How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-1%

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%

Absolute Difference

-0.3"

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
4039.6-0.4
5049.5-0.5
6059.4-0.6
7069.3-0.7
8079.2-0.8
Tire 1Dia. 30.04 ″Rim 20 ″5.02 ″Width 10.04 ″Revs/Mile 671.4Circum. 94.4″Tire 2Dia. 29.74 ″Rim 20 ″4.87 ″Width 10.83 ″Revs/Mile 678.1Circum. 93.4″
Tire 1
Specification
Tire 2
30.04"
Diameter
29.74"
10.04"
Width
10.83"
5.02"
Sidewall
4.87"
94.371"
Circum.
93.444"
671.4
Revs/Mile
678.1

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