How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

1.14%

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

1.14%

Absolute Difference

0.35"

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
2020.2+0.2
3030.3+0.3
4040.5+0.5
5050.6+0.6
6060.7+0.7
7070.8+0.8
8080.9+0.9
Tire 1Dia. 30.81 ″Rim 20 ″5.40 ″Width 12.01 ″Revs/Mile 654.7Circum. 96.8″Tire 2Dia. 31.16 ″Rim 20 ″5.58 ″Width 12.40 ″Revs/Mile 647.2Circum. 97.9″
Tire 1
Specification
Tire 2
30.81"
Diameter
31.16"
12.01"
Width
12.4"
5.4"
Sidewall
5.58"
96.783"
Circum.
97.896"
654.7
Revs/Mile
647.2

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