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Size comparisons

245/75R17 vs 285/70R17 — base trim to off-road trim

The two tires a fourth-generation Tacoma is sold with, and a case where the casing widens far more than the tire grows.

245/75R1731.47 in tall9.65 in wide285/70R1732.71 in tall11.22 in wide+1.24 in · +3.94%
Drawn to scale from the size codes. Solid outline is 245/75R17; dashed outline is 285/70R17. The bars below each tire are section width at the same scale.

285/70R17 is 1.24 in taller than 245/75R17 (3.94%) and 1.57 in wider. A speedometer calibrated for 245/75R17 would under-read by 3.94%.

Measured against each other

  • Overall diameter

    31.47 in32.71 in+1.24 in (+3.94%)

  • Section width

    9.65 in / 245 mm11.22 in / 285 mm+1.57 in (+16.33%)

  • Sidewall height

    7.23 in7.85 in+0.62 in

  • Circumference

    98.9 in102.8 in+3.9 in

  • Revolutions per mile

    640.9616.6-24.3

  • Ride height at the axle

    ——+0.62 in

  • Speedometer reading

    ——+3.94% under-read

Calibrated for 245/75R17, running 285/70R17

Dial reads 30
31.2 mph
Dial reads 50
52.0 mph
Dial reads 70
72.8 mph
Odometer counts 100
103.9 mi covered

Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.

The same truck, two trim levels, and two tires that differ more in shape than in size.

The width leads

1.57 in of extra section width against 1.24 in of extra diameter. That ordering is unusual in this comparison set — normally the height change is the larger of the two — and it happens because the aspect ratio drops from 75 to 70 at the same time as the width climbs by 40 mm.

The drop in profile eats some of the height the extra width would otherwise have produced. What survives is 1.24 in, or 3.94%.

Two changes pulling against each other

  • 245 mm to 285 mm: 40 mm, four increments of the metric width scale in one move
  • 75-series to 70-series: a five-point reduction, worth roughly 0.55 in of sidewall on a 285

Sidewall ends up at 7.85 in against 7.23 in — only 0.62 in more, despite the casing being an inch and a half wider. The profile change is doing most of the moderating.

What the truck sees

Multiply anything the cluster says by 1.0394 and you have the truth. One factor does for every readout on the dash, because none of them is measuring independently — each is a rotation count converted into whatever unit its display wants.

Four per cent is enough to be measurable and not enough to require anything to be done about it, which is where most factory-to-factory steps land by design.

Why the trims differ this way

Because the off-road package wants footprint rather than ride height. Forty millimetres of casing changes how the tire behaves on soft ground far more than an inch of diameter does, and keeping the diameter increase modest keeps the gearing, the speedometer and the driveline within the calibration already homologated for the base truck.

Buying the tire is not buying the assembly

An off-road trim is a package, and the tire is one item in it. Wheel width is another, and it is the one no size code records — not in 245/75R17, not in 285/70R17, and not in this site's data either. So a base truck fitted with the wider casing on its existing wheels is not automatically running what the higher trim runs, and the difference is unquantifiable from here.

The direction of the effect is at least predictable. The narrower the rim relative to what a casing was specified against, the more the shoulders draw inward: it measures under its nominal 11.22 in and stands a shade over its computed 32.71 in. Roughly 0.2 in of measured section per half inch of rim is the working figure, which the rim width reference sets out.

The same base tire, aimed higher

If the destination is a 35 rather than the off-road trim's size, 245/75R17 against 35x12.50R17 is nearly three times the diameter change and a much larger project. The 33-inch middle option sits between the two.

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