Truck Tire Size Guide
Tire size is part marketing, part math. This guide breaks down how to read the numbers on the sidewall, what diameter actually fits each common lift, and where most fitment mistakes happen.
Reading a Tire Sidewall
Every tire size follows the same pattern. Take LT285/75R17 121S as an example.
- LT - Light Truck construction. Heavier ply rating than P-metric.
- 285 - Section width in millimeters across the widest part of the tire.
- 75 - Aspect ratio. Sidewall height equals 75% of the section width.
- R17 - Radial construction, mounts on a 17-inch wheel.
- 121 - Load index. 121 = 3,297 lbs per tire at max pressure.
- S - Speed rating. S = 112 mph max sustained.
Tire Size by Lift Height
Guidelines for half-ton trucks. Heavy-duty trucks usually clear one size larger at the same lift height.
| Lift Height | Recommended Tire | Notes |
|---|---|---|
| Stock height | 265/70R17 (~31.6") | OEM size on most half-ton trucks. No mods needed. |
| Leveling kit (2") | 275/70R18 (~33.2") | Clears with minor trimming on most 1500 trucks. |
| 3" lift | 285/75R17 (~33.8") | Classic 33" combo. Fits most F-150, Silverado, Ram 1500. |
| 4" lift | 35x12.50R17 (~35") | True 35s. May need bumpstops on 1500s; Super Duty fits clean. |
| 6" lift | 37x12.50R17 (~37") | Aggressive look. Re-gear strongly recommended (4.56-4.88). |
| 8"+ lift | 40x13.50R17 (~40") | Show or off-road build. Custom flares, fender trim, gear swap required. |
Tire Sizing Questions
What does 285/75R17 mean?+
285 is the tire width in millimeters. 75 is the aspect ratio - sidewall height as a percent of width (so 75% of 285mm). R17 means radial construction on a 17-inch wheel. Total diameter ≈ 17 + 2×(285×0.75/25.4) ≈ 33.8 inches.
Will bigger tires hurt my mileage?+
Yes. Heavier tires take more energy to spin and aggressive tread adds rolling resistance. Expect 1-3 mpg loss going from stock to 33s, and 3-5 mpg from stock to 37s. Re-gearing the axles recovers some of that on heavy lifts.
Do I need to re-gear after going bigger?+
Usually yes once you cross 33". Bigger tires raise effective gear ratio, sapping low-end power and confusing the trans. We re-gear most 35" builds to 4.10 or 4.56 and most 37"+ builds to 4.88 or 5.13.
Will my speedometer be wrong?+
Yes - larger tires read low on the speedometer. A handheld programmer (SCT, HP Tuners, Edge) or our shop ECM tune corrects it. Plan on a calibration any time you change tire diameter more than ~3%.
What's the difference between LT and P tires?+
P-metric tires are designed for cars and light-duty use. LT (Light Truck) tires have heavier sidewall plies, higher load ratings, and better puncture resistance. If you tow, haul, or run gravel/off-road, get LT-rated.
Can you mount and balance the tires I bought online?+
Yes. Kustom Truck Fab and Kustoms & Service mount, balance, and road-force test tires daily. Bring your wheels, your tires, and your truck and we'll handle the rest.
Need help picking a size?
We mount, balance, and test-fit wheel and tire packages daily. Bring the truck and we'll measure clearance before you spend a dollar.