How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

2.06%

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

2.06%

Absolute Difference

0.63"

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
2020.4+0.4
3030.6+0.6
4040.8+0.8
5051+1
6061.2+1.2
7071.4+1.4
8081.6+1.6
Tire 1Dia. 30.61 ″Rim 21 ″4.80 ″Width 12.01 ″Revs/Mile 659.0Circum. 96.2″Tire 2Dia. 31.24 ″Rim 21 ″5.12 ″Width 12.80 ″Revs/Mile 645.7Circum. 98.1″
Tire 1
Specification
Tire 2
30.61"
Diameter
31.24"
12.01"
Width
12.8"
4.8"
Sidewall
5.12"
96.153"
Circum.
98.131"
659
Revs/Mile
645.7

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