Ford F-150 13th gen: five factory sizes and four axle ratios
The widest set of recorded factory variables of any platform here — and the reason two owners of the same truck get different answers to the same question.
Ford F-150 · 13th generation · 2015–2020
- Factory tire sizes
- 245/70R17 · 265/70R17 · 275/65R18 · 275/55R20 · LT265/70R17
- Factory wheel diameters
- 17 in · 18 in · 20 in
- Factory axle ratios
- 3.15:1 · 3.31:1 · 3.55:1 · 3.73:1
- Commonly stepped up to
- 285/70R17 · 33x12.50R20 · LT285/65R20 · 35x12.50R20
- Sizes owners of this platform are widely observed to run. An observation about a community, not a finding about your vehicle.
Transmission gear ratios are not recorded for this platform. A figure that could not be confirmed against a source is left out rather than printed: an invented ratio yields engine-speed arithmetic that looks exactly as authoritative as a measured one. How these numbers are calculated sets out the rule.
The arithmetic of the sizes owners commonly step up to, measured from 245/70R17
| Size | Diameter | Change | Speedometer | Ratio restoring original rpm |
|---|---|---|---|---|
| 285/70R17 | 32.71 in | +2.20 in | +7.23% | 4.00:1 (nearest sold 3.91:1) |
| 33x12.50R20 | 33.00 in | +2.50 in | +8.18% | 4.04:1 (nearest sold 4.10:1) |
| LT285/65R20 | 34.59 in | +4.08 in | +13.38% | 4.23:1 (nearest sold 4.27:1) |
| 35x12.50R20 | 35.00 in | +4.50 in | +14.74% | 4.28:1 (nearest sold 4.27:1) |
Ratio column computed from the tallest factory axle ratio recorded for this platform (3.73:1). Change any input in the gear ratio calculator.
Five sizes, and two of them are the same tire
The recorded factory fitments are 245/70R17 (30.50 in), 265/70R17 (31.61 in), 275/65R18
(32.07 in), 275/55R20 (31.91 in) and LT265/70R17 (31.61 in).
That last one is geometrically identical to the second: same diameter, same section width, same sidewall, same 638.1 revolutions per mile. What differs is the casing standard and therefore the load index and the reference inflation. On a truck offered in configurations from a fleet regular cab to a loaded crew cab, that distinction is the whole reason both exist.
The spread from the smallest to the largest is 1.57 in, or 5.15%. A base truck on 17-inch wheels and a mid-trim on 18s do not read the same speed at the same road speed.
Four recorded axle ratios turn one question into four
3.15:1, 3.31:1, 3.55:1 and 3.73:1 are all recorded for this generation. The 3.15 is low enough that this truck was clearly geared for highway economy in at least one configuration, and it is the reason a tire change here can have a much larger effect than the same change on a differently geared truck.
Take 35x12.50R20 — 35.00 in — fitted from the mid-size 265/70R17. That is +3.39 in and
10.74%, and an indicated 70 mph becomes 77.5.
| Factory ratio | Ratio restoring engine speed | Nearest ratio sold |
|---|---|---|
| 3.15:1 | 3.49:1 | 3.55:1 |
| 3.31:1 | 3.67:1 | 3.73:1 |
| 3.55:1 | 3.93:1 | 3.91:1 |
| 3.73:1 | 4.13:1 | 4.10:1 |
Every one of those is a genuine ring-and-pinion change, and the two bottom rows land almost exactly on catalogue ratios. The top row does not: a 3.15-geared truck asking for 3.49:1 has to accept 3.55:1 and slightly overshoot.
The base truck is the one that gets surprised
From 245/70R17 — the smallest recorded fitment — the same 35x12.50R20 is +4.50 in, 14.74%.
An indicated 70 mph is 80.3, and 100 miles of odometer is 114.7 miles covered.
The 20-inch owner making what sounds like the identical modification is making a 9.69% change from
275/55R20, and the 18-inch owner a 9.12% change. Three trucks, one tire size, three materially
different outcomes.
Where the effective final drive ends up
The most useful way to see what a big tire does to a low-geared truck is to fold the two together.
A 3.15-geared truck on the base 245/70R17, fitted with 35x12.50R20, is left with an effective
final drive of about 2.7:1 once the tire's extra circumference is accounted for.
That is numerically lower than any ratio this platform was offered with. The engine has not changed, the gearbox has not changed, and the truck is now geared beyond the bottom of its own factory range — which is why the symptom owners report is not a slow speedometer but a transmission that will not settle in top gear.
The 33 that is barely a change
Against all of that, 33x12.50R20 from 275/65R18 is +0.93 in and 2.88%, and from 275/55R20
just +1.09 in. On the 18 and 20-inch trims a nominal 33 is close to a substitution rather than an
upgrade, which is exactly why it is so common on this platform: it changes appearance and tread
pattern without moving the arithmetic far.
LT285/65R20 at 34.59 in sits between that and a 35, at +2.51 in from the 18-inch size.
The usual caveat, and it is the important one
Everything above is geometry computed from the size codes, and none of it says a tire will fit. Cab configuration, bed length, suspension, wheel offset and steering travel all decide contact on this platform, and they vary more within one model year than the tire sizes do. The whole factory range covers 1.57 in of rolling diameter; the range of bodies that diameter was fitted to covers several distinct wheelbases and two bed lengths.
Where these figures come from
Manufacturer's published specifications for the model year range shown, cross-checked against the size printed on the vehicle's tire and loading information placard.
Last checked 2026-08-10.
Your vehicle's own tire and loading information placard — the sticker on the driver's door jamb — is the authoritative record of what left the factory on your specific vehicle, including the inflation pressures its speedometer and load ratings were set against. Read that before you trust any table, including this one.