How this calculator works
Diameter change = new diameter − current diameter. Clearance change ≈ diameter change ÷ 2. Circumference = π × diameter. Sidewall height = (diameter − wheel diameter) ÷ 2. Theoretical actual speed = indicated speed × new diameter ÷ current diameter.
Worked example
A change from 25 × 8–12 to 27 × 9–12 adds 2 inches of nominal diameter, about 1 inch of ground clearance, and 1 inch of width. At an indicated 30 mph, the theoretical speed is 32.4 mph.
30, 32 or 35 inches: compare one change at a time
These are calculated examples using nominal diameters, unchanged speed calibration and a 4:1 starting gear ratio. They are not measured tire tests or fitment recommendations.
30 → 32 inches
+1 inch theoretical radius
Diameter: +6.7%
Indicated 50 mph: approximately 53.3 mph
Equivalent reduction: 3.75:1
30 → 35 inches
+2.5 inches theoretical radius
Diameter: +16.7%
Indicated 50 mph: approximately 58.3 mph
Equivalent reduction: 3.43:1
32 → 35 inches
+1.5 inches theoretical radius
Diameter: +9.4%
Indicated 50 mph: approximately 54.7 mph
Equivalent reduction: 3.66:1
Equivalent reduction is starting ratio × old diameter ÷ new diameter. A smaller equivalent number means taller effective gearing, not that the gears themselves changed. Use the gearing calculator for your ratio.
Build a comparison from your own measurements
- Record the exact tire model, size marking, axle, wheel, cold pressure and vehicle load.
- On level ground, record installed height from ground to top of the tire without mixing that method with an unloaded catalog diameter.
- Repeat on the new setup with the same method and conditions. Enter those diameters to compare the geometry; rolling circumference and speed still need separate verification.
- Record actual overall width separately. Tire width alone cannot predict it when wheel offset or spacers change.
For sourced stock baselines, see the Pioneer 700 Deluxe or RZR Pro R Sport. After a wheel change, use overall-width planning before checking the trailer.
Common questions
Does a larger tire always fit?
No. Check approved wheel widths, rim diameter, offset, suspension clearance, steering clearance, and the tire maker’s measured dimensions.
Should I compare front and rear tires separately?
Yes, when their dimensions or wheel requirements differ. Keep each axle’s stock and proposed size in its own comparison; do not average front and rear widths.
Use the right tire measurements
A marking such as 30 × 10–14 describes nominal overall diameter, width and wheel diameter. The 14-inch wheel number is not the tire diameter. Enter front and rear tires separately when their widths differ. The tire-size reading guide explains the markings before you compare them.
This calculation uses labeled sizes. Two tires with the same label can have different installed dimensions; inflation, load and the measuring method also matter. For a more realistic comparison, use consistently measured installed diameters or the tire maker’s dimensional data, and identify which you entered. Do not combine an unloaded catalog diameter with a loaded measurement as if both had the same basis.
A positive clearance result is a theoretical radius gain, not proof of room in the wheel well. Check the width and wheel measurement guide before reviewing offset, steering clearance and approved rim width. The 30, 32 and 35-inch comparison shows how diameter changes also affect speed and gearing.
Reading this estimate
The first number changes the nominal outside diameter; half that difference is the theoretical radius gain. Width and wheel diameter answer separate questions. Compare the current and new values on the same measurement basis, then check installed width and fit separately.
Does the wheel diameter affect the radius comparison?
When the entered outside diameters are unchanged, radius difference is unchanged. Wheel diameter changes nominal sidewall height and must match the tire. It does not substitute for outside tire diameter.
What should I use after changing wheel offset?
Review overall width with the wheel-offset tool and measure the completed vehicle. A tire-diameter comparison cannot predict where the tire sits relative to the hub.
Supporting explanation reviewed October 8, 2026.