Acceleration
0 to 60 mph vs 0 to 100 km/h: Why the Numbers Never Match
60 mph is 96.56 km/h, and American tests start the clock one foot off the line. Those two conventions together separate the same car by three to six tenths.
The two figures measure different things. 60 mph is 96.56 km/h, so a 0 to 100 km/h run covers 3.44 km/h more than a 0 to 60 mph run. On top of that, American figures normally deduct the time spent covering the first foot off the line, which is worth another 0.26 to 0.42 s depending on how hard the car accelerates. Put together, the same car can honestly be quoted at 3.0 s in Europe and 2.6 s in the United States without anybody lying.
Start with the unit, because it is not a rounding error
One mile per hour is 1.609344 km/h, so 60 mph is 96.56 km/h and 100 km/h is 62.14 mph. A car that reaches 96.56 km/h still has 3.44 km/h to find before the European clock stops. That sounds trivial until you notice where in the run it happens: at the top, where the car is heaviest against aerodynamic drag and usually in a higher gear, so those last few km/h are the slowest ones of the whole exercise.
If acceleration were constant, the ratio between the two times would be fixed at 96.56 divided by 100, or 0.9656. A 3.00 s run to 100 km/h would be a 2.90 s run to 60 mph, a gap of 0.10 s. A 6.00 s run to 100 km/h would be 5.79 s to 60 mph, a gap of 0.21 s. The slower the car, the larger the gap in absolute terms, because it spends longer in that final slice of speed.
The first foot, and why it is missing
The second difference is a convention borrowed from drag racing. On a drag strip the timing beam does not fire when the driver reacts, it fires when the front wheel has rolled all the way past the staging beam, which is roughly one foot of travel. United States magazines adopted the same reference so that their published times could be compared with strip data, and the practice is now standard there. European manufacturers generally quote from a true standstill.
The size of that deduction is not fixed either. Time to cover a fixed distance from rest falls as acceleration rises, so a quick car loses less of its run to the first foot than a slow one. Working it through for a distance of 0.3048 m:
| 0 to 100 km/h, no rollout | Average acceleration | Time inside the first foot | Speed when the clock starts | 0 to 60 mph, with rollout |
|---|---|---|---|---|
| 3.00 s | 9.26 m/s², 0.94 g | 0.257 s | 8.6 km/h | 2.64 s |
| 4.00 s | 6.94 m/s², 0.71 g | 0.296 s | 7.4 km/h | 3.57 s |
| 5.00 s | 5.56 m/s², 0.57 g | 0.331 s | 6.6 km/h | 4.50 s |
| 6.00 s | 4.63 m/s², 0.47 g | 0.363 s | 6.0 km/h | 5.43 s |
| 8.00 s | 3.47 m/s², 0.35 g | 0.419 s | 5.2 km/h | 7.30 s |
Read the fourth column first, because it is the one that surprises people. When the American clock starts, the car is already moving at 5 to 9 km/h. It is not a standing start in any physical sense, it is a very slow rolling start, and the slower the car the slower that entry speed. Published rollout adjustments of 0.2 to 0.3 s are the values you get for genuinely quick cars, and the deduction grows past 0.4 s for ordinary ones.
Putting both differences together
The two effects pull in opposite directions, which is why the confusion survives. The unit difference makes the 0 to 60 time shorter than the 0 to 100 time. The rollout makes it shorter again. Neither is a mistake, and neither is disclosed in the headline.
- A car quoted at 3.00 s to 100 km/h in Europe is around 2.64 s to 60 mph by American convention. The gap is 0.36 s, or 12 % of the original figure.
- A car quoted at 6.00 s to 100 km/h is around 5.43 s to 60 mph. The gap is 0.57 s, or 9.5 %.
- Reverse the calculation before comparing anything. A US figure of 3.5 s corresponds to roughly 3.9 s to 100 km/h from a genuine standstill.
These are approximations, and they assume constant acceleration, which no real car delivers. A car with a violent first gear and a long second gear behaves differently from one with a close ratio set, and a gearshift landing just before 96.56 km/h can distort the relationship in either direction. The point is not that the conversion is exact, it is that the difference is systematic and large enough to change a ranking.
How to read a spec sheet without being caught
Three questions settle almost every argument about acceleration figures.
- Which speed? 60 mph and 100 km/h are 3.44 km/h apart. If the source does not say, look at the unit it used for everything else.
- With or without rollout? American publications and drag strips deduct it, European manufacturers usually do not. A figure quoted to two decimal places without a stated method is a marketing number.
- Manufacturer or independent? A factory claim is a promise about a production car in a defined condition. An independent test is one car, on one day, on one surface, at one temperature.
The habit worth building is to convert everything to a single convention before comparing. Pick 0 to 100 km/h without rollout, because it is the harder of the two and cannot be flattered by a timing choice, then adjust every American figure upward before it joins the table.
Questions readers ask
Is 0 to 60 mph the same as 0 to 100 km/h?
No. 60 mph is 96.56 km/h, so the 0 to 100 km/h run covers 3.44 km/h more. Assuming constant acceleration, the 0 to 60 mph time is about 0.9656 times the 0 to 100 km/h time, which is 0.10 s on a three second car and 0.21 s on a six second car.
What is rollout and why does it make cars look faster?
Rollout is the first foot, 305 mm, of travel off the line. American timing convention starts the clock only after the car has covered it, following drag strip practice. By then the car is already doing 5 to 9 km/h, so the published time is 0.26 to 0.42 s shorter than a run timed from a true standstill.
Do European manufacturers use rollout?
Generally no. European factory figures for 0 to 100 km/h are normally quoted from a standstill without a rollout deduction. That is the main reason a European claim and an American test result can differ by several tenths on the same car without either being wrong.
How do I convert a 0 to 60 mph time to 0 to 100 km/h?
Divide by 0.9656 to correct the speed difference, then add back the rollout, roughly 0.26 s for a car under 4 s and around 0.36 s for a six second car. A 3.5 s figure with rollout becomes approximately 3.9 s to 100 km/h from rest.
Which figure is the honest one?
Both are honest as long as the method is stated. The 0 to 100 km/h figure without rollout is the stricter measurement, because nothing about the timing convention can flatter it, which makes it the better basis for comparing cars from different markets.
Why do slower cars gain more from rollout?
Because the time needed to cover a fixed distance from rest rises as acceleration falls. A car pulling 0.94 g spends 0.257 s inside the first foot. A car pulling 0.35 g spends 0.419 s there, so it has more time deducted from a run that was already longer.
Sources
- Rollout (drag racing), on the one foot staging convention and its origin at the drag strip.
- 0 to 60 mph, on the difference between the American and European benchmarks and typical rollout deductions of 0.2 to 0.3 s.
- 0-60 Specs, testing methodology, on how published acceleration figures are verified and which conventions are applied.
Conversions and worked examples in this article use 1 mph = 1.609344 km/h and a rollout distance of one foot, 0.3048 m, with constant acceleration assumed for the arithmetic.