The speeding cost benefit calculator above leads with the only number most people have never actually worked out: how many minutes a higher speed saves on a real journey. The answer is almost always smaller than the intuition that drives the behaviour. Thirty miles at 80 mph instead of 70 mph saves three minutes and thirteen seconds — and that is the theoretical best case, before a single traffic light, roundabout or queue takes it back.
Arb Digital builds free calculators that put the honest arithmetic first rather than the flattering one. This page exists because the time saving is the finding. Everything below it — the extra fuel, the penalty figures you enter — is context for a number that is usually too small to have been worth thinking about in the first place.
What This Calculator Does
It compares one journey done at two speeds. You enter the distance, the speed limit for that road, and the higher speed you want to compare against it. The headline is the difference in journey time, expressed in minutes and seconds, because seconds are the unit this question genuinely lives in.
Below the headline, three supporting figures. Journey time at each speed, so the comparison is visible rather than asserted. Extra fuel cost, calculated from your own economy figure, your own fuel price, and a drag model described in full further down. And a cost-per-minute-saved figure that divides everything — extra fuel plus whatever penalty amounts you entered — by the minutes gained.
Two things this page deliberately does not do. It publishes no fine amounts, no penalty point tables and no licence thresholds, because those are set by jurisdiction, differ enormously between them, and change without notice; a table baked into a web page would be wrong somewhere and out of date everywhere. And it does not estimate the probability of being caught, because a probability field turns the tool into a way of rationalising a decision rather than a way of measuring one.
How to Use It
- Enter the distance of the stretch where the higher speed would actually be sustained. This is where most people overstate the saving: a 200-mile trip that includes 40 miles of urban driving only offers 160 miles of possible gain.
- Enter the speed limit for that road and the higher speed you are comparing. Read the time saved at the top before you read anything else.
- Enter your vehicle's economy at the limit and your local fuel price. Both are your figures; nothing is assumed.
- Set the aerodynamic share of road load. Sixty per cent is a reasonable starting point for a typical car at motorway speed, and raising it makes the higher speed cost more fuel.
- If you want the penalty side, enter the fine amount and any other consequential cost that would apply where you drive. Leave them at zero to see the fuel comparison alone.
The Formula and How It Is Calculated
Journey time is distance divided by speed. Time saved is the difference between the two. That is the whole of the headline calculation, and its simplicity is the point — this is arithmetic anyone could do, which is why it is striking that so few people have.
Work through the defaults. Thirty miles at 70 mph takes 30 ÷ 70 hours, which is 25 minutes and 43 seconds. The same thirty miles at 80 mph takes 22 minutes and 30 seconds. The saving is 3 minutes and 13 seconds. Put another way, at these two speeds you would need to drive 560 continuous miles to save a single hour — and you can check that directly, since 560 miles at 70 mph is exactly eight hours and at 80 mph exactly seven.
The fuel side needs a model, and this one is stated rather than hidden. The force resisting a car at speed is roughly the sum of rolling resistance, which is nearly constant with speed, and aerodynamic drag, which rises with the square of speed. Energy used per mile therefore scales as (1 − a) + a × (v₂ ÷ v₁)², where a is the aerodynamic share of road load at the reference speed.
At the defaults, (80 ÷ 70)² is 1.306, so the ratio is 0.40 + 0.60 × 1.306 = 1.184. Economy falls from 40 mpg to 40 ÷ 1.184 = 33.79 mpg. The thirty miles costs 0.75 gallons at the limit and 0.888 gallons at the higher speed, which at $4.00 a gallon is $3.00 against $3.55 — an extra 55 cents to save three minutes and thirteen seconds.
That model is an approximation and says so. It ignores gearing, engine efficiency maps, headwinds and the fact that many cars are geared to sit efficiently at a particular cruise. It is good enough to show the direction and rough magnitude, which is all it claims.
Why the Real Saving Is Smaller Than the Calculated One
Everything the calculator reports is a best case, and four things erode it in practice.
The first is that no journey is run at one speed. Junctions, queues, roadworks, slower vehicles and the simple fact that traffic sets the pace mean the higher speed is available for a fraction of the distance. A saving computed over thirty uninterrupted miles is not the saving available on thirty ordinary miles.
The second is convergence. On busy roads, driving faster mostly means arriving sooner at the back of the same queue. Anyone who has overtaken a line of cars and then sat behind them at the next set of lights has already run this experiment.
The third is that the saving shrinks as speeds rise. Going from 30 to 40 mph over ten miles saves five minutes; going from 70 to 80 mph over the same ten miles saves just over one. Time saved depends on the difference between the reciprocals of the speeds, so the faster you already are, the less each additional mile per hour buys. The most time-efficient speed increases are the ones on slow roads, where they are also the most dangerous.
The fourth is that stops dominate. A single five-minute fuel or coffee stop wipes out the entire gain from an hour of sustained speeding on most journeys. If the goal is genuinely to arrive earlier, leaving earlier or planning around traffic beats driving faster by an order of magnitude — and our speed distance time calculator exists for exactly that kind of planning, while the commute cost calculator puts a figure on the trip you make most often.
What This Page Prices, and What It Cannot
Be clear about the boundary. This tool prices two things: some extra fuel, and a fine plus any consequential cost you chose to enter. That is not the cost of speeding. It is the cost of the two consequences that happen to be arithmetic.
The consequence that is not in the number is a collision, and it is not omitted because it is small. It is omitted because it cannot honestly be reduced to an expected value on a calculator page. Two physical facts sit behind that. Kinetic energy rises with the square of speed, so a car at 80 mph carries about thirty per cent more energy to dissipate than the same car at 70. And stopping distance grows faster than speed, because thinking distance rises in proportion to speed while braking distance rises with its square — which is why the UK's Highway Code publishes stopping distances as a curve rather than a line, and states in its general rules for drivers and riders that "the speed limit is the absolute maximum and does not mean it is safe to drive at that speed irrespective of conditions".
The World Health Organization's factsheet on road traffic injuries puts a figure on the relationship at a population level: every one per cent increase in mean speed produces roughly a four per cent increase in fatal crash risk and a three per cent increase in serious crash risk. That is an epidemiological finding about traffic systems, not a prediction about any individual trip, and it is quoted here for exactly that reason — it describes the shape of the risk that the fuel-and-fine arithmetic on this page leaves out.
Speed limits are legal requirements, not suggestions, and they are set for the road rather than for the vehicle. Nothing on this page changes that, and the calculator is not a decision aid for whether to exceed one.
Where the Time Really Goes on a Journey
If arriving earlier genuinely matters, the levers are not on the accelerator. Departure timing is the largest by a wide margin: leaving thirty minutes before or after a peak can change a commute more than any legal speed change could. Route choice is second, particularly where a slightly longer road avoids junctions. Stops are third — consolidating two stops into one usually beats an hour of pushing the speed.
There is also a cost side to the same question. Steady driving at or below the limit is where fuel economy peaks for most cars, and the difference compounds over a year of commuting. Our fuel cost calculator works out what a given journey costs at a given economy, the MPG calculator turns your own fill-up records into a real economy figure rather than the manufacturer's, and the road trip cost calculator handles multi-leg trips. For unit changes, the speed converter and fuel economy converter are quicker than doing the conversion by hand.
One more framing worth keeping. The cost-per-minute-saved figure in the grid is a useful sanity check because it converts the whole question into a wage. If a single fine would make each saved minute cost more than a minute of your time is worth — and with the defaults, that figure runs to tens of dollars per minute — then the arithmetic has answered itself without anyone needing to make an argument about safety.
Arb Digital designs and builds free interactive calculators that state their assumptions, show their arithmetic and earn links because they are genuinely useful. Browse what we have already published, or tell us what your audience keeps searching for.
Browse the Free Tools Hub Talk to Arb DigitalCommon Mistakes to Avoid
- Using the whole journey distance. Only the stretch where the higher speed is actually sustainable counts, and on most trips that is a fraction of the total.
- Treating the calculated saving as achievable. It is a theoretical best case that assumes constant speed with no junctions, no traffic and no stops.
- Assuming the saving scales with the speed difference. It does not. Ten mph added to 70 saves far less time than ten mph added to 30 over the same distance.
- Reading a low fuel cost as the full cost. This page prices fuel and the penalty figures you entered. It does not price a collision, and it cannot.
- Looking for a fine table on this page. There is not one, on purpose. Penalties vary by jurisdiction and change, so the amount has to be the one that applies where you are driving.
Related Free Tools From Arb Digital
For journey planning use the speed distance time calculator, and the reaction time calculator for the thinking-distance side of stopping. For running costs, the fuel cost calculator, the MPG calculator and the road trip cost calculator cover a single trip through to a full itinerary, and the speed converter plus fuel economy converter handle unit changes. Everything else is on the free online tools hub.
Frequently Asked Questions
Far less than most people expect. Over 30 miles, driving at 80 mph instead of 70 mph saves 3 minutes and 13 seconds. At those two speeds you would have to drive 560 continuous miles to save one full hour, and that assumes no traffic, junctions or stops.
Because journey time depends on the reciprocal of speed, not on speed itself. Adding 10 mph to 30 mph cuts a ten-mile leg by five minutes; adding 10 mph to 70 mph cuts the same leg by just over one minute. The faster you already are, the less each extra mile per hour buys.
It depends on your car, but the direction is consistent because aerodynamic drag rises with the square of speed while rolling resistance barely changes. With the defaults here, going from 70 to 80 mph drops economy from 40 mpg to about 33.8 mpg, which over 30 miles is roughly half a dollar of extra fuel.
Because penalties are set by jurisdiction, differ enormously between them, and change without notice. Any table published here would be wrong somewhere and out of date everywhere, so the fine and any other consequential cost are figures you enter for the place you are driving.
No. Speed limits are legal requirements rather than suggestions, and this page is not a decision aid for breaking one. It prices two consequences that happen to be arithmetic — extra fuel and the penalty figures you entered — and it explicitly does not price a collision.
Because it does not scale evenly. Thinking distance rises in proportion to speed, but braking distance rises with the square of it, so total stopping distance grows as a curve. Kinetic energy also rises with the square of speed, so there is more energy to dissipate in any impact.
Departure timing, route choice and cutting stops, in that order. Leaving before or after a peak changes a typical journey by more than any legal speed change can, and consolidating a single five-minute stop usually saves more than an hour of sustained speeding would.
Speed limits are legal requirements and this page is an arithmetic tool, not advice about how to drive. It prices only extra fuel and the penalty figures you enter; it does not and cannot price a collision, and stopping distance and collision energy both rise faster than speed does.
