Affiliate disclosure: As an Amazon Associate, we earn from qualifying purchases, at no extra cost to you. We research rather than test, and commissions never change our rankings. This site is reader-supported: those commissions pay for hosting, research and keeping every guide up to date, so if a guide helped you, buying through our links is an easy way to support it. How we rank · Full disclosure
Enter a stock and a new tire size below. The calculator shows the change in overall diameter, your true speed at an indicated 60 mph, revolutions per mile, and the effective axle ratio. For a flotation size such as 35x12.50R17, use the manufacturer's published overall diameter: the size code gives it directly rather than as width and aspect ratio.
How This Is Calculated
Diameter (in) = wheel diameter + (2 × width × aspect ratio ÷ 100 ÷ 25.4). True speed = indicated speed × new diameter ÷ stock diameter. Revolutions per mile = 63,360 ÷ (π × diameter). Effective axle ratio = axle ratio × stock diameter ÷ new diameter; the ratio that restores stock gearing = axle ratio × new diameter ÷ stock diameter.
Size-code format per the Tire and Rim Association (TRA) conventions described in Toyo Tires' load-and-inflation guidelines.
These are geometric figures from the size code. Real tires differ: Nitto lists several sizes 0.05 to 0.5 inch off the calculated or nominal diameter. Tires also squat under load and lose diameter as they wear.
Reading the Size Code
Toyo's guide to the TRA standards breaks down the three formats on light-truck tires:
- P-metric (P265/70R17): passenger-type tire; section width in millimeters, aspect ratio (sidewall height as a percentage of width), R for radial, rim diameter in inches.
- LT-metric (LT285/75R17): the same layout with a light-truck designation and a load range letter.
- Flotation (35x12.50R17LT): overall diameter and section width in inches, then the rim diameter, LT, and the load range.
The calculator's width and aspect fields take P-metric and LT-metric sizes. For a flotation size, start from its stated diameter, or better, the manufacturer's measured one.
Worked Examples
| Size | Calculated or nominal diameter | Manufacturer's listed diameter | vs 31.6 in stock (265/70R17) | True speed at indicated 60 mph | Effective ratio with 3.73 axle |
|---|---|---|---|---|---|
| LT285/70R17 | 32.7 in | 32.76 in (Nitto Recon Grappler A/T) | +3.6% | ≈ 62 mph | ≈ 3.60 |
| LT285/75R17 | 33.8 in | 34.09 in (Nitto Trail Grappler M/T) | +7.9% | ≈ 65 mph | ≈ 3.46 |
| 35x12.50R17LT | 35 in (nominal) | 34.53 in (Recon Grappler) / 34.84 in (Trail Grappler) | +9.3% to +10.2% | ≈ 66 mph | ≈ 3.38–3.41 |
The last three columns use the manufacturer's listed diameter, which is why the 35-inch tire comes out below 11%. Two tires sold under the same size code can differ by about a third of an inch.
Why Diameter Is the Number That Matters
The vehicle travels one tire circumference (π × diameter) per wheel turn. A 31.6-inch tire turns about 638 times a mile (63,360 inches ÷ 99.3); a 34.5-inch tire turns about 585 times. The speedometer and odometer count turns, so they read low by the diameter ratio. The engine turns the wheels through a fixed axle ratio, so each engine revolution now moves the truck farther: the effective gearing is taller. Width doesn't enter into either. A wider tire with a lower aspect ratio can have the same diameter as a narrower, taller one.
Gearing: What the Effective Ratio Tells You
The effective-ratio output is the axle ratio the engine now works against with the new tires. Taller effective gearing means lower engine speed at a given road speed and less torque at the wheels, most noticeable when towing, climbing or crawling. The "restore" figure is the axle ratio that would return the original engine-speed-to-road-speed relationship. Real axles come only in specific ratios, so you'd pick the nearest one your axle offers. See the regear calculator to compare available ratios.
Correcting the Speedometer and Odometer
A bigger tire makes the speedometer read low and the odometer log fewer miles than you drove. On a 7% larger tire, the odometer records about 93 miles for every 100 driven, which stretches mileage-based service intervals by the same amount. Ask the vehicle maker's dealer whether your truck's tire-size setting can be reprogrammed after a size change. Until it is, apply the true-speed figure above. See off-road maintenance and DIY for mileage-based service schedules.
Size Isn't the Only Spec That Changes
A new size also needs to carry the vehicle's load at the pressure you'll run. Toyo's guidance is that a replacement must carry at least the original tire's load according to the load-and-inflation tables. The all-terrain and mud tires hub covers load range and the P-metric to LT-metric pressure change. Check fender and suspension clearance at full lock and full compression too.
Keep All Four Diameters Matched on a 4x4
Ford's owner's manual warns that uneven tread depth between front and rear tires can degrade 4WD engagement and disengagement, add noise from the 4WD system, or cause damage, and it says 4WD vehicles should have all four tires replaced at once. Mixing sizes has the same effect as uneven wear. See how to rotate 4x4 tires.
Recommended gear
Gear for airing down and back up
Affiliate disclosure: As an Amazon Associate, we earn from qualifying purchases, at no extra cost to you. Purchases through these links help keep this site running and up to date. Learn more
Best for airing all four tires down to a set pressure at once.
- Automatic deflators preset between 6 and 30 PSI.
- Air all four tires down at once, then stop at the set pressure.
Best for budget-friendly airing down before sand or rock.
- Adjustable 4-pack of deflators, 10–30 PSI.
- Screw on, set the pressure, and they stop automatically.
Best for airing back up before you hit pavement.
- Portable 12V compressor rated to 150 PSI.
- Sized for truck and SUV tires.
Best for fast inflation of larger off-road tires.
- Heavy-duty 12V off-road compressor.
- Built for inflating larger off-road tires quickly.
We research products from manufacturer specs and owner feedback; we don't test them ourselves. How we choose.
Sources
- Toyo Tires — Guidelines for the Application of Load and Inflation Tables (Version 2020-7) — P-metric, LT-metric and flotation size-code formats (TRA); replacement tires must carry at least the original load per the load-and-inflation tables.
- Nitto — Recon Grappler A/T Specifications — Listed overall diameters: 35x12.50R17LT at 34.53 in; LT285/70R17 at 32.76 in.
- Nitto — Trail Grappler M/T Specifications — Listed overall diameters: 35x12.50R17LT at 34.84 in; LT285/75R17 at 34.09 in.
- Ford Owner's Manual — Tire Care (Ford Service Content) — Uneven front-to-rear tread depth can degrade 4WD engagement or cause damage; 4WD vehicles should have all four tires replaced at once.