Free Automotive Calculators

These calculators cover common car questions: will new wheels fit, how much does an EV cost to charge at home, why does a battery go flat overnight and which spark plug suits a tuned engine. Enter your own figures and get a number to check against your manual or a mechanic. Everything runs in your browser and nothing is uploaded.

5 free tools 2 groups Runs in your browser Nothing uploaded

Electric Vehicles

2 tools

Battery health over time and the cost of charging at home compared with running a gas car.

  • Electric Vehicle Owners

    EV Battery Degradation Estimator

    Select your vehicle, enter your mileage, and see estimated battery health, current and projected range at 5 and 10 years, plus fleet average comparison.

    Battery health + range projection + fleet average
  • EV Owners

    EV Home Charging Cost Calculator

    Compare real cost per mile and annual cost between an EV charged at home and a gas car using your own electricity rate.

    EV vs gas cost per mile + annual savings

Wheels, Battery & Engine

3 tools

Wheel and tire changes, a battery that will not hold charge and spark plug choice after an engine change.

  • Car Enthusiasts

    Wheel & Tire Fitment Calculator

    Enter stock and new wheel specs to check overall diameter change, speedometer error, and whether the new setup will rub.

    Rub check + speedo error + clearance diff
  • Car Owners / DIY Mechanics

    Parasitic Battery Drain Calculator

    Enter your measured key-off draw in milliamps and your battery's reserve capacity, and see how many days until the car won't crank, plus the safe-draw threshold for that battery.

    Days until no-start + safe draw threshold
  • Tuners / Hot Rodders

    Spark Plug Heat Range Calculator

    Enter your stock heat range, the power you added and the elevation you drive at to get the shift in steps, the plug number to order for NGK, DENSO, Champion or Bosch, and the firing tip temperature it changes.

    Heat range steps + plug number to order

Which tool do I need?

  • Check that a wheel and tire swap will fitWheel & Tire Fitment Calculator

    Enter the stock and new wheel specs. The result shows the change in overall diameter, the speedometer error that change causes and whether the new setup will rub.

  • Correct a speedometer after changing tire sizeWheel & Tire Fitment Calculator

    The same tool reports the speedometer error, so you can see how far the reading will be off before you buy.

  • Find out how far an electric car's battery has degradedEV Battery Degradation Estimator

    Select the vehicle and enter the mileage. It returns estimated battery health and the current range, then projects range at 5 and 10 years.

  • Compare running an EV at home with a gas carEV Home Charging Cost Calculator

    It uses your own electricity rate to give cost per mile and annual cost for both cars.

  • Work out why a car battery goes flat while parkedParasitic Battery Drain Calculator

    Measure the key-off current in milliamps, then enter it with the battery's reserve capacity. The result is the number of days until the car will not crank.

  • Check whether a key-off draw is too high for a batteryParasitic Battery Drain Calculator

    The calculator also shows the safe-draw threshold for the battery you enter, so you can compare your measured figure against it.

  • Pick a spark plug after adding powerSpark Plug Heat Range Calculator

    Enter your stock heat range and the power you added. It returns the shift in steps and the plug number to order.

  • Adjust the spark plug heat range for elevationSpark Plug Heat Range Calculator

    The elevation you drive at is one of the inputs, so it feeds into the recommended shift.

Automotive FAQ

Will bigger wheels change my speedometer reading?

A wheel and tire with a different overall diameter changes how far the car travels per wheel revolution, so the speedometer reads differently. The wheel fitment calculator compares stock and new specs and reports the overall diameter change and the speedometer error.

How can I tell whether new wheels will rub?

Enter the stock and new wheel specs into the wheel fitment calculator. It flags whether the new setup will rub, based on the numbers you give it. It does not replace a test fit for a car with modified suspension.

How much does it cost to charge an EV at home?

Enter your electricity rate in the EV home charging cost calculator. It returns the cost per mile and the annual cost, and sets them beside the same figures for a gas car.

How do I estimate EV battery degradation?

Select your vehicle and enter the mileage in the EV battery degradation estimator. It shows estimated battery health, current and projected range at 5 and 10 years, and a fleet average comparison.

How much current can drain a car battery while it is parked?

The parasitic battery drain calculator compares your measured key-off draw with the battery's reserve capacity. It shows the number of days until the car will not crank and the safe-draw threshold for that battery.

What does spark plug heat range mean?

Heat range describes how quickly a plug moves heat away from its tip. The spark plug heat range calculator takes your stock range, the power you added and your elevation, and returns the shift in steps plus the plug number to order for NGK, DENSO, Champion or Bosch.

Should I rely on these calculators for safety decisions?

No. They estimate from the values you enter. Check your vehicle manual or ask a qualified mechanic before changing wheels, electrical systems or engine parts.

Last updated: September 30, 2026