# Why do 04 GTO's have better EPA numbers than 05-06?



## masospaghetti (Jul 3, 2011)

Hey all, first post...i've been wanting a GTO since I was 17, and I'm finally almost within my means to buy one.

I know that the 04 has the LS1 and less HP, but the EPA numbers are quite a bit better: 16/26 for the M6 compared to 15/23 for the 05-06 models. Both are adjusted post-2008 figures. I know the GTO is not an economy car nor do I expect it to be one, but I am curious why there is a difference. And either way, a 15% improvement in highway economy does matter with gas at $4.00 a gallon.

Is it gearing? or is the LS1 a more economical motor? Or is it something else (or nothing at all)?

Many thanks all!!


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## Falco21 (May 11, 2010)

Well for one the LS1 is a smaller motor putting out less HP. Also, the 2004 has a way to put the car in "lean" cruise mode. Not sure how exactly that works, someone with an 04 can chime in on that. But basically it allows for better gas mileage.


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## masospaghetti (Jul 3, 2011)

So the difference is within the motor - not the gearing - is that right?


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## Poncho Dan (Jun 30, 2009)

19/26 is about what I get. My combined average is usually 21 or 22.


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## Falco21 (May 11, 2010)

Not to mention those numbers are estimated. I have an 05 and can easily get more than 15 in the city. Depends how I drive it.


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## RJ_05GTO (Jul 6, 2008)

Im averaging 18.9mpg on my monitor right now. If I hammer on it a lot it will drop down but i have never seen anything under 15mpg on my monitor. If I take it down the interstate for a trip I usually get about 21-22mpg. Mine is a A4 so a m6 will get better. Not bad for what it is if you ask me. My 3.5l Impala is rated like 29mpg on the highway but I actually get 32-33 mpg. I think the EPA mpg ratings are slightly underated.


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## svede1212 (Nov 1, 2005)

With lean cruise enabled (thru tuning) you should be able get 30 highway with a M6. I don't think HP has a lot to do with it. Peak HP doesn't occur at 1,800 RPM where you cruise. To maintain the same AFR the cubic inch disparity could account for roughly 5%. I'm not sure what other definitive differences could make up the rest. One could possibly be the exhaust pipe size difference (2 1/4" vs 2 1/2"). At lower RPM a bigger pipe would make the system less efficient. There could be others. With roughly 200 more horse than stock and 3.91 gears I still get 25-26 highway with my LS1.


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## masospaghetti (Jul 3, 2011)

Does that mean the LS2 can also have lean cruise enabled through software, or is this a LS1 thing only? And is lean cruise enabled by default on the 04?


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## HP11 (Apr 11, 2009)

Only on the 2004's from what I've read. With tuning software it can be enabled. It "leans out" the air fuel ratio from 14.7-1 up to 16 or 17/1 (I foget which) under light load highway type conditions. Default it would be disabled. 

Just curious: how would cruising at that level of 'lean-ness' affect the engine in the long run? Wouldn't it run hotter, poor lubrication, detonation, deposits, i.e.; the things normally associated with running an engine lean?


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## svede1212 (Nov 1, 2005)

There are no ill effects. Because of the low load and RPM the engine just loafs along. The Australian version uses it and the 04 came over with their computer with the LC just disabled. The LS2s got new, different computers with no such provision. There are no AFR numbers in the tables when you enable it. That is one of the tables that you decide where and how much it does it. It also advances the timing as that's needed when it kicks in. The other tables are speeds and delays. There are some Monaro tables you could look at to get an idea of what to do or now there are many 2004 GTOs that have it enabled too.


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## masospaghetti (Jul 3, 2011)

Thanks for the information...really good to know this is available.

However, I'm assuming this lean-burn mode was not enabled for the EPA tests - so the 16/26 rating is without lean-burn, right?


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## HP11 (Apr 11, 2009)

I'm not sure but I would think that the lean cruise would have been used for the tests.


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## svede1212 (Nov 1, 2005)

HP11 said:


> I'm not sure but I would think that the lean cruise would have been used for the tests.


It wasn't. Lean cruise is not allowed in the U.S. by the EPA due to nitrogen oxide emissions exceeding their limits. Lean conditions produce the best gas mileage but produce more of the nitrogen oxide. Rich conditions produce more power but cause more hydro carbon emissions. 14.68 to 1 air fuel gives the best balance between the two and that's why that's used in our cars. Under WOT our cars _DO_ go to a richer condition (~12-13 to 1) to provide power (contributing to pollution). When you're cruising you don't need power and lean cruise does the same thing in the other direction. It is heavier than air and in places that are prone to it can contribute to smog (like LA). It isn't a problem in the open spaces tho. With it enabled a stock '04 M6 can get 30 MPG highway.


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## HP11 (Apr 11, 2009)

Didn't know that. I was about to add *'unless it caused problems emissions wise in the U.S.'* to my post....


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## svede1212 (Nov 1, 2005)

It's really amazing the gas mileage our cars can get even with pretty big power. My car with about 550 HP flywheel HP and 3.91 gears can get the same gas mileage my wife's 252 HP G6 GTP gets on the highway (26). The LS1 was touched by the hand of God.


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## Sport The War (Jul 3, 2011)

I wish I could get the EPA listed numbers


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## Poncho Dan (Jun 30, 2009)

svede1212 said:


> It wasn't. Lean cruise is not allowed in the U.S. by the EPA due to nitrogen oxide emissions exceeding their limits. Lean conditions produce the best gas mileage but produce more of the nitrogen oxide. Rich conditions produce more power but cause more hydro carbon emissions. 14.68 to 1 air fuel gives the best balance between the two and that's why that's used in our cars. Under WOT our cars _DO_ go to a richer condition (~12-13 to 1) to provide power (contributing to pollution). When you're cruising you don't need power and lean cruise does the same thing in the other direction. It is heavier than air and in places that are prone to it can contribute to smog (like LA). It isn't a problem in the open spaces tho. With it enabled a stock '04 M6 can get 30 MPG highway.


Yep, richer than Stoich = more CO2 & CO, leaner = more NOx

I'm interested to know at what parameters (like MAP & TPS) that Lean Cruise is enabled. Is it available in the custom 2-bar OS? I've got 2 more credits to burn.  Though I am getting 26-27 on a forced open loop tune. And I've cut a substantial amount of weight, though you wouldn't know it from looking at it.


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## masospaghetti (Jul 3, 2011)

So through tuning, its not possible to have a lean-burn mode for the 05-06 LS2's?

I'm just still amazed that the 04 model gets 26++ mpg and the 05-06 only gets 23.


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## Falco21 (May 11, 2010)

No only 04s


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