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Originally Posted by Big Dave
Darcane:
Your point about wheels/tires is taken but it actually reinforces the superiority of the diesel/stick configuration. The truck has tires with a 31.7 inch outside diameter while the OD on the Impala tires is 27.0 inches. Since rotational moment of inertia goes up with the square of radius, I would expect the truck tires to have 38% higher rotational moment.
We have been over this before, but in the real world where stops lights and signs enforce stopping the higher rotational moment of inertia penalty of bigger OD tires rapidly overwhelms the lower engine RPM. With trucks the equation is always the same: Bigger tires = Lower MPG. Now that’s conventional drive trains I’m talking. It might be different with hybrids, but I notice that both Toyota and Honda put small-diameter tires on their MPG flagships.
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I agree there is a penalty for the higher moment of taller tires... but I don't agree it outwieghs the change in gear ratios. This winter I changed to 5% taller tires and saw about a 5% increase in mpgs. It's the simplest way to change your overall gearing ratio and generally taller tires has meant higher mpg in the posts I remember reading.
Quote:
Originally Posted by Big Dave
I hear you about the disappearing manual transmission. Let’s face it, ecomodders/hypermilers are a rare breed. The vast majority are just lazy. Talking on the cell phone is more important to most people than being aware of how you are driving. Also, I think an automatic is a way for the manufacturer tio hold the engine into a regime where NOx emissions are minimized. The very low RPM – high load regime that give optimum MPG tends to have high cylinder temps that generate a lot of NOx. Hence the rev-happy way most cars and trucks drive these days.
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The other issue that increases emissions is letting off the throttle in between shifts. My wife's Vue "solves" this by having the drive-by-wire throttle stay open for a second or so and slowly close after you let off.