How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

2.14%

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

2.14%

Absolute Difference

0.65"

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
4040.9+0.9
5051.1+1.1
6061.3+1.3
7071.5+1.5
8081.7+1.7
Tire 1Dia. 30.39 ″Rim 21 ″4.69 ″Width 10.43 ″Revs/Mile 663.6Circum. 95.5″Tire 2Dia. 31.04 ″Rim 21 ″5.02 ″Width 10.04 ″Revs/Mile 649.8Circum. 97.5″
Tire 1
Specification
Tire 2
30.39"
Diameter
31.04"
10.43"
Width
10.04"
4.69"
Sidewall
5.02"
95.472"
Circum.
97.513"
663.6
Revs/Mile
649.8

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