The speedometer error calculator above answers a question that becomes urgent the moment someone fits larger tyres or changes an axle ratio: what is the car actually doing when the dial says 60? A taller tyre covers more ground per revolution, so the instrument — which counts revolutions, not distance — under-reports. The needle says 60 and the vehicle is doing 64.
Arb Digital builds free calculators that state the convention they use and flag the consequences rather than just returning a percentage. Here that matters, because the direction of the error is not a detail. A speedometer reading above true speed is merely annoying. One reading below true speed is a regulatory problem in a great many jurisdictions, and this page says so on the result rather than burying it.
What This Speedometer Error Calculator Does
You give it the original tyre size and the new one, optionally the original and new axle ratios, and the speed currently showing on the dial. It reports the true road speed, the percentage error and its direction, how far you have really travelled for every hundred indicated miles, the revolutions per mile of the new tyre, and — for vehicles that still use a cable-driven mechanical speedometer — the driven gear tooth count that would restore accuracy.
The odometer figure matters more than people expect. Speedometer error and odometer error are the same error, so a setup that under-reads speed by seven per cent also under-records mileage by seven per cent. Over a lease term, a service interval or a resale, that adds up in a way the speed reading does not.
One boundary worth stating plainly. Our tire size calculator is the tool for decoding a tyre code into its dimensions — diameter, circumference, sidewall height, and the percentage difference between two sizes. This page starts where that one stops: it converts a size change into speed, distance and gear-tooth consequences, and it adds the axle ratio, the indicated-speed reading and the mechanical gear correction, none of which are size questions at all.
How to Use It
- Enter the three numbers from your original tyre code. In 245/75R16 the section width is 245 mm, the aspect ratio is 75 per cent and the rim is 16 inches.
- Enter the same three numbers for the new tyre.
- If you have changed the differential ratio, enter both the old and new figures. If not, leave them equal and the axle term drops out of the maths.
- Enter the speed showing on the dial. Read the true speed and, just as importantly, which way the error runs.
- For a mechanical cable speedometer, enter the drive gear tooth count to get the driven gear you would need. Set it to zero on any vehicle with an electronic sensor.
The Formula and the Tyre Geometry Convention
The tyre convention used here is the standard metric one, and it is worth spelling out because everything else follows from it. Section width is in millimetres. Aspect ratio is the sidewall height expressed as a percentage of that width. Rim diameter is in inches. So sidewall height in millimetres is width × aspect ÷ 100, and overall diameter is rim × 25.4 + 2 × (width × aspect ÷ 100), in millimetres.
Take the defaults. A 245/75R16 has a sidewall of 245 × 0.75 = 183.75 mm and an overall diameter of 406.4 + 367.5 = 773.9 mm, which is 30.47 inches. A 285/70R17 has a sidewall of 199.5 mm and a diameter of 431.8 + 399 = 830.8 mm, or 32.71 inches. The new tyre is 7.35 per cent taller.
True speed is then indicated speed × (new diameter ÷ old diameter) × (old axle ratio ÷ new axle ratio). At an indicated 60 mph on the taller tyre with no gear change, true speed is 60 × 1.0735 = 64.41 mph. Expressed against true speed, the error is (60 − 64.41) ÷ 64.41 = −6.85 per cent: the instrument is under-reading by nearly seven per cent, which at motorway speeds is more than four miles per hour.
Revolutions per mile comes from the same geometry: there are 63,360 inches in a mile, so revolutions per mile is 63,360 ÷ (π × diameter in inches), which simplifies to the widely used 20,168 ÷ diameter. The original tyre turns 661.9 times per mile; the new one turns 616.6.
The mechanical gear correction uses the classic cable-speedometer relation, described in industry references such as the JEGS guide to choosing the correct transmission speedometer gears: driven gear teeth = drive gear teeth × axle ratio × tyre revolutions per mile ÷ 1,001, where 1,001 is the cable revolutions per mile the instrument expects. With 8 drive teeth, a 3.55 axle and the original tyre, that gives 18.78 — a 19-tooth gear. On the taller tyre it becomes 17.49, so a 17-tooth gear is the correction. Different manufacturers used slightly different cable constants, so confirm the figure for your gearbox before buying a gear.
Why Under-Reading Is the Direction That Matters
Vehicle speedometers are not designed to be accurate. They are designed to be accurate or optimistic, never pessimistic, and that asymmetry is written into regulation. UN Regulation No 39, which governs speedometer and odometer approval across the countries that apply UNECE vehicle rules, requires that the indicated speed never falls below the true speed. Its published form, reproduced in the official text of UN Regulation No 39, is 0 ≤ (V₁ − V₂) ≤ 0.1 V₂ + 4 km/h, where V₁ is the displayed speed and V₂ the true speed. In plain terms: the display may over-read by up to ten per cent plus four kilometres per hour, and may never under-read at all.
That is why a factory speedometer usually shows a little more than you are doing. It also means a modification that shifts the reading downward can move a compliant instrument outside the approved window. In jurisdictions that apply this regulation, and in many that apply their own equivalent, a speedometer that under-reads can fail a roadworthiness inspection, and it removes the safety margin that manufacturers deliberately build in.
The practical consequence is the one drivers care about. If your instrument under-reads by seven per cent, then holding an indicated 70 means travelling at about 75. You have not chosen to speed, but the enforcement equipment measures true speed, not what your dashboard believes. This is arithmetic, not legal advice — the rules that apply to your vehicle are set by where it is registered and driven, and only they govern.
What a Taller Tyre Actually Changes
The speedometer is the visible symptom, but a diameter change ripples through several systems. Effective gearing changes: a tyre seven per cent taller behaves like an axle ratio seven per cent numerically lower, so the engine turns more slowly at a given road speed. That can help economy at a steady cruise and hurt acceleration and towing, which is why people who fit large tyres often regear to compensate — and why this calculator carries both axle ratio fields.
The odometer under-records by the same percentage, which quietly changes service intervals, warranty mileage and anything measured in miles. On a leased vehicle it also means the recorded mileage understates real use, which is not the direction anyone should rely on.
Systems that infer road speed from wheel speed sensors are affected too, including cruise control set points and any function that compares wheel speeds. Some vehicles allow the tyre size to be reprogrammed in the instrument cluster, some need an external correction module, and older mechanical setups need the gear swap this page calculates. Which of those applies is a vehicle-specific question.
Fitment is a separate matter that this calculator does not address. A larger diameter can foul suspension components or bodywork at full lock or full compression, and load index and speed rating have to be appropriate for the vehicle. Diameter arithmetic tells you nothing about whether the tyre fits.
How This Fits With the Other Driving Tools
Use the tire size calculator first if you want to compare tyre specifications themselves — diameter, circumference and sidewall height for a given code. Come here for the consequences of the change: true speed, odometer drift, axle ratio interaction and gear teeth.
For the rest of a journey, the speed distance time calculator works out timings from a true speed, the speed converter moves between mph and km/h, and the MPG calculator turns fill-up records into a real economy figure. Remember that an under-recording odometer also distorts calculated fuel economy, since the distance term is wrong — the fuel economy converter is useful once you have corrected it. On the cost side, the fuel cost calculator prices a trip and the percentage difference calculator handles the general case of comparing two figures.
Arb Digital designs and builds free interactive calculators that state their conventions, 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
- Assuming a bigger tyre makes the speedometer read high. It reads low. A taller tyre covers more ground per revolution, so the instrument counts fewer revolutions for the same distance.
- Ignoring the odometer. The same percentage error applies to recorded mileage, which affects service intervals, warranty and lease terms as well as any economy figure calculated from distance.
- Using placard diameter instead of the fitted tyre. Calculate from the size actually on the vehicle, not the one on the door sticker, if they differ.
- Forgetting that the axle ratio pushes the other way. A numerically higher ratio makes the instrument read high, which can partly or wholly offset a taller tyre.
- Buying a speedometer gear on this number alone. Cable constants and available tooth counts vary by gearbox, so confirm the correct part for your specific transmission.
Related Free Tools From Arb Digital
Pair this with the tire size calculator for the tyre specifications themselves, the speed distance time calculator and speed converter for journey maths, the MPG calculator and fuel economy converter for economy once the odometer is corrected, and the fuel cost calculator for running costs. The percentage difference calculator covers the general comparison case. Everything else is on the free online tools hub.
Frequently Asked Questions
Slow. A taller tyre travels further per revolution, and a speedometer counts revolutions, so it under-reports both speed and distance. In the default example a 7.35 per cent taller tyre makes an indicated 60 mph actually 64.4 mph.
The standard metric one. Section width is in millimetres, aspect ratio is sidewall height as a percentage of that width, and rim diameter is in inches, so overall diameter equals rim times 25.4 plus twice (width times aspect divided by 100), in millimetres.
It is a problem in many jurisdictions. UN Regulation No 39 requires that the indicated speed never falls below the true speed, allowing an over-read of up to ten per cent plus four kilometres per hour and no under-read at all. Whether and how that applies to your vehicle depends on where it is registered and driven, and your local rules govern.
Driven gear teeth equals drive gear teeth multiplied by the axle ratio, multiplied by the tyre's revolutions per mile, divided by 1,001. That constant is the cable revolutions per mile a typical instrument expects, and it varies slightly between manufacturers, so confirm the figure for your gearbox.
Yes, because both are driven from the same revolution count. A setup that under-reads speed by seven per cent also under-records distance by seven per cent, so real mileage runs ahead of what the dashboard shows.
It works in the opposite direction to tyre diameter. A numerically higher ratio spins the driveshaft faster for the same road speed and makes the instrument read high, so regearing can partly or completely offset the error a taller tyre introduces.
That tool decodes tyre codes and compares specifications: diameter, circumference and sidewall height. This one converts a change into its consequences — true speed at a given indicated reading, odometer drift, axle ratio interaction and the corrected speedometer drive gear.
This is an arithmetic tool, not a compliance check or legal advice. Speedometer accuracy requirements, roadworthiness testing and tyre fitment rules are set by the jurisdiction where the vehicle is registered and driven, and only those rules govern.
