# Reply to "MTH Engine Gear Ratios"

Mark I'm amazed at some of the stuff you've already come up with!  I'm pretty sure the byte 0x08 by itself tells a lot about the effective gear ratio, taking into account the driving wheel diameter.  If you take that value as an 8-bit integer and divide by 22.7, it will yield the approximate motor revolutions per inch.

I've counted the gear ratios on a lot of locos, and at one point several years ago, Dave Hikel shared a table with some common gear ratios.  Every MTH diesel I've ever measured is geared at 10.5:1.  I thought I measured the wheels at 0.875"; that loco may have had a lot of mileage with its tires worn smooth.  I no longer have physical access to all of the locos I measured.  In trying to relate the hex value to distance traveled, I had to guess at driving wheel diameters, so my suggested divisor of  22.7 is approximate.  It's known that slight speed variation exists among DCS locos; traction tire thickness could be one of the variables responsible for this variation.  I also know that MTH specifies the same tire for wheels having slightly different diameters.  In some cases, getting the tire on is a stretch!

The other variable is the flywheel diameter, and especially, the number of stripes.  Perhaps the flywheel diameter doesn't matter much, since the reader sees the stripes in linear fashion.  I'm pretty sure Gunrunner John said 28 stripes is standard.  There is probably a minimum stripe width required for reliable operation.  If the flywheel is too small to accommodate a tape of 28 stripes (at their minimum width), it's possible that MTH fudges the odometer variable so that the sensor's returned value at any given RPM matches that of other "standard" locos.  Maybe that's the purpose of byte 0x09h?

I know it's possible to "trick" a loco by using a tape with more stripes.  Some of my PS2 locos are jerky below an indicated 4 mph.  Folks here on the Forum have claimed they could force a balky loco to run slower-- at a measured 1 or 2 scale mph-- by using a custom tape with more stripes.  (Of course at higher speeds, the actual speed would be very different from the indicated speed, and also from that of "factory" unmodified locos.)

I hope you're able to find out more on this issue, and I look forward to reading about it on the Forum!

Last edited by Ted S
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