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Driving costs are easy to underestimate because they arrive in small pieces: a fill-up here, a tire there, a longer trip once or twice a year. The tools in this category turn those pieces into numbers you can plan around. The MPG calculator works out your real-world fuel economy from the distance you drove and the fuel you used; the fuel-cost and road-trip calculators project what a journey will cost; the EV charging and gas-versus-EV tools compare energy costs per mile or per kilometer; and the tire-size calculator shows how a different tire changes overall diameter and speedometer accuracy.

A few ideas explain most of the results. Fuel economy measured at the pump is usually a little lower than the sticker rating because of speed, weather, cargo and driving style. Charging an electric vehicle costs more than the battery capacity suggests because some energy is lost as heat, so the energy drawn from the outlet is higher than the energy stored. And a tire with a slightly larger diameter makes the speedometer read lower than your true speed, which is why tire changes are usually kept within a few percent of the original diameter.

The estimates assume steady conditions. Towing, mountain driving, cold weather and heavy loads all reduce efficiency, so add a margin when you plan a demanding trip. For vehicle-specific safety information such as load index and speed rating, rely on your owner's manual and the tire placard.

Which tool fits which driving question

The six tools here split along two lines: what you already know and what you want to find out. The MPG Calculator works backward from the past, taking the distance you drove and the fuel you put in and returning your actual fuel economy in miles per gallon, or liters per 100 kilometers in metric mode. The Fuel Cost Calculator works forward, taking a distance, a fuel economy figure and a fuel price and returning what the trip will cost. The Road Trip Cost Calculator extends that by adding a daily amount for food and lodging multiplied by the number of days, so it answers a budgeting question rather than only a fuel question.

For electric vehicles, the EV Charging Cost Calculator prices a charge from battery capacity, starting and target charge levels, electricity price and a charging-loss percentage. The Gas vs. EV Cost Calculator sets a gasoline car beside an electric one at the same annual mileage and returns yearly energy costs for each. The Tire Size Calculator is the odd one out; it compares two tire sizes written in the standard width, aspect ratio and rim format and reports the change in overall diameter and what your speedometer would then show.

A useful order of operations for a new car decision is to measure what you actually get (MPG), project your typical year (Fuel Cost or Gas vs. EV), and only then price the occasional big trip (Road Trip). The guides on this site about measuring MPG, charging an electric car, reading tire sizes and building a road-trip budget cover the same ground in more narrative form.

Measuring real fuel economy and what it does to a budget

Fuel economy is distance divided by fuel, and the only way to get it right is to measure both honestly. Fill the tank until the pump first clicks off, note your trip meter or odometer, drive normally, and fill again the same way. If you went 312 miles and the second fill took 11.2 gallons, your fuel economy is about 27.9 miles per gallon. Averaging several tanks smooths out one-off differences such as a long idle or a cold week, and very short trips tend to look worse because a cold engine and catalytic converter use more fuel before they reach normal operating temperature.

That measured figure then feeds the budget tools. A 450 mile drive at 28 miles per gallon uses about 16.1 gallons, and at 3.50 dollars a gallon the Fuel Cost Calculator shows about 56.25 dollars. The tool applies one constant figure to the whole distance, so a combined city and highway rating gives a more realistic result than a highway-only number, and a round trip needs the full there-and-back distance entered, because the tool cannot tell whether your number already includes the return leg.

The Road Trip Cost Calculator multiplies its daily food and lodging figure by every day you enter, not only driving days. For a 1,200 mile trip at 30 miles per gallon, 3.40 dollars per gallon, five days and 150 dollars a day, fuel is 136 dollars and daily costs are 750, so the total is 886 dollars. Mountain grades, stop-and-go traffic, heavy loads and trailers lower real fuel economy, so leave a margin when the route is demanding.

Electric vehicles: charging cost and the loss factor

Charging an electric vehicle costs more than the energy that ends up in the battery, because some energy is lost on the way. The EV Charging Cost Calculator handles this by dividing rather than subtracting: energy drawn from the wall equals energy added divided by one minus the loss percentage. That makes the effect larger than the percentage suggests. A 15 percent loss adds about 17.6 percent to the energy you pay for, not 15.

Consider a 60 kWh battery charged from 20 to 80 percent. The battery gains 36 kWh. With a 10 percent loss, the outlet supplies 40 kWh, and at 0.15 dollars per kWh the charge costs 6.00 dollars. Raise the loss to 15 percent and the same charge draws about 42.4 kWh and costs about 6.35 dollars. The number you enter for loss is an assumption, and home charging and public fast charging can behave differently, so use a figure that matches how you usually charge.

Two cautions apply. If you leave the starting or target charge blank, the tool assumes a full charge from 0 to 100 percent, so check those fields. And fast charging slows noticeably as the battery fills, which affects how long a session takes but not the cost this tool reports, since it applies one price per kilowatt-hour throughout. If you use a public network with a different price, run the calculation again with that rate.

Gas versus electric: comparing cost per mile

The fairest way to compare a gasoline car with an electric car is cost per mile. A gas vehicle's cost per mile is the fuel price divided by its miles per gallon; an electric vehicle's is the electricity price divided by its miles per kilowatt-hour. At 3.50 dollars per gallon and 28 miles per gallon, gasoline costs 12.5 cents per mile. At 0.15 dollars per kWh and 3.5 miles per kWh, electricity costs about 4.3 cents per mile. Multiply by 12,000 miles a year and the Gas vs. EV Cost Calculator shows about 1,500 dollars against about 514, a difference near 986 dollars. Those prices and efficiencies are examples, so substitute your own.

Because both costs scale with distance, annual mileage changes the size of the dollar gap but not the per-mile advantage. A driver covering 6,000 miles saves half as much as one covering 12,000. The comparison also covers energy only. Purchase price, insurance, maintenance, tires, depreciation and charger installation are separate parts of total cost of ownership, and a time-of-use electricity plan can lower the electric figure while heavy use of public fast charging can raise it. Treat the output as one line in a larger spreadsheet, not the whole answer.

Tires: diameter, speedometer and plus sizing

A tire size such as 225/45R17 packs three numbers. The 225 is the section width in millimeters, the 45 is the sidewall height as a percentage of that width, and the 17 is the wheel diameter in inches. The Tire Size Calculator turns a pair of these into overall diameters: sidewall height is width times the aspect ratio, doubled and converted to inches, plus the rim size. Moving from 225/45R17 to 225/40R18 keeps the diameter within about 0.5 percent, which is the point of plus sizing, and at a 60 mph reading you would be traveling about 60.3 mph.

Larger changes show up in the speedometer and the odometer. Going from 265/70R17 to 285/70R17 increases the diameter by about 3.5 percent, so a speedometer reading 60 mph corresponds to a true speed near 62.1 mph, and the odometer under-counts distance by the same proportion. The tool notes that a difference above roughly 3 percent can noticeably affect speedometer accuracy and gearing, and anti-lock, traction and stability systems are calibrated to the factory tire. The calculator compares geometry only; load index, speed rating, wheel width, clearance and offset belong to your owner's manual, the door placard and a tire professional.