Are you looking for engine rpm’s or mph?
The flywheel sensor for rpm’s is fairly accurate (once you get it working), but the software for determining mph in a nitro powered rc with a two speed transmission leaves a lot to be desired and with the current available software is very inaccurate.
Until the software is enhanced for more input parameters the telemetry for mph in most nitro powered rc’s is a bust and your money would be better spent on a onboard gps unit.