How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-0.68%

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

-0.68%

Absolute Difference

-0.21"

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
2019.9-0.1
3029.8-0.2
4039.7-0.3
5049.7-0.3
6059.6-0.4
7069.5-0.5
8079.5-0.5
Tire 1Dia. 30.99 ″Rim 18 ″6.50 ″Width 10.83 ″Revs/Mile 650.7Circum. 97.4″Tire 2Dia. 30.78 ″Rim 18 ″6.39 ″Width 11.61 ″Revs/Mile 655.3Circum. 96.7″
Tire 1
Specification
Tire 2
30.99"
Diameter
30.78"
10.83"
Width
11.61"
6.5"
Sidewall
6.39"
97.365"
Circum.
96.684"
650.7
Revs/Mile
655.3

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