Start with the quantity you need
These pages separate six different automotive questions. Fuel cost starts with a route and a consumption rate. Mileage turns distance and US gallons into a rate. The MPG converter changes the unit of an already known rate. A trip split allocates fuel spend among people. Tyre size derives a nominal diameter from the marking on the sidewall. EV charging prices energy moved from the grid into a battery.
Similar words do not make these interchangeable. A vehicle may average 30 MPG yet have a costly mountain journey because distance, fuel price, load, and speed are different facts. A tyre with a larger diameter can change speedometer behaviour without saying anything about litres used. An electric charging receipt can show energy bought without predicting motorway range.
Keep the measurement with its unit
Record kilometres or miles, litres or US gallons, and the period over which they were measured. US and Imperial gallons are different volumes. L/100 km and MPG point in opposite directions: lower consumption is better in the first unit, higher is better in the second. A pump price needs its currency and whether fees are included. Tyre markings use millimetres, percentages, and inches in the same code.
For driving costs, a fill-to-fill log is usually more informative than a brochure figure. For charging, retain the tariff period and any session fees. For a group journey, write down which legs were shared before dividing the bill. These records let a later estimate be repeated instead of reconstructed from memory.
Know what the arithmetic does not cover
Fuel and electricity are operating inputs, not the full cost of travel. Parking, tolls, maintenance, insurance, depreciation, and driver time require separate treatment. Likewise, nominal tyre diameter is not a fitment approval: clearance, load rating, wheel offset, and the vehicle placard still govern a safe choice. Use the cards above to select the physical or cost relationship that actually matches the decision.
Fuel consumption begins with one measured interval
A fuel log becomes useful when its numerator and denominator describe the same interval. A fill-to-fill entry needs the odometer distance since the previous comparable fill and the fuel bought at the current fill. A 480 km interval supplied with 36 litres is 7.5 L/100 km; at a price of 1.90 per litre it represents 68.40 of fuel. The same data can also be expressed as distance per fuel unit, but it should not be combined with an unrelated dashboard average or a partial top-up. Weather, road speed, towing, tyre pressure, and warm-up time belong in the note beside the record because they help explain a changed result without altering the formula.
Cost, economy, and reimbursement answer separate questions
Fuel price turns an observed consumption rate into cash for a route. Economy compares how much fuel is used for distance. A group split decides how one receipt is allocated among people. None proves what driving ought to be reimbursed at: an employer or tax authority may use a policy rate that includes expenses beyond fuel. For example, a 600 km trip at 7.5 L/100 km and 1.70 per litre creates a 76.50 fuel receipt, while a mileage policy could produce a different number because it measures another obligation. Label the calculation before sharing it so a receipt, a consumption record, and a claim are never treated as interchangeable evidence.
Energy at the meter has a different boundary
An EV battery percentage is a share of usable battery capacity, whereas an electricity bill records energy delivered by the grid. Charging efficiency connects those quantities: storing 42 kWh at 90 percent efficiency requires about 46.67 kWh at the meter. The tariff then prices meter energy, but a station may add time, connection, parking, membership, or idle charges that a pure per-kWh comparison does not include. Charging power in kW affects elapsed time; it is not the same unit as kWh accumulated during a session. Keep a station receipt, tariff period, and state-of-charge change together when checking a result.
Tyre markings are geometry before they are a purchase decision
A sidewall code mixes width in millimetres, aspect ratio as a percentage, and rim diameter in inches. The geometry calculator exposes how these components produce a nominal outside diameter, so a proposed change can be screened for its likely speedometer and clearance effect. It does not see wheel offset, brake clearance, suspension travel, axle load, tread depth, or local approval requirements. The vehicle placard and manufacturer documentation remain the source for approved dimensions, load capacity, and cold pressure. Treat the numerical result as a question to take to that vehicle-specific information, not as permission to fit a tyre.
Turn one output into a checkable record
Write the date, distance unit, fuel or electricity unit, price basis, and the observation that supplied the consumption or efficiency figure. For a tyre comparison, preserve both complete size markings and the vehicle configuration. For shared travel, record the legs and travellers before settling. This small record permits a later calculation to be repeated with a new pump price, seasonal consumption, or tariff without guessing which part changed. The useful outcome is not a very precise number by itself; it is a stated relationship whose assumptions can be tested against a receipt, meter, odometer, placard, or actual route. It also makes a later correction traceable to one input instead of a vague recollection.