How it works

TireFlows

Get diameter, speedometer impact, and alternate sizes — free, no signup.

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-1.16%

Safety Rating

Safe

Within the recommended 3% safety margin.

The 3% rule is a common reference — not vehicle-specific advice. How it works

Speedometer Error

-1.16%

Absolute Difference

-0.36"

Speedometer

Assumes Tire 1 is what the speedometer is calibrated for. Shows approximate actual speed if you switch to Tire 2.

Reading (mph)Actual (mph)Difference
2019.8-0.2
3029.7-0.3
4039.5-0.5
5049.4-0.6
6059.3-0.7
7069.2-0.8
8079.1-0.9
Tire 1Dia. 31.10 ″Rim 21 ″5.05 ″Width 11.22 ″Revs/Mile 648.5Circum. 97.7″Tire 2Dia. 30.74 ″Rim 21 ″4.87 ″Width 10.83 ″Revs/Mile 656.0Circum. 96.6″
Tire 1
Specification
Tire 2
31.1"
Diameter
30.74"
11.22"
Width
10.83"
5.05"
Sidewall
4.87"
97.699"
Circum.
96.585"
648.5
Revs/Mile
656

How results are calculated

Tire dimensions are calculated from the metric size format (width / aspect ratio / rim diameter). Diameter, circumference, and revolutions per mile are derived mathematically — no fitment or vehicle data is used.

The 3% rule is a general reference for acceptable diameter difference, not a guarantee. Always check your vehicle manual or consult a tire professional.

How it works