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Size comparisons

245/70R17 vs 265/70R17 — 20 mm on the same wheel

The Grand Cherokee's step up within its own factory range, and the 1.4 multiplier that governs every fixed-aspect swap.

245/70R1730.50 in tall9.65 in wide265/70R1731.61 in tall10.43 in wide+1.10 in · +3.61%
Drawn to scale from the size codes. Solid outline is 245/70R17; dashed outline is 265/70R17. The bars below each tire are section width at the same scale.

265/70R17 is 1.10 in taller than 245/70R17 (3.61%) and 0.79 in wider. A speedometer calibrated for 245/70R17 would under-read by 3.61%.

Measured against each other

  • Overall diameter

    30.50 in31.61 in+1.10 in (+3.61%)

  • Section width

    9.65 in / 245 mm10.43 in / 265 mm+0.79 in (+8.16%)

  • Sidewall height

    6.75 in7.30 in+0.55 in

  • Circumference

    95.8 in99.3 in+3.5 in

  • Revolutions per mile

    661.2638.1-23.1

  • Ride height at the axle

    ——+0.55 in

  • Speedometer reading

    ——+3.61% under-read

Calibrated for 245/70R17, running 265/70R17

Dial reads 30
31.1 mph
Dial reads 50
51.8 mph
Dial reads 70
72.5 mph
Odometer counts 100
103.6 mi covered

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

The aspect ratio does not move, so this pairing is governed by a single relationship and nothing else.

Only one dimension was chosen

Somebody specifying 265/70R17 in place of 245/70R17 is choosing 20 mm of width. The 1.10 in of extra height arrives unrequested, because the 70 in the code makes the sidewall a fixed proportion of whatever width it is given.

The consequence is that a swap people describe as "a bit wider" is in fact a bigger change in the dimension nobody mentioned than in the one they did.

What it costs the instruments

3.61% is just past the threshold at which people stop calling an error negligible. It is measurable against a GPS, it accumulates visibly over a year — a thousand indicated miles are 1,036 on the ground — and it is small enough that essentially nobody corrects it.

At motorway pace the needle understates road speed by roughly two and a half miles an hour.

The Grand Cherokee context

Both sizes appear on the WK2 across its trim range, so this is an internal comparison rather than an aftermarket one. A buyer looking at two used examples with different wheels and tires is looking at a 1.10 in difference in how tall the vehicle stands, half of which — 0.55 in — is at the axle.

Sidewall, and where it goes

0.55 in more sidewall per side, 6.75 in against 7.30 in. On an independently sprung SUV with a relatively tight front arch, the extra 0.55 in above the tire is generally the tighter constraint than the 0.79 in of extra casing, which splits roughly half inboard and half out on an unchanged wheel offset. That ordering is the reverse of the usual pattern on solid-axle vehicles.

The multiplier, stated once

Every fixed-aspect swap obeys one rule: the diameter moves by the width increment multiplied by twice the profile expressed as a fraction. At 70-series that factor is 1.4, which is why 0.79 in of casing returns 1.10 in of height. It also means the number in the middle of the code decides how expensive width is — on a 60-series tire the same 20 mm would have returned less, and on a 75-series tire more.

Going further on the same wheel

The next size up in the same family is 285/70R17, which adds another 1.10 in for another 20 mm of casing. The rungs are evenly spaced, which makes this particular ladder easy to plan and easy to climb one step too far.

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