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Old 02-06-2006, 06:31 PM   #4
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Re: Gas Mileage myths or facts?

Originally Posted by ice745
I've heard that both A/C and Overdrive bring down gas mileage.

Personally, I think Overdrive would help gas mileage, because (as far as I know, I may be wrong) it just allows the transmission to go into 4th gear. A 4th gear would help decrease RPM, and therefore help gas mileage? Am I wrong? I really don't know that much about what Overdrive does if not just allowing the use of the 4th gear.
Many modern cars (especially with automatic transmissions) are geared so that fuel efficiency at highway speeds becomes a priority. For instance, the first three gears on an automatic transmission might be closely spaced for acceleration at midrange speeds. The car is then programmed to tend to shift into 4th gear right around the normal speed limit (60-65 mph for most cars in the USA.) The 4th gear size is designed so that the RPMs are as low as possible, while the keeping the engine spinning fast enough to power the car along at 60mph.

For manual transmissions, this is of course different, because you can select your own shift points. However, the same gear-ratio concept still applies: at cruising speed, the gearing is selected to minimize engine RPM. Some cars with manual transmissions can have gearing which allows them to hit their top speed in 4th or 5th gear, where 5th or 6th gear is used for economy cruising.

The other thing is with the A/C I don't understand how it affects gas milage. The compressor is belt driven, and its not like the compressor itself consumes gas. Since the belt is always going to spin whether the A/C is on or not, I don't see how it can affect gas mileage. I understand the pully will only spin if the A/C or Defroster is on, but does the amount of force needed to rotate a pully really affect gas mileage? Or is there something else I'm missing.
Anything in your car that uses power must get power from the engine. If you want to maintain a certain speed in your car after turning on your A/C system and 1000 Bagigawatt sound system, your engine must produce more power. Producing more power means more gas must go to the engine, hence, for every mile traveled, you're using more gas.

As for the actual impact of using A/C: in some cases, the gas mileage hit from A/C is smaller than rolling down the windows.
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