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A theory on why the 6.1L-powered LX cars get lousy mileage

Old 01-29-2007, 09:06 PM
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Default A theory on why the 6.1L-powered LX cars get lousy mileage

Now I have heard for a long time that the reason for the LX cars mileage is because of its weight. I've never completely accepted this theory and now I think I have found a possible explanation.

My theory is it is the W5A580 transmission and here is why.

Some LX models had a different transmission known as the 42RLE (a RWD variant of the transmission used in LH cars) namely the 3.5L models. This set up on the 3.5L-powered LX cars originally had the 42RLE 4-speed (having a .69 overdrive) with a 3.64 axle ratio. This gave it around a 2.51 combined-top gear ratio. When they switched over to the MB-designed tranny, they switched to a 2.87 axle ratio and the W5A580 (having a .83 overdrive). This has about a 2.38 combined-top gear ratio, yet the fuel economy remained the same. I remember test driving a 3.5L powered Charger and noticed that despite this taller top gear, it was cruising at the same RPM as the 42RLE equipped models.

Think this is odd, it gets worse. The Dodge Durango and Jeep Commander (both 2WD models) have the same base V6 engine, but the slightly heavier Durango gets 1 mpg better mileage on the highway. What's the difference? The Durango has the 42RLE with a 3.92 axle-ratio (a 2.70 combined ratio) while the Commander has the W5A580 with a 3.07 (a 2.55 combined ratio). There is no logical reason why the Durango should get better mileage.

Another example to demonstrate this. The new Sebring has the 3.5L offered in it with a new transmission the 62TE. The axle-ratio on the LH cars was a 3.66 with the 42LE. The two transmissions have the same top-gear ratio, but the difference is a 2.37 combined ratio on the Sebring vs. a 2.53 in the LH cars. The difference is 19/28 for the Sebring and 18/27 for a 300M. The weight is similar due to the fact that cars keep weighing more and more so it isn't that. Why did it increase for the Sebring but not the LX cars?

So what does this have to do with explaining why a 6.1L HEMI can only get 14/20 mpg? This proves that the W5A580 needs taller gearing to get similar mileage and sometimes taller gearing isn't enough.

Then another observation I made between GM and DCX. An LS1 equipped GTO with a 4L60-E transmission with a 3.46 axle ratio (a combined 2.42 ratio) gets only 16/21; meanwhile, a 02 LS1 equipped Trans Am got 18/26 with the same transmission and a 2.73 axle ratio (a combined 1.91 ratio). The fuel mileage didn't change at all and neither did the gearing when they switched to the LS2. The gearing of a GTO is taller than an SRT-8 (they have the same 3.07 axle-ratio and combined 2.55 as the Grand Cherokee V6 has). The only difference is 2/1 below the GTO and the LX weighs more than the GTO. The 6.1L has the potiental to get better fuel mileage as well. The Firebird could get 18/28 when equipped with a T-56 Tremec, granted I don't think it will do that good due to weight but it can certainly do better than 14/20.

It isn't that the design of a 6.1L HEMI is a gas guzzler design, its the gearing. What do you think?
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Old 02-04-2007, 06:26 PM
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Default RE: A theory on why the 6.1L-powered LX cars get lousy mileage

Definetely the gearing.. what is heavier needs to be pushed easier right?
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