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Speedometer error

Speedometer calculator

With 285/75R17s on a 2026 Toyota Tacoma set up for 265/70R17s, the speedometer reads 6.6% low: 56.1 mph at a true 60, and 60 on the dial is really 64.2 mph. Set your tires and gears for the full table.

Set for

Stock
mm
%
in
31.6 in · 803 mm tall

The size the speedometer was set up for: the stock tire, unless it has been recalibrated.

On the truck

mm
%
in
33.8 in · 859 mm tall
Maker

For my vehicle

Fills in the stock tire size and axle ratio the speedometer was set up for.

2026 Toyota Tacoma TRD Off-Road Double Cab 5-ft bed 4x4

Stock tire
265/70R17
Maker
Axle ratio
3.583
Maker

Source: 2026 Tacoma Product Information (opens in a new tab)

285/75R17 on a dial set for 265/70R17

Math on the nominal sizes

At a true 60

56.1mph

what the dial shows

Error

−6.6%

reads low

Dial shows 60

64.2mph

your true speed

Odometer

93.4mi

counted per 100 real miles

True speed

020406080100TRUE 6056.1MPH ON THE DIAL
Needle: what the speedometer shows. Laser mark: your true speed. The lit arc between them is the error.
What the dial shows
True speedDial showsOff by
25 mph23.4 mph−1.6
45 mph42.0 mph−3.0
60 mph56.1 mph−3.9
75 mph70.1 mph−4.9
How fast you’re really going
Dial showsTrue speedkm/h
25 mph26.8 mph43
45 mph48.2 mph78
60 mph64.2 mph103
75 mph80.3 mph129

What to do

Guidance, not advice

  • Speedometer

    Recalibrate

    Reads 6.6% low: at an indicated 70 you're really doing 74.9. Over 3% is worth fixing. A programmer or the dealer sets the new tire size (and axle ratio, if it changed); use the tire maker's revolutions per mile for the most accurate result, then check against GPS.

  • Odometer

    Counts wrong

    It counts 93.4 miles for every 100 you drive: 10,000 on the display is 10,704 real miles, so oil changes and warranty miles come due sooner than it says.

Compare these tires →Gears for these tires →Toyota Tacoma spec plate →

Formulas

How we calculate

A speedometer counts turns and multiplies by the distance one turn covers on the tire it was set up for. Change the tire, or the gears between the sensor and the wheel, and the count means something else.

Tire diameter

wheel + 2 × width × aspect ÷ 100 ÷ 25.4 (inch sizes state it)

e.g. 265/70R17 = 31.61 in, 285/75R17 = 33.83 in

Dial factor

old diameter ÷ new diameter

e.g. 31.61 ÷ 33.83 = 0.9342

After a regear

factor × new axle ratio ÷ old axle ratio (speed read at the transmission; at the wheels it stays the same)

e.g. 0.9342 × 3.91 ÷ 3.583 = 1.0195

Dial shows

true speed × factor

e.g. 60 × 0.9342 = 56.1 mph

True speed

dial ÷ factor

e.g. 60 ÷ 0.9342 = 64.2 mph

Error

(factor − 1) × 100

e.g. (0.9342 − 1) × 100 = −6.6%

Odometer

100 × factor miles counted per 100 driven

e.g. 100 × 0.9342 = 93.4 mi

We call it worth recalibrating above 3% off: about 2 mph at 60, and the odometer off by 3 miles in every 100. That’s a rule of thumb, not a legal limit.

Stock tire sizes and axle ratios come from the makers’ documents, each linked. The rest is math on nominal sizes; a real tire’s measured diameter and revolutions per mile are on its maker’s spec sheet, and those are the numbers to program.

FAQ

Questions people ask

How fast am I really going on bigger tires?

Faster than the dial says, by the ratio of the diameters. On 285/75R17s with a speedometer set for 265/70R17s, an indicated 60 is 64.2 mph and an indicated 70 is 74.9. The dial shows 56.1 at a true 60.

Does regearing fix the speedometer?

Only if the speedometer reads speed at the transmission or transfer case. Then a regear changes the reading too: going from 3.583 to 3.91 with 285/75R17s moves the error from −6.6% to +2.0%. If speed comes from the ABS wheel sensors, a regear changes nothing and only a recalibration does.

How much speedometer error is OK?

Under 1% you won’t notice. Between 1% and 3% it’s a mile or two an hour at highway speed. Above 3% it’s worth fixing: 3% is about 2 mph at 60, and the odometer is off by the same 3%, so service intervals and resale miles drift too.

How do I recalibrate the speedometer?

On most modern trucks you program the new tire size (as revolutions per mile or diameter) and axle ratio with a handheld programmer or the dealer’s scan tool. Older trucks with a cable-driven speedometer need a different driven gear. Either way, check the result against GPS at a steady highway speed.

Does tread wear change the speedometer?

A little. Tread comes off both sides of the diameter, so a tire with about 15/32 in of tread worn to 2/32 in is 2 × 13/32 = 0.81 in shorter: on a 285/75R17 that is 2.4%. Calibrate to a tire with some wear, or to the maker’s revolutions per mile.

What is the error for a 33 or 35 on stock 31s?

From 265/70R17 (31.6 in): 285/70R17 (32.7 in) reads 3.4% low, 58.0 at a true 60; 35x12.50R17 reads 9.7% low, 54.2 at a true 60 and 77.5 mph when it shows 70.

Data

Stock sizes and axle ratios

What each speedometer leaves the factory set for, from the makers' own documents.

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