How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-1.46%

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

-1.46%

Absolute Difference

-0.45"

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
2019.7-0.3
3029.6-0.4
4039.4-0.6
5049.3-0.7
6059.1-0.9
7069-1
8078.8-1.2
Tire 1Dia. 30.74 ″Rim 21 ″4.87 ″Width 10.83 ″Revs/Mile 656.0Circum. 96.6″Tire 2Dia. 30.29 ″Rim 21 ″4.65 ″Width 11.61 ″Revs/Mile 665.8Circum. 95.2″
Tire 1
Specification
Tire 2
30.74"
Diameter
30.29"
10.83"
Width
11.61"
4.87"
Sidewall
4.65"
96.585"
Circum.
95.163"
656
Revs/Mile
665.8

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