Guide

Why speed in gear is an estimate

The speed table multiplies engine RPM by the distance the tire covers in one turn, then divides by overall gear reduction. In miles and minutes that is MPH = RPM × circumference in miles × 60 ÷ overall reduction. Overall reduction is primary × gearbox × (rear teeth ÷ front teeth).

That equation assumes the rear tire rolls without slip at the circumference you supplied. A geometric size such as 180/55-17 is an unloaded estimate. A tape around a mounted tire is closer. Neither figure includes the way a tire grows with speed, the way it squats under load, or the way a chain motorcycle can slip the clutch.

What the table is for

The table is for comparing two sprocket pairs, or for seeing whether sixth gear at 4,500 RPM is a 70 MPH cruise or a 60 MPH cruise on your numbers. It is not a substitute for a GPS run on a quiet road, and it is not a claim about indicated speed on the cluster.

If you take MPH from the table at 5,000 RPM and enter that speed in the RPM calculator with the same ratios and tire, you should return to about 5,000 RPM. That round trip is the check that both pages share one model.

Use speed in gear for the table and RPM at speed for the inverse. The tire assumption is written on the tire size guide.