ReckonBoxWhat the auto tools do
An auto calculator turns the numbers on a window sticker or a spec sheet into the payment, the running cost, or the performance figure you actually want. Nine calculators split into two jobs. Five deal with money: the auto loan payment, the lease payment, cash back versus a low APR, fuel cost, and IRS mileage reimbursement. Four deal with the machine: horsepower from torque, horsepower from trap speed, gas mileage in four units, and tire size.
None of them returns a bare number. Each shows the arithmetic, because the figure that decides a car deal is usually the one the showroom states least clearly.
Buying and running costs
The costliest surprises in a car deal are the parts that are not the sticker price. Sales tax is the clearest example: most states tax the price minus your trade-in, so a $10,000 trade-in at 8% keeps $800 in your pocket, but eight states plus DC tax the full price and hand that saving back. Leasing hides its rate in plain sight, as a money factor rather than an APR, and multiplying that money factor by 2,400 converts it, so a 0.0025 factor is a 6% rate. And the choice between a rebate and a low APR is usually either-or: on $30,000 over 60 months, a 1.9% rate beats a 6.9% rate by more than a $2,000 rebate is worth, though a big rebate against a small rate gap flips it back to cash.
Running costs are simpler arithmetic. Fuel cost per mile is the pump price divided by MPG, a clean way to compare two cars, and the IRS business mileage rate of 76 cents a mile from 1 July 2026 covers fuel, maintenance, insurance, and depreciation, not just gas.
The performance side
The physics tools trade in defined constants. Horsepower equals torque in lb-ft times RPM divided by 5,252, and because 5,252 is a unit conversion the two curves are equal at 5,252 RPM, which is why every dyno chart crosses there. Trap speed estimates power through the Hale formula, weight times the cube of trap speed over 234, and the result is wheel horsepower, with crank power about 15 to 20% higher. Tire size is geometry: diameter is the rim plus twice the sidewall, and keeping a swap within about 3% of the original diameter preserves the speedometer reading, the gearing, and the clearance.
What these tools do not decide
They are planning estimates, not the final word. Real fuel economy usually trails the EPA sticker by 10 to 20% in mixed driving, so the honest figure comes from your own trip computer. The horsepower estimates are empirical and land within a few percent, not a dyno reading. And a lender or dealer can quote a rate or fee that differs from a default, so confirm the numbers on your own paperwork before signing.
Frequently asked questions
What can I calculate in the auto section? Nine calculators cover a car from purchase to performance: auto loan and lease payments, cash back versus a low APR, fuel cost, gas mileage, IRS mileage reimbursement, horsepower from torque or trap speed, and tire size. Each one shows the formula behind the answer, not just a final number.
How does a trade-in change the sales tax on a car? Most states tax the price minus the trade-in, so a $10,000 trade-in at an 8% rate saves $800 in sales tax. Eight states plus DC are the exception and tax the full price: California, DC, Hawaii, Kentucky, Maryland, Michigan, Montana, and Virginia.
What is a lease money factor? The money factor is the lease version of an interest rate, and multiplying it by 2,400 gives the APR, so a 0.0025 money factor is a 6% rate. A lease payment is depreciation plus a rent charge on the money factor plus tax, which is why it is not a simple loan payment.
Is cash back or a low APR the better deal? It depends on the numbers, since they are usually an either-or choice. On a $30,000 loan over 60 months, a 1.9% rate beats a 6.9% rate by more than a $2,000 rebate is worth, while a large rebate against a small rate gap favors the cash. The tool runs both and shows the cheaper one.
What is the current IRS mileage rate? The business standard mileage rate is 76 cents a mile from 1 July 2026, up from 72.5 cents earlier in the year, so 1,000 business miles is $760. The medical and moving rate is 23.5 cents, and the charity rate is fixed by law at 14 cents. The rate covers fuel, maintenance, insurance, and depreciation, not just gas.