How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-1.15%

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

-1.15%

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
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. 30.39 ″Rim 21 ″4.69 ″Width 10.43 ″Revs/Mile 663.6Circum. 95.5″Tire 2Dia. 30.04 ″Rim 21 ″4.52 ″Width 10.04 ″Revs/Mile 671.5Circum. 94.4″
Tire 1
Specification
Tire 2
30.39"
Diameter
30.04"
10.43"
Width
10.04"
4.69"
Sidewall
4.52"
95.472"
Circum.
94.359"
663.6
Revs/Mile
671.5

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