# MTH Engine Gear Ratios

As part of my RTC program, I've been trying to understand how the odometer in the engine works.

Part of the answer must somehow include the gear ratio between the motor and the wheel. I took a diesel, marked the tach cylinder and the wheel position. Then I twirled the tach wheel until I got an integral number of tach revolutions and wheel revolutions. I got 42 tach revolutions for 4 wheel revolutions which gives a gear ratio of 10.5 to 1.

My question is for the next step, trying to understand the number of tach pulses to distance traveled. The tach strip in this engine has 24 black bars. Then I need to know the circumference of the wheel.

The diesel has the typical wheel that uses 22mm traction tires. But when I measure the solid wheel without the tire, I get about 22.6 mm. It's a little tough to measure the wheel with the tire since the measurement depends on how well the tire is installed. I measured a few times and I could say that it might average around 22.6 mm.

Does anyone have any information as to whether or not I am using the right values for the wheel or the gear ratio?

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