# Reply to "MTH Engine Gear Ratios"

I did some calculations using the measured gear ratio, measured diameter of the driving wheel and the observed number of stripes on the tach strip.  I was able to determine the number of tach stripes per Smile. For a typical diesel, this was 119000 stripes per Smile (it is different for each gear ratio and wheel diameter).

I ran the engine for as close to 1 Smile on my layout as I could (1 Smile is 110 feet). I read up the raw DTO value and raw DOD value at the beginning and end of that run.

For DTO, the raw value delta over 1 Smile is 237012, for DOD, the raw value delta over 1 Smile is 462.

Looking at these numbers shows that 237012 is about 2 * 119000 and 462 is about 119000 / 256. I confirmed this general pattern with two other engines with different gear ratios and wheel diameters.

This tells me that for the DTO, each tach stripe generates two raw tach counts, possibly a count for the white to black edge and a count for the black to white edge.

The DOD raw value is just the DTO raw value divided by 512 (8 fewer significant bits, yes 8, not 9).

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