# Thread: How to scan speeds faster than 158 mi/h (255km/h)?

1. Originally Posted by Blue70SS
What is the "336"? Is this the gear ratio or a multiplier used on the "gear_ratio"? (so if my rear end gear ratio is 3.25, would I use 325?)
Or, if I'm interpreting correctly from previous posts, the "gear_ratio" is the currently selected gear's ratio, and the 336 is the differential gear ratio. Does a separate PID need to be created for "gear_ratio"?

Thanks
Hi Blue70SS,

The 336 is simply the constant of conversion from mph to rpm and inches (and/or vice-versa); it is not related to rear axle ratio.

so for example (using say 6th gear):

TR6060 6th = 0.63

using engine RPM:
gear_ratio = 3.25 * 0.63 = 2.0475
road_speed[mph] = (engine_speed[rpm] * tire_diameter[inch]) / (2.0475 * 336)

using have transmission output rpm:
road_speed[mph] = (trans_output_speed[rpm] * tire_diameter[inch]) / (3.25 * 336)

I don't know if there's a pid for the M6 trans output speed in RPM.

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