Ford Explorer 6th gen: one recorded fitment shrinks the tire

Two fitments that match to a hundredth of an inch

255/65R18 measures 31.05 in. 255/55R20 measures 31.04 in. Same 255 mm section width, two inches of extra wheel on the second, and the aspect ratio drops ten points to absorb all of it.

275/45R21 breaks the run at 30.74 in — 0.99% shorter than the 18-inch fitment, 0.79 in wider, and with only 4.87 in of sidewall against the base tire's 6.53 in.

So the 18-inch package carries the tallest tire of the three and the 21-inch package the shortest — the ranking runs backwards against the wheel size.

A recorded upgrade that is a downgrade in diameter

Two sizes are recorded as commonly fitted, and they run in opposite directions:

• 265/65R18 at 31.56 in — +0.51 in and 1.65% from 255/65R18.
• 265/60R18 at 30.52 in — −0.53 in and −1.71% from the same starting point.

The second is a reduction. Owners fitting it are gaining 0.39 in of section width while losing half an inch of height, and the speedometer would begin to read slightly high rather than slightly low.

That combination is characteristic of a second wheel-and-tire set assembled for winter use, where a slightly smaller overall diameter, a narrower or equal tread and a cheaper wheel are all deliberate choices. It is the same substitution logic as plus sizing, applied in reverse — and plus sizing covers why the tolerance works in both directions.

From the 21-inch fitment

Because 275/45R21 starts shorter, both recorded sizes read differently against it: 265/65R18 is +0.82 in and 2.66%, while 265/60R18 is −0.22 in and −0.73%.

An owner leaving the 21-inch package is also gaining sidewall on a large scale — from 4.87 in to 6.78 in on the taller option — which on a unibody crossover is felt over broken surfaces more than anything the diameter does.

Why the changes here are all small

This platform is a unibody, independent-suspension vehicle with wheel wells packaged around its factory sizes and an interior floor that constrains what can be raised. Nothing in the recorded upgrade set exceeds 2.7%, and that is not an accident of what people happen to fit — it is what the architecture accommodates.

The practical consequence is that speedometer error is a non-issue on this platform, and the reasons for changing tires here are grip, tread pattern, sidewall and cost rather than clearance.

Gearing, which barely enters into it here

Nothing on this page moves the effective final drive enough for a ratio to matter: a 1.65% diameter increase behaves like a 1.65% numerically lower final drive, which no transmission notices.

The gear ratio calculator will produce the exact engine-speed figure if you have your own ratio to hand.

The 21-inch fitment and the tire-shopping problem

Sizes at 21 inches are rare, and the selection in 275/45R21 is a fraction of what exists at 18. An owner who wants a winter tire, an all-terrain pattern, or simply a cheaper replacement is usually looking at a second set of wheels rather than a second set of tires.

The arithmetic above is what makes that decision straightforward. The two rolling diameters differ by less than one part in a hundred, so nothing the instruments read moves. What does move is the sidewall: 6.53 in against 4.87 in, a gain of 1.65 in of casing between the rim flange and the road, on top of a catalogue several times larger.

What is computed here and what is not

Diameter, width, sidewall and speedometer effect are exact consequences of the size codes. Whether a wheel clears the brakes is the live question on this particular vehicle, because dropping from a 21-inch rim to an 18 is the change the arithmetic most encourages here, and it is the change most likely to run into a front disc. That is measured with a straightedge across the hub face, not computed from a size.