What may be causing a 5mpg drop? - Pontiac GTO Forum
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post #1 of 11 (permalink) Old 05-13-2010, 10:30 AM Thread Starter
 
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What may be causing a 5mpg drop?

At idle my Throttle Position Sensor (TPS) it's at 16%.

During a freeway cruise it will read 27% - 35%.

Are these within specs?

I'm asking since my GTO is averaging 15MPG since my rebuild.

That is down from 20MPGs that I was getting before.

There are a few things that was changed that may have reduced the fuel economy:

- lower 10.5:1 compression ratio
- upgraded to Ferrea beehive springs (higher spring rates than stock?)
- swapping out the Maggie 112 stock 2.8 pulley for a 2.6 pulley.

Tuning my GTO for the new mods didn't make a difference in MPGs.

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post #2 of 11 (permalink) Old 05-13-2010, 01:36 PM
 
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Well your compression could have an effect on MPG's and if the engine is still tight then that can have an effect too. The supercharger is pretty much unloading when crusing. Also the time of year and the change in fuel blends could have an effect too. My MPG vary all over the place and I don't really change my driving habbits. Most of it has to do with the season and fuel blends. I go fro 14-17.5 MPG city like driving. I don't do much highway but its usally around 23-26mpg if I do.



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post #3 of 11 (permalink) Old 05-13-2010, 01:59 PM
 
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My fuel economy is about the same all year. 18-19.5 with mixed driving, probally 60-70% highway. I do think it goes down a little in the summer, not because of the fuel, but because I drive it more agressive since the roads are nice and clean.

2003 Corvette Z06 - MGW shifter, Corsa X pipe, Corsa Xtreme exhaust, Texas Speed long tubes w/o cats, PowerStop rotors/pads, Conti DWS tires, C6Z51 F&R sways, C5R headlights, tuned

04 GTO - sold
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post #4 of 11 (permalink) Old 05-13-2010, 02:51 PM Thread Starter
 
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Originally Posted by GM4life View Post
Well your compression could have an effect on MPG's and if the engine is still tight then that can have an effect too. The supercharger is pretty much unloading when crusing. Also the time of year and the change in fuel blends could have an effect too. My MPG vary all over the place and I don't really change my driving habbits. Most of it has to do with the season and fuel blends. I go fro 14-17.5 MPG city like driving. I don't do much highway but its usally around 23-26mpg if I do.
I've got 3k miles on the engine so far using dyno oil for break-in.

Nothing has changed for the better. Maybe 1MPG improvement. My highway used to average 28MPGs at 2k RPMs in 6th gear. And city was 15mpg. Now everything has dropped where my average is like my city driving.

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post #5 of 11 (permalink) Old 05-13-2010, 07:13 PM
 
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You have to realize that you went from an unknown tune that was possably really lean to begain with to a different setup with possably a better tune.



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post #6 of 11 (permalink) Old 05-14-2010, 12:02 AM
 
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Going from a 2.8 to a 2.6 will lower your mileage for sure.

<-- Guess which ones quicker?.....Nope! Not even close.
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post #7 of 11 (permalink) Old 05-14-2010, 10:45 AM Thread Starter
 
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Going from a 2.8 to a 2.6 will lower your mileage for sure.
Even during cruising with no boost? Just wondering what the averages are out there.

I understand that more boost will consume more fuel.

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post #8 of 11 (permalink) Old 05-14-2010, 04:09 PM
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Even during cruising with no boost? Just wondering what the averages are out there.

I understand that more boost will consume more fuel.
Swap it back and see what you get. It's not that hard to change, is it?
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post #9 of 11 (permalink) Old 05-14-2010, 04:20 PM Thread Starter
 
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Swap it back and see what you get. It's not that hard to change, is it?
I may do just that.

First I need to have him check my TPS since the readings are higher than normal.

Dunno how much that effects MPGs.

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post #10 of 11 (permalink) Old 05-17-2010, 11:22 AM
 
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Quote:
Originally Posted by batmans View Post
At idle my Throttle Position Sensor (TPS) it's at 16%.

During a freeway cruise it will read 27% - 35%.

Are these within specs?

I'm asking since my GTO is averaging 15MPG since my rebuild.

That is down from 20MPGs that I was getting before.
TPS readings are totally normal. You will also notice that at WOT, TPS reads 88%. Explanation here HP Tuners Bulletin Board - View Single Post - 2005 A4 GTO Help please.

As for your mileage, your tune might be a little on the rich side. Without being able to look at a wideband, who knows? What do you know about your tune?
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