A speedometer error calculatorsettles the argument you have with your own dashboard: the needle says 70, your phone's GPS says 72, and one of them is wrong. If you've put on tires that aren't the size on the door-jamb placard, it's the needle. Swap the 265/70R17s your truck left the factory with for 285/70R17s and the speedometer starts under-reading by 3.49%— an indicated 70 is really 72.4 mph, and the odometer quietly loses 419 miles a year.
So Which One Is Lying — the Dash or Your Phone?
Your car has no idea how big its tires are. None. It counts wheel revolutions through the ABS sensor and multiplies by a circumference number stored in the ECU at the factory. Change the tire and that stored number becomes a lie, but the car keeps using it anyway. GPS doesn't care about any of that — it derives speed from the Doppler shift of the satellite signals, which is why published GPS accuracy is measured in feet, not car lengths. On open highway, the phone wins.
The arithmetic behind the whole thing is one line:
- True speed = indicated speed × (new diameter ÷ stock diameter)
- Metric tire diameter (inches) = rim + 2 × (section width × aspect ÷ 100) ÷ 25.4
Run the truck example through it. A 265/70R17 stands 31.61 inches tall; a 285/70R17 stands 32.71. That's a difference of 1.10 inches — about the thickness of your thumb, which is why nobody eyeballs it correctly. Divide 32.71 by 31.61 and you get 1.0349, so every speed on the dial is 3.49% low. The gap is a percentage, not a fixed offset, so it barely registers at 25 mph (0.9 mph out) and becomes 2.6 mph at 75. Our tire diameter calculator handles the first half of that math if you just want the height of a size you're considering.
Your Speedometer Was Already Wrong the Day You Bought the Car
Here's the part that changes how you should read your result. Factory speedometers are biased optimistic on purpose, and in most of the world they're legally required to be. UN Regulation 39 governs speedometers across Europe and much of Asia — it's named directly in UK road vehicle law as an accepted standard — and it states that the indicated speed may never be lower than the true speed, and may not exceed it by more than 10% plus 4 km/h. Read that carefully: reading fast is legal, reading slow is not. Manufacturers build in a safety cushion, typically 1–3%, so nobody ever ships a car that flatters the driver.
The United States has no equivalent rule. NHTSA rescinded FMVSS 127, the standard covering speedometers, back in 1982, and no federal accuracy requirement for passenger-car speedometers replaced it. Automakers keep the optimistic bias anyway, because they build one car for every market.
Now combine the two effects, because almost no one does. Say your truck read 2% fast from new — a perfectly ordinary factory bias. Bolt on those 3.49% taller tires and the two errors work against each other: 1.02 ÷ 1.0349 = 0.9855. Your real-world error is 1.4% slow, not 3.5%. At an indicated 70 you're doing 71.0 mph, not 72.4. A slightly larger tire can genuinely make a speedometer morehonest. That's why the calculator above has a GPS mode — the tire math gives you the change, but only a measured run against GPS gives you the absolute truth about your particular car.
How Far Off Are the Swaps People Actually Make?
These are the five changes that show up most often in driveways, run at an indicated 70 mph with 12,000 miles a year:
| Stock → Fitted | Height change | Error | True speed at 70 | Odometer / year |
|---|---|---|---|---|
| 205/55R16 → 225/45R17 | +0.09" | +0.38% | 70.3 mph | −46 mi |
| 265/70R17 → 285/70R17 | +1.10" | +3.49% | 72.4 mph | −419 mi |
| 245/75R17 → 285/70R17 | +1.24" | +3.94% | 72.8 mph | −473 mi |
| 235/65R17 → 265/60R18 | +1.49" | +5.14% | 73.6 mph | −617 mi |
| 265/70R17 → 245/70R17 | −1.10" | −3.49% | 67.6 mph | +419 mi |
Two rows deserve a second look. The 205/55R16 to 225/45R17 jump is textbook plus-sizing — a whole inch of extra wheel, and the overall height moves 0.09 inches. Done properly, going up a rim size costs you nothing on the dial, and our tire size calculator is built to find those matches. The 235/65R17 to 265/60R18 row is the same idea done badly: the buyer picked the wheel first and let the tire fall where it landed, and now the crossover is 3.6 mph faster than it claims and hiding 617 miles a year.
That last column matters more than people expect. Bigger tires turn fewer times per mile — 638 revolutions for the 31.61-inch tire, 617 for the 32.71 — and the odometer counts revolutions. Over five years the truck example hides 2,095 miles of genuine wear. Your oil-change reminders arrive 3.5% late, your warranty runs out later than it should on paper, and when you sell, the number on the dash understates what the car has actually done.
When This Calculator Won't Give You the Real Number
Every speedometer error calculation on the internet, including this one, works from geometric diameter — the height of an unloaded tire, calculated from the sidewall stamp. Reality is messier in four specific ways:
- A loaded tire squats.Put 1,200 pounds of corner weight on it and the rolling radius shrinks. That's why manufacturers' published revs-per-mile figures usually run 2–3% higher than pure geometry predicts. Both tires squat, so most of it cancels out of the ratio — unless you're comparing a stiff load-range-E truck tire against a soft passenger tire, where it won't.
- Tread wears away.A tire that starts at 10/32" and reaches the 2/32" wear bars has lost a quarter-inch of radius, or half an inch of diameter. On a 31.61-inch tire that's 1.58% — your error drifts by that much over the life of the set, which is more than some people's entire tire-swap error. Our tire tread depth calculator shows where in that 8/32" window you are sitting today.
- Pressure changes the radius too. Running 10 psi below placard flattens the contact patch and shortens the rolling radius. Get that right first with a tire pressure calculator and the NHTSA tire safety guidance, because underinflation is a safety problem long before it's a speedometer problem.
- Flotation sizes lie a little.A tire sold as 33x12.50R15 is nominal, not measured. Plenty of "33-inch" tires mic out at 32.2 inches. Use the manufacturer's spec sheet rather than the name on the sidewall, and enter it in the calculator's inches mode.
Do You Actually Need to Recalibrate?
Not every error is worth money to fix. Use the size of the gap to decide:
- Under 1% — ignore it.You're inside the noise of your factory bias and normal tread wear. There is nothing to correct.
- 1–3% — just know your number.This is the window tire shops treat as acceptable, and the range where a slightly taller tire may be cancelling the factory's optimistic bias rather than adding to it. Remember that an indicated 70 is really about 72 and move on.
- 3–5% — fix it if the car does the thinking.Cruise control, adaptive cruise following distance, and automatic transmission shift points all key off wheel speed. A dealer ECU reflash with the correct tire size usually runs $80–150; handheld tuners and standalone calibration modules cost more but let you change the number again next time. Taller tires also change your cruising RPM, which our RPM calculator will show you.
- Over 5% — recalibrate before you drive it far. Beyond this point the ABS and stability-control modules are working from meaningfully wrong wheel speeds, the odometer drift becomes a disclosure issue at resale, and you are more than 3.5 mph past where you think you are at highway speed.
One thing the gap won't buy you is a way out of a ticket. Officers measure your actual speed with radar, lidar, or a pace car, and telling a court your speedometer read low is an admission that you knowingly drove an uncalibrated vehicle. In most states the driver carries the duty to know their speed. Run the numbers before the traffic stop, not after it.