SMG SPEED SHOP • FITMENT LAB
Tire Size Calculator
Compare tire dimensions, visualize the change, calculate speedometer error, check staggered setups and find alternate sizes—all in one place.
Metric + flotation
Speedometer error
AWD / staggered check
Alternate sizes
1
275/35R20
Current / OEM tire
2
305/35R20
Proposed tire
Diameter comparison
Excellent diameter match
0.0%
diameter change
Scaled visualizer
See the difference
Current
Proposed
Ride height
Ground clearance
Width change
At a glance
Key changes
Actual speed at 70
when the speedometer reads 70 mph
Revolutions per mile
change from current tire
Effective axle ratio
—
with the proposed tire
Diameter difference
overall tire height
Full comparison
Tire specifications
| Measurement | Current | Proposed | Difference |
|---|
Speedometer correction
Indicated speed vs. actual speed
Alternate size finder
Similar diameter on another wheel
Suggestions are calculated against the current tire diameter. Click a size to load it as the proposed tire.
Ready to put it together?
Find the tire—or get a real fitment check.
Calculated dimensions are a starting point. Wheel width, offset, suspension, brakes, body clearance, load rating and manufacturer specifications still matter.
Rolling diameter check
AWD and staggered tire compatibility
Compare front and rear rolling diameter. Manufacturer tolerances vary, so this tool flags the difference—it does not replace the vehicle or tire manufacturer’s specification.
Verify your exact vehicle’s allowable front-to-rear variance before purchase.
F
275/35R20
Front tire
R
305/30R20
Rear tire
Front-to-rear comparison
diameter variance
Front diameter
Rear diameter
Revs/mile difference
Diameter difference
Important: Some AWD systems require extremely tight matching, including tread depth and tire model—not just labeled size. Always follow the vehicle manufacturer’s specification.
Metric example
305 / 35 R20
305Section width in millimeters
35Sidewall height as % of width
R20Radial tire for a 20-inch wheel
Flotation example
35 × 12.50 R20
35Approximate overall diameter in inches
12.50Approximate section width in inches
R20Radial tire for a 20-inch wheel
What changes?
Bigger diameter affects more than appearance.
- Speedometer and odometer readings
- Effective gearing and acceleration
- Ground clearance and fender clearance
- ABS, traction control and AWD behavior
Before you buy
Size math is only one part of fitment.
- Confirm approved wheel-width range
- Check wheel offset and brake clearance
- Check suspension and body clearance
- Verify load and speed ratings
The metric diameter formula
((Width × Aspect Ratio ÷ 2540) × 2) + Wheel Diameter
Example: 285/75R16 calculates to approximately 32.8 inches in overall diameter. Actual mounted dimensions vary by manufacturer, model, wheel width, pressure and load.
Fitment disclaimer: Calculated dimensions are estimates based on labeled tire size. Actual dimensions vary by manufacturer, tire model, wheel width, inflation pressure, load and tread wear. Always verify approved wheel width, load rating, speed rating, vehicle clearances and drivetrain requirements before purchasing or installing tires.