4/10 guideline
A widely used rule of thumb for keeping a car affordable: put 20% down, finance for no more than 4 years, and keep total transport costs under 10% of gross income. It is deliberately conservative, and its real value is the middle term. Stretching to a six or seven year loan lowers the payment and quietly guarantees you will owe more than the car is worth for most of it, because depreciation outruns the principal in the early years.
Trade-ins change the tax, and it matters
In most US states, sales tax on a vehicle is charged on the price after the trade-in is deducted rather than on the full sticker. On a $10,000 trade at a 7% rate that is $700 you do not pay, which is easily large enough to change which deal is better. The car loan calculator models this, capped at the tax actually due, because a trade-in cannot save more tax than the purchase would have generated in the first place.
Fuel economy is measured, not quoted
The figure on the window sticker is a test result under controlled conditions. Your own number comes from arithmetic on real tanks: miles driven divided by gallons used, which is what the MPG calculator does. Fill the tank fully, reset the trip meter, drive normally, then fill it fully again. Real-world economy commonly runs below the quoted figure, and the gap widens with short trips, cold weather, roof boxes and a heavy right foot.
Auto & Vehicle Calculators: common questions
How much car can I afford?
A common guideline is 20% down, a term no longer than 4 years, and total transport costs under 10% of gross income. The car affordability calculator works backwards from your budget, and the term limit matters more than the down payment does.
Does a trade-in reduce the sales tax I pay?
In most US states, yes. Tax is charged on the price after the trade-in is deducted, so a $10,000 trade at 7% saves $700. A few states tax the full price, so check your own state before relying on it.
How do I calculate my real MPG?
Fill the tank completely, reset the trip meter, drive normally, then fill it completely again. Divide the miles shown by the gallons it took. Repeat over a few tanks, since a single fill is sensitive to where the pump cuts off.
Why is my fuel economy worse than the official figure?
Official figures come from a standardised test. Short trips, cold weather, city traffic, roof racks, low tyre pressure and hard acceleration all reduce real economy, and a loaded roof box alone can cost 10 to 25% at highway speed.