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As far as I understand, the IS uses an engine similar to the GS300; why is it then that the GS gets marginally better gas mileage even though it weighs 300 lbs more and has 5 more hp? I is confused...
 

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Here's the wierdest thing, the GS400 actually gets better mileage than the GS300 and the IS300. That is odd, huh?
 

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nope, GS300 doesn't get better mileage than IS. I can tell you this because my dad has the GS and I have the IS, and I am getting at least 1 mpg better than him, and he is the more conservative driver of two.
 

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Alot of people have noticed that and wondered.

The factory stickers show that the GS300 has slightly better "EPA" highway mileage.

I have heard the following stories from Lexus people:

1> The IS300 has shorter gearing (different differential ratio) which accounts for the reduced mileage (higher engine RPMs in the same gear at the same speed).

2> EPA testing is a really funny business and any number of small differences between the cars could impact the results. "Real world" mileage may be different than the EPA numbers.
 

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Long ago, I posted a link to a URL that describes how the EPA does mileage testing...and get this, they don't actually measure the volume of gasoline consumed. Instead, they measure the emissions from the exhaust, and I guess there's some formula to figure out the mileage. How wacked out is that?!

Here's the URL (you'll need a .pdf viewer like Acrobat): http://www.epa.gov/oms/cert/factshts/gasmel.pdf
 
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