How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

-2.06%

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

-2.06%

Absolute Difference

-0.63"

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
2019.6-0.4
3029.4-0.6
4039.2-0.8
5049-1
6058.8-1.2
7068.6-1.4
8078.4-1.6
Tire 1Dia. 30.61 ″Rim 21 ″4.80 ″Width 12.01 ″Revs/Mile 659.0Circum. 96.2″Tire 2Dia. 29.98 ″Rim 21 ″4.49 ″Width 11.22 ″Revs/Mile 672.8Circum. 94.2″
Tire 1
Specification
Tire 2
30.61"
Diameter
29.98"
12.01"
Width
11.22"
4.8"
Sidewall
4.49"
96.153"
Circum.
94.174"
659
Revs/Mile
672.8

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