Knowing how much rpm will drop when shifting gears will help you to hit a shift point just right. This requires that the gear ratios of the gears being shifted between is known. Below is the ratio of each transmission gear for late model CR125’s that use the 1997 gear set.

1st 2.357
2nd 1.867
3rd 1.526
4th 1.286
5th 1.130
6th 1.000

The percent change in rpm when up shifting from one gear to the next is proportional to the ratio of ratios of the two gears. So for up shifts between the gears the engine rpm will drop according to the following percentages after the gear change is made.

1st to 2nd 79.2%
2nd to 3rd 81.7%
3rd to 4 th 84.2%
4th to 5th 87.9%
5th to 6th 88.4%

Notice that the rpm will drop more in the lower gears than in the upper gears. That is designed to keep the engine nearer peak horsepower rpm at the higher speeds and air drag encountered in the upper gears.

The shift rpm in each gear is different to take best advantage of the rpm range available in each gear. The shift from 2nd to 3rd will drop the rpm from 12500 to 10212 rpm, which is about right for a stock SKUSA S3 CR125. However, the shift from 5th to 6th will drop the rpm from 12300 to 10873 rpm, again about right for SKUSA S3 CR125.

A simple rule of thumb for SRS stock CR125’s: When in 1st, 2nd, or 3rd , shift up to the next gear is about 12500 rpm. In gears 4th and 5th. shift up about 200 rpm sooner (12300 rpm) to keep the engine output closer to peak horsepower.

For more detailed information about SRS Shifter Kart engine recommendations, call us at 903-769-4140.