How it works

TireFlows

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

Tire 1
/R
Tire 2
/R

Result

Diameter Difference

2.91%

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

2.91%

Absolute Difference

0.87"

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
2020.6+0.6
3030.9+0.9
4041.2+1.2
5051.5+1.5
6061.7+1.7
7072+2
8082.3+2.3
Tire 1Dia. 29.91 ″Rim 18 ″5.95 ″Width 10.83 ″Revs/Mile 674.3Circum. 94.0″Tire 2Dia. 30.78 ″Rim 18 ″6.39 ″Width 11.61 ″Revs/Mile 655.3Circum. 96.7″
Tire 1
Specification
Tire 2
29.91"
Diameter
30.78"
10.83"
Width
11.61"
5.95"
Sidewall
6.39"
93.963"
Circum.
96.684"
674.3
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