Car Specification Units Explained: How to Compare Listings Consistently

To compare cars fairly, convert every figure to one unit system and confirm that each figure comes from the same kind of measurement. A spec only compares with another spec when both share a unit and a measurement basis. Pick one unit per attribute, such as kW for power, Nm for torque, L/100 km for fuel and kWh/100 km for electricity. Convert from the original stated value, and record the source and test conditions next to each number. Many apparent differences between listings come from units, rounding or test cycles rather than from the cars. A blank field means the information is missing. It does not mean the car is worse.

What do car specification units actually measure?

Car specification units describe a few physical quantities, and each quantity can be written in several metric and imperial units:

• Power is how quickly the drivetrain can do work. It is written in kW, hp, bhp or PS.
• Torque is turning force. It is written in Nm, lb-ft or, in some older material, kgf·m.
• Energy use is written in L/100 km, mpg, km/L, kWh/100 km, Wh/km, miles/kWh or MPGe.
• Size and mass are written in mm or inches, litres or cubic feet, and kg or lb.

Unit conversion is simple arithmetic. The harder part is checking that two figures measure the same thing under the same test conditions.

Engine power: what is the difference between hp, bhp, PS and kW?

kW is the SI unit of power, and hp, bhp and PS are horsepower variants with slightly different definitions.

• kW (kilowatt) is the official unit in EU type approval documents and is unambiguous.
• hp (mechanical or imperial horsepower) equals about 745.7 W. It is common in the US.
• PS (metric horsepower, Pferdestärke) equals about 735.5 W, so 1 PS is roughly 1.4% less than 1 hp. CV and ch in French, Spanish and Italian listings mean the same unit as PS.
• bhp (brake horsepower) means power measured at the engine output shaft before drivetrain losses. UK listings often write bhp for figures that the manufacturer quoted in PS, so the label alone does not tell you which unit was used.
• whp (wheel horsepower) is measured at the wheels on a dynamometer. It is lower than engine power because of drivetrain losses, so it does not compare with manufacturer figures.

Power conversion factors

From	To kW	To hp	To PS
1 kW	1	1.341	1.360
1 hp	0.7457	1	1.014
1 PS	0.7355	0.9863	1

Example: a car rated at 150 PS produces about 110.3 kW, or about 147.9 hp. If one listing says 150 hp and another says 150 PS, the real difference is about 2 hp. That gap is too small to notice when driving.

Which power differences actually matter?

A power gap of a few percent matters less than the weight the engine has to move. Power-to-weight ratio, usually written in kW per tonne, says more about real acceleration than power alone. It only works if both cars' weights use the same definition, as explained in the dimensions section below. Also check whether the figure is the system output of a hybrid or the engine alone. Hybrid combined power is not always the sum of the engine and motor figures.

Torque: how do Nm and lb-ft compare?

Torque is turning force at the crankshaft or motor shaft. One pound-foot (lb-ft) equals 1.3558 newton-metres (Nm), and 1 Nm equals 0.7376 lb-ft.

• 300 Nm ≈ 221.3 lb-ft
• 250 lb-ft ≈ 339.0 Nm
• 1 kgf·m ≈ 9.81 Nm, a unit found in some older Japanese and Asian specifications

For combustion engines, peak torque is usually quoted with an rpm range, for example 300 Nm at 1,600–3,500 rpm. A wide range at low rpm usually means the engine pulls strongly in everyday driving. Electric motors deliver their peak torque from very low speed. EV torque figures can be very high, but they describe a different delivery pattern, so comparing them directly with combustion figures can mislead. For towing or hilly routes, the torque curve often matters more than the peak number.

Fuel consumption: how do you convert between mpg and L/100 km?

L/100 km measures how much fuel a car uses to cover a fixed distance, so lower is better. mpg (miles per gallon) measures how far a car goes on a fixed amount of fuel, so higher is better. Because one is the inverse of the other, the conversion is a division rather than a multiplication:

• L/100 km = 282.48 ÷ UK mpg
• L/100 km = 235.21 ÷ US mpg
• km/L = 100 ÷ L/100 km

Why UK mpg and US mpg are not the same

A UK (imperial) gallon is about 4.546 litres, and a US gallon is about 3.785 litres. The same car therefore shows a higher mpg figure in UK units than in US units. One US mpg equals about 1.201 UK mpg. Example: 6.0 L/100 km equals about 47.1 UK mpg, or about 39.2 US mpg. Before comparing an mpg figure, confirm which gallon the listing uses.

Why L/100 km makes savings easier to see

mpg understates gains at the low end and overstates them at the high end, because fuel used per distance falls off with the inverse of mpg. Examples in US mpg, over 10,000 km:

• Going from 10 to 20 mpg (23.5 to 11.8 L/100 km) saves about 1,176 litres.
• Going from 30 to 40 mpg (7.84 to 5.88 L/100 km) saves about 196 litres.

Fuel cost scales directly with L/100 km. Converting everything to L/100 km, or to litres for your own annual distance, shows the real cost difference between cars.

Electricity consumption: what do kWh/100 km, Wh/km and miles/kWh mean?

EV electricity consumption describes how much energy a car uses per distance, or how much distance it covers per unit of energy.

• kWh/100 km is energy per 100 kilometres, and lower is better.
• Wh/km carries the same information. Multiply kWh/100 km by 10 to get it, so 18 kWh/100 km equals 180 Wh/km.
• miles/kWh is distance per energy, and higher is better. Convert with miles/kWh = 62.14 ÷ kWh/100 km. Example: 18 kWh/100 km ≈ 3.45 miles/kWh, and 4 miles/kWh ≈ 15.5 kWh/100 km.
• MPGe is a US rating that treats 33.7 kWh as one gallon of petrol. Convert with kWh/100 miles = 3,370 ÷ MPGe, then divide by 1.609 to get kWh/100 km.

Checks before comparing EV figures

1. Measurement point. Check whether the figure is measured from the charging point, which includes charging losses, or from the battery. Official test figures are typically measured at the plug. Trip computer readings usually reflect energy taken from the battery.
2. Battery capacity. Check whether the listed capacity is gross or usable kWh. Range depends on usable capacity.
3. Charging power. Make sure you compare AC with AC and DC with DC. Peak kW alone says little, so where available, a 10–80% charging time is more useful.

Test conditions: why do figures from different sources not line up?

Consumption, emissions and range figures depend on the test procedure that produced them. Figures from different test cycles should not be compared directly, even after unit conversion. The common procedures are:

Procedure	Where it is used	What to keep in mind
WLTP	EU, UK and many other markets for newer cars	Replaced NEDC in Europe. Figures can differ by trim, wheel size and options.
NEDC	Older European figures	Generally more optimistic than WLTP. Do not compare NEDC with WLTP.
EPA	United States	A different test method and adjustment approach, quoted in US units.
CLTC	China	EV range figures tend to come out higher than WLTP or EPA figures for the same car.
JC08 / WLTC	Japan	Usually quoted in km/L. Note which of the two procedures applies.

Even within one procedure, a trim with larger wheels or extra equipment can have a different official figure from the base model. A used-car listing may also repeat a figure from a different trim. When a figure looks unusually good or bad, check the cycle, the exact version and the model year before treating it as a real difference.

Vehicle dimensions, space and weight: what should you check?

Vehicle dimensions convert easily, at 1 inch = 25.4 mm, so 4,500 mm ≈ 177.2 in. The measurement definitions vary more than the units do.

• Width. Check whether the figure is with or without mirrors. This matters for narrow garages and parking spaces.
• Height. Check whether it includes roof rails or an antenna.
• Turning circle. Kerb-to-kerb figures are smaller than wall-to-wall figures.
• Boot or cargo volume. 1 cubic foot ≈ 28.3 litres, so 500 L ≈ 17.7 cu ft. European figures typically use the VDA method, and US figures use different methods, so a converted cu ft figure may still not compare. Also check whether the figure is measured to the parcel shelf or to the roof, and with the seats up or folded.
• Weight. 1 kg ≈ 2.205 lb. The EU "mass in running order" includes a 75 kg driver and a 90% full tank. US curb weight excludes the driver. Unladen and dry weights can be lower still.
• Towing and payload. Braked and unbraked towing limits are separate figures. Note which one a listing gives.

Performance figures: are 0–60 mph and 0–100 km/h the same?

No. 100 km/h is about 62.1 mph, so a 0–60 mph time is slightly quicker than a 0–100 km/h time for the same car. Some US magazine tests also subtract a short "rollout" from the start, which lowers the figure further. Top speed converts at 1 mph = 1.609 km/h. Compare acceleration times only when the target speed and method match.

How do you build a consistent side-by-side comparison?

A comparison sheet works best when every row is one attribute in a single chosen unit and every column is one candidate car. Build it in these steps:

1. Choose your units first. For example: kW, Nm, mm, litres, kg, L/100 km and kWh/100 km. Use mph or miles/kWh instead if those feel more natural, but use the same unit across every car.
2. Record the original value exactly as listed. Include its unit, such as "148 bhp" or "47 mpg", so you can check your conversions later.
3. Add the source and conditions. Note the listing, brochure or official data, plus the test cycle, trim, wheel size and model year.
4. Convert from the original value. Round only at the end, to one decimal place. Never convert an already-rounded conversion.
5. Mark each cell as stated, converted, estimated or unknown. This keeps your own estimates separate from published figures.
6. Add your own information. Useful extras are your annual mileage, a local fuel or electricity price, your garage width and anything you measured during a viewing.
7. Calculate figures that reflect your use. For example: annual litres = your km ÷ 100 × L/100 km, or annual kWh = your km ÷ 100 × kWh/100 km.
8. Flag rows that are not comparable. If two cars' figures come from different cycles or methods, note it instead of ranking them on that row.

How do you tell missing information from a genuine drawback?

Missing information is a gap in the listing, while a genuine drawback is a confirmed figure that falls short of your needs. Keep them separate in your sheet, for example with "unknown" and "confirmed" labels.

• Missing. The listing does not mention boot volume with seats folded, battery usable capacity or towing limit. Look up the official specification for that exact version, or ask the seller. Do not count the gap as a negative score.
• Not comparable. One car has a WLTP figure and another an NEDC or EPA figure. Neither figure is wrong, but they cannot rank the two cars against each other.
• Genuine drawback. A confirmed figure fails a requirement. Examples: a width with mirrors larger than your garage opening, or a confirmed braked towing limit below your trailer's weight.
• Within tolerance. Very small gaps can come from rounding or unit labels, such as 150 PS against 148 hp. Treat these as equal.

How should you compare options against your own priorities?

The best way to compare options is to rank the attributes that affect your use. Then judge each car only on those rows, using comparable figures. No single spec sheet result makes one car right for everyone. The following hypothetical cases show how priorities change what matters:

• High annual mileage. A difference of 1 L/100 km over 30,000 km a year is about 300 litres. That may outweigh a small power advantage.
• Low annual mileage, mostly in town. A consumption difference adds up to little. Turning circle, length and width with mirrors may matter more.
• Regular towing or full loads. Braked towing limit, payload and the torque curve come before peak power or acceleration.
• EV with home charging. kWh/100 km and AC charging power matter for daily cost. A buyer who often drives long trips may care more about usable battery capacity and DC 10–80% times.

A practical method is to split your rows into two groups. "Must meet" rows are pass-or-fail, such as fitting the garage or towing the trailer. "Nice to have" rows are weighted by how much they matter to you. Remove any car that fails a confirmed must-meet row. Resolve unknown values before deciding, and only then compare the remaining cars on the weighted rows. The resulting shortlist reflects your needs, and each figure in it has been converted and checked rather than copied from listings that use different units.