265/50R20 vs 275/55R20 — a taller sidewall does the work
Two Grand Cherokee fitments on the same 20-inch wheel, where a modest change on paper produces an inch and a half of tire.
275/55R20 is 1.48 in taller than 265/50R20 (4.85%) and 0.39 in wider. A speedometer calibrated for 265/50R20 would under-read by 4.85%.
Measured against each other
Overall diameter
30.43 in31.91 in+1.48 in (+4.85%)Section width
10.43 in / 265 mm10.83 in / 275 mm+0.39 in (+3.77%)Sidewall height
5.22 in5.95 in+0.74 inCircumference
95.6 in100.2 in+4.6 inRevolutions per mile
662.7632.0-30.7Ride height at the axle
——+0.74 inSpeedometer reading
——+4.85% under-read
| Dimension | 265/50R20 | 275/55R20 | Change |
|---|---|---|---|
| Overall diameter | 30.43 in | 31.91 in | +1.48 in (+4.85%) |
| Section width | 10.43 in / 265 mm | 10.83 in / 275 mm | +0.39 in (+3.77%) |
| Sidewall height | 5.22 in | 5.95 in | +0.74 in |
| Circumference | 95.6 in | 100.2 in | +4.6 in |
| Revolutions per mile | 662.7 | 632.0 | -30.7 |
| Ride height at the axle | — | — | +0.74 in |
| Speedometer reading | — | — | +4.85% under-read |
Calibrated for 265/50R20, running 275/55R20
- Dial reads 30
- 31.5 mph
- Dial reads 50
- 52.4 mph
- Dial reads 70
- 73.4 mph
- Odometer counts 100
- 104.9 mi covered
Change either size in the comparison tool, or see the full speed table in the speedometer error calculator.
Two sizes on one wheel, and a difference in ride quality that the 10 mm of width in the codes gives no hint of.
The sidewall is the story
Three-quarters of an inch more sidewall standing under the rim — 5.22 in against 5.95 in — on a wheel that is unchanged.
Most of that comes from the profile step rather than the width step. Fifty per cent of 265 mm is 5.22 in; fifty-five per cent of 275 mm is 5.95 in. Of the 0.74 in gained, roughly a quarter is attributable to the extra 10 mm of casing and the rest to the profile step.
Diameter and its consequences
1.48 in of it, or 4.85% — enough that the needle runs light by nearly three and a half miles an hour at 70 and that every thousand recorded miles are 1,049 real ones.
Ride height at the axle and the sidewall gain are both 0.74 in, and that is not a coincidence. When the wheel diameter is held, they are the same quantity measured from two different places, and this pairing is the clearest place on the site to see it.
Why 20-inch fitments vary so much
Because there is very little sidewall to work with, so a five-point profile step is a large proportion of what is there. Fifty to fifty-five is a ten per cent relative increase in a dimension that was already short, and that is why two sizes which look adjacent on a spec sheet feel so different from the driver's seat.
What stays the same
Section width barely moves: 10.43 in to 10.83 in, 3.77%. Four-tenths of an inch of nominal casing is a smaller quantity than the wheel underneath can introduce by itself — roughly 0.2 in of measured section per half inch of rim width — so on one particular pair of wheels the two figures can end up closer together than the codes say, or further apart.
That is as far as this page goes on the subject. Neither the rim widths these casings are approved for nor the room available at the WK2's steering stop is data this site holds, and both are measurements rather than inferences.
Both are original fitments
Neither of these is an aftermarket size. Both appear on the WK2 across its trim range, which means the manufacturer certified a 1.48 in spread within its own catalogue and calibrated the vehicle to tolerate it. A used buyer comparing two examples is comparing two factory configurations, not a stock car against a modified one.
The tool, with the pair loaded
The speedometer error calculator will produce the full speed table for this pair, and the aspect ratio reference explains why a five-point step is worth so much more at the bottom of the scale than at the top.