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by rdrainer on August 11th, 2007
A very high gear ratio used to conserve fuel once route speed is achieved. It is not to be used when towing another vehicle or trailer over hilly country or when the trip will be mostly uphill because it will:
a) require frequent gear changes, or
b) lug the engine.
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Comments
Lug the engine?
by Smurphy on August 11th, 2007
Lugging an engine means to have it labor excessively at a low rpm while pulling a heavy load. When the engine rotation speed drops below a certain level, lubricating oil is not circulating adequately, and load-bearing surfaces heat up accelerating wear, leading to premature engine failure.
Also, the respiration (i.e. flow rate) of the engine's fuel+air mix is not sufficient to provide adequate cooling of the combustion chamber and related parts.
The combination of heat build-up and heavy loads can cause dieseling, i.e. "pinging", which further contributes to heat build-up and mechanical damage to piston tops, among other components.
For all these reasons, the overdrive gear should never be used at a low engine speed or with excessive loading.
Automatic transmissions equipped with OD can wear excessively, or break components, by trying to downshift constantly from OD, then engage OD as engine speed builds up.
by rdrainer on August 11th, 2007
Thanks.
by Smurphy on August 11th, 2007