"33s," "35s," "37s" and "40s" are shorthand for overall tire diameter in inches, and they're the fastest way truck owners size up a build at a glance. But the number alone doesn't tell you what it takes to run that tire, or what changes once it's on your truck. Here's what each size actually means mechanically, and how to think about which one is realistic for your setup.
What the number actually measures
In flotation sizing (see how to read a tire size for the full breakdown), the first number is the tire's overall diameter, mounted and inflated. A "35" is roughly 35 inches tall; a "37" roughly 37. That single number drives almost everything else in this comparison — clearance needs, gearing math, and how the truck rides and sounds.
Metric equivalents for each flotation size
Most trucks these days are sold with metric-sized tires (like 275/65R18), so it helps to know roughly where the common flotation sizes land in that language:
| Flotation size | Approx. metric equivalent | Approx. diameter |
|---|---|---|
| 33X12.50R20 | ~305/55R20 | 32.8–33.1 in |
| 35X12.50R20 | ~325/60R20 | 34.7–35.1 in |
| 37X12.50R20 | ~305/70R20 (close, not exact) | 36.5–37.1 in |
| 40X13.50R20 | no common direct metric match | 39.5–40.2 in |
These pairings are approximate — two tires with the same calculated diameter can still ride, sound and clear differently depending on tread design and sidewall construction. Use them as a starting point for comparison, not an exact substitution.
Size-by-size comparison
| Size | Typical wheel width | Typical lift needed | Gearing / MPG impact | Ride & noise | Cost tier |
|---|---|---|---|---|---|
| 33s | 9–11 in | Often none to a leveling kit | Minimal on most gas trucks; barely noticeable MPG hit | Close to stock, quiet | Lowest |
| 35s | 10–12 in | Leveling kit to 2–3 in lift, trimming common | Noticeable MPG drop; re-gear optional, common on diesels | Slightly firmer, some added road noise | Moderate |
| 37s | 12–14 in | 3–6 in lift plus trimming, often fender work | Re-gearing frequently recommended, especially towing | Firmer ride, more road noise, more unsprung weight | High |
| 40s | 14+ in | 6 in+ lift, major fender/bumper modification, often a full build | Re-gearing essentially required; real MPG and power loss | Noticeably heavier steering feel, most road noise | Highest |
Speedometer error, worked out
Speedometers are calibrated to the factory tire's revolutions per mile. A taller tire covers more ground per revolution, so the speedometer under-reports your real speed. The rough formula: percent error ≈ (new diameter ÷ old diameter − 1) × 100.
- Factory 31-inch tire to a 33: 33 ÷ 31 = 1.065, about a 6.5% error — the dash shows roughly 60 mph when you're actually doing about 64.
- Factory 31-inch tire to a 35: 35 ÷ 31 = 1.129, about a 13% error — dash shows 60, real speed is closer to 68.
- Factory 33-inch tire to a 37: 37 ÷ 33 = 1.121, about a 12% error.
- Factory 31-inch tire to a 40: 40 ÷ 31 = 1.29, about a 29% error — dash shows 60, real speed is closer to 77.
This isn't just a curiosity — it throws off cruise control accuracy, trip odometer mileage, and any speed-based safety systems tied to wheel speed sensors. A tuner that recalibrates for tire diameter, or a re-gear paired with a tuner correction, brings the reading back in line. Without correction, both your speedometer and odometer read low by roughly the percentages above.
Weight per tire and what it changes
Tire weight climbs fast with size, and that added rotating mass affects more than just how the truck feels off the line:
| Size | Typical weight per tire | Approx. added weight vs. factory 31–33in tire |
|---|---|---|
| 33x12.50 | 55–65 lb | baseline to +10 lb |
| 35x12.50 | 65–75 lb | +10 to +20 lb |
| 37x12.50 | 75–90 lb | +20 to +35 lb |
| 40x13.50 | 90–110 lb | +35 to +55 lb |
Multiply that difference by four (or six on a dually) and a 37-inch setup can be carrying 150-plus extra pounds of unsprung, rotating weight versus a factory tire. That mass has to be accelerated, steered and stopped every time you touch the pedals — it's a real factor in softer acceleration, longer braking distances, and reduced fuel economy, independent of the rolling resistance change from a more aggressive tread pattern.
Wheel width and offset pairing by size
Tire section width has a manufacturer-recommended wheel width range, and running outside it changes the tire's measured footprint and how it sits relative to the fender. As a general starting point:
- 33x12.50: commonly paired with 9–11 inch wide wheels, close to many factory setups.
- 35x12.50: commonly paired with 10–12 inch wide wheels, often with less backspacing (more offset toward the fender) than stock to clear suspension components.
- 37x12.50: commonly paired with 12–14 inch wide wheels; offset selection becomes more critical since there's less margin before the tire contacts a control arm or frame rail at full steering lock.
- 40x13.50: typically 14-inch-plus wide wheels on a purpose-built package, chosen alongside the specific suspension lift rather than off a general chart.
Browse 20x10, 20x12, or the full wheel size hub to see what's available once you've settled on a target tire size — offset needs to be checked against your specific truck, not assumed from a general range.
Stock, leveled, or lifted: what's realistic
Stock height
Most half-ton and mid-size trucks can run a mild size bump — often up to a 33 — on factory ride height with the factory wheel or a similar offset, depending on backspacing and fender liner clearance. This is the size range where "fits with no changes" is most plausible, but it's still truck- and trim-specific.
Leveled (1.5–3 inches of lift)
A leveling kit is the most common upgrade paired with 35s, since it corrects the factory rake and opens up front clearance. Many owners get there with a leveling kit plus minor trimming of the front fender liner or mud flap. Whether it's rub-free at full lock and full compression depends on wheel width, offset and tire brand — this is exactly the kind of detail we flag as Potential Match — Fitment Not Yet Verified until it's confirmed against your specific vehicle.
Mid lift (3–6 inches)
This is the practical range for most 37-inch builds. A mid-travel kit gives enough clearance at the upper control arm and fender to run 37s with a wider wheel and adjusted offset, though trimming the fender liner or lip is still common. Trucks in this range are also where re-gearing conversations start in earnest, particularly if the truck tows or the engine is a diesel with tall factory gearing.
Big lift (6 inches and up)
37s generally want a real suspension lift, not just a leveling kit, to clear control arms, frame components and the fender opening without heavy trimming, and 40s essentially live here. At this height, sheet-metal trimming, long-arm suspension conversions and a purpose-built wheel and tire package are the norm rather than the exception. Most setups in this range also pair a wider wheel with more backspacing adjustment than stock to keep the tire from hitting suspension components at full turn.
Why 40s are a build, not a purchase
40-inch tires don't have a dedicated size landing page in most catalogs because there's no single common bolt-up spec for them the way there is for 33s or 35s — they show up on heavily built trucks with long-arm suspension kits, cut and reworked fenders, and wheels chosen specifically to clear that individual truck's geometry. If you're planning a 40-inch build, treat it as a project with a suspension shop and a wheel/tire package built around your exact truck, not a tire order. Start that conversation from our tire catalog and shop by vehicle so fitment gets checked against your actual truck rather than assumed.
Re-gearing thresholds
Axle gearing determines how hard the engine works to turn a given tire diameter at a given speed. Bigger, heavier tires require more torque multiplication to move the same way the factory-size tire did, and every step up shifts that math. As a rough example, a truck running a factory 3.55 axle ratio on a 31-inch tire behaves, torque-wise, closer to a 3.73–3.90 ratio once it's carrying 35s, and closer to a 4.10–4.56 ratio once it's carrying 37s — the engine has to work harder at the same road speed even though the axle hardware hasn't changed:
- 33s: rarely need re-gearing on gas engines; the factory ratio usually absorbs it with a modest MPG and acceleration hit.
- 35s: a gray area. Plenty of trucks run 35s on stock gears and live with softer throttle response; towing-focused or diesel owners re-gear more often.
- 37s: re-gearing becomes the common recommendation, especially for anyone towing or hauling regularly, to keep the transmission out of a low gear constantly.
- 40s: re-gearing is close to mandatory to keep the drivetrain in a usable operating range and avoid excessive heat and wear.
Rubbing and trimming realities
Rubbing shows up in predictable spots: the front upper control arm at full compression and full steering lock, the inner fender liner, the front sway bar end links, and the rear wheel well lip at full articulation. Trimming the plastic fender liner is common and low risk; cutting sheet metal is a bigger commitment and usually only comes up at 37s and above. Wheel offset and backspacing change this equation as much as tire size does — a narrower backspacing setup can let a 35 clear where a poorly chosen offset makes even a 33 rub.
Buying decision by truck state
- Stock height, want a subtle upgrade: 33s are the low-risk starting point — minimal drivetrain impact, usually no lift required, easiest to reverse if you change your mind.
- Already leveled: 35s are the natural next step and the size most leveling kits are designed around; check offset and trimming needs before ordering.
- Running a mid-size lift (3–6 in): 37s become realistic, but budget for re-gearing consideration and expect some fender trimming even with adequate lift.
- Committed to a big-lift build: 40s only make sense once the suspension, fenders and gearing are already part of the project plan — buying the tire first and figuring out clearance after is the expensive way to do it.
Once you've picked a target size, browse 33X12.50R20, 35X12.50R20, 35X12.50R17, or 37X12.50R20 to see what's available, or start from our full tire catalog if you're still narrowing things down.