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GR-RRR!
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Discussion Starter #1
When the GTO was pretty much stock, I took it on a road trip to Indianapolis. On the stretches that were entirely highway driving I got 30 MPG at times, other times very close.

I recently drove the now bolt-ons and tuned GTO to a race track about 2 hours away. I gassed up just before I left and was on the interstate within a mile and a half. On the trip there it struggled to get to 25 MPG on the read out and when filling up later I found that the readout was wildly optimistic by about 4-5 MPG which made the MPG performance even worse than I thought. So, my question is, is there something messed up with the tune? When cruising shouldn't the tune be pretty much the stock tune and thus deliver better MPGs than I'm getting? On the stretch where I just barely got to 25 on the readout I was cruising at 75, sometimes slower so nothing outrageous speedwise.

Maybe I drove at slower speeds on the trip to Indy, I know Iowa and Illinois especially had 65 speed limits at the time. But still, a drop from 30 to 20 seems pretty dramatic.
 

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Powdercoat all of it
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I've heard of lean cruise in reference to gtos, and my understanding is that's not normally kept in an aftermarket tune. Is it possible lean cruise was turned off on yours? The scenario you describe (light, constant throttle on the highway) seems like lean cruise territory.

But yeah, you modified your car for performance. If you wanted mpg, well, no.
 

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GR-RRR!
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Discussion Starter #3
But yeah, you modified your car for performance. If you wanted mpg, well, no.
Yeah, I get that. I was just in the understanding that the cruise mapping is pretty much left stock in situations such as mine. I'll have to see what my tuner has to say about it.

Now that I think about it, I probably had a Diablo Sport tune canned tune in the car before that trip to Indy. Maybe the Diablo tune had a stronger lean cruise setting than stock and than what I have now.
 

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Rambler is stuck in 2nd gear
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There is not a lean cruise tune.
O2 sensors are there to correct the air/fuel to stoich when you are cruising. If on the trip to Indy, you were watching the mpg thing on the dash, it is useless for actual mpg. As long as your tuner has the MAF and ve table close, the O2's will keep the mixture correct.---as long as you don't have a vacuum leak somewhere. The only way to run it leaner is to turn off fuel trims and run in open loop and set the maf and ve to run lean while you are cruising. Probably not a good idea for a street car.
 

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When cruising shouldn't the tune be pretty much the stock tune and thus deliver better MPGs than I'm getting?
No, and if it were, you’d be running lean with headers, FAST intake, CAI and 1.7 rockers.

In short: more air being delivered to the motor requires more fuel added to get you back to stoich during part throttle driving. More fuel burned per ignition cycle is going to affect overall MPG’s, but 25 on the highway is damn good for your mods.
 

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Lean cruise is not enabled in a stock tune. You have to enable it. Its a bitch to tune but does slightly help. You can change how optimistic (or not) the average mpg is by going in the tune and changing the Cylinder volume. If you go down about 10% in cylinder volume, your average mpg will go up by 2-4mpg and if you increase the cylinder volume by 10% it will go down (indicated, not your ACTUAL mpg) 2-4mpg.
 

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GR-RRR!
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Discussion Starter #7
No, and if it were, you’d be running lean with headers, FAST intake, CAI and 1.7 rockers.

In short: more air being delivered to the motor requires more fuel added to get you back to stoich during part throttle driving. More fuel burned per ignition cycle is going to affect overall MPG’s, but 25 on the highway is damn good for your mods.
This makes a lot of sense, I think I was just overthinking it. (I think you meant my 1.8 rockers.)

My actual mileage calculated by miles/gallons for the highway portions was probably more like 20 or 21. The onboard computer was waaaay off. It used to be more accurate.
 

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2005 GTO MT Red, Red, Pee the bed.
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Lean cruise is not enabled in a stock tune. You have to enable it. Its a bitch to tune but does slightly help. You can change how optimistic (or not) the average mpg is by going in the tune and changing the Cylinder volume. If you go down about 10% in cylinder volume, your average mpg will go up by 2-4mpg and if you increase the cylinder volume by 10% it will go down (indicated, not your ACTUAL mpg) 2-4mpg.
I'm not sure if changing the cylinder volume is a good thing for the ECU to work with. You would be better off mapping the MAF in the cruise range.
 

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Worthless reject member
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Lean cruise is not enabled in a stock tune. You have to enable it. Its a bitch to tune but does slightly help. You can change how optimistic (or not) the average mpg is by going in the tune and changing the Cylinder volume. If you go down about 10% in cylinder volume, your average mpg will go up by 2-4mpg and if you increase the cylinder volume by 10% it will go down (indicated, not your ACTUAL mpg) 2-4mpg.
It's not even available to turn on the 05 or 06.
 

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GR-RRR!
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Discussion Starter #12
Maybe I need to try another highway cruise and keep it at 70 or below. I know on my daily, the MPGs drop pretty fast if I cruise at 75 or above but it gets great mileage at 70. If I can ever finish the shifter cup mod job I'll take it for a longer cruise.
 

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I'm not sure if changing the cylinder volume is a good thing for the ECU to work with. You would be better off mapping the MAF in the cruise range.
If he wants the average mpg to be accurate, its his only choice and its not a bad idea at all. The tune MAY need to be touched up but l did it 10% the other day and it may have made 2-3% difference in my fuel trims. I would definately run a log on it, and touch up what needs to be done.

mapping the maf will not change the estimate mpg. It will correct the afr errors if there is any. And if he did bolt ons and no tune then dialing in the maf would be recommended. But that still wont fix mpg errors. Changing cylinder volume directly affects the estimated mpg's.
 

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This makes a lot of sense, I think I was just overthinking it. (I think you meant my 1.8 rockers.)

My actual mileage calculated by miles/gallons for the highway portions was probably more like 20 or 21. The onboard computer was waaaay off. It used to be more accurate.
Did they change injectors?
After I did my mods and changed injectors (ID1050X) Said I was getting almost 40MPG. There is a fuel density value that is only used by the trip computer.
I adjusted it by what the injector scaling was and it got me close. I monitored the mileage over a few tanks and compared the trip computer with my actual calculated MPG, and changed the value by the percentage difference I had. It's pretty damn close now. Filled up today and calc'd out to 17.6MPG, dash said 17.5

502569
 

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Did they change injectors?
After I did my mods and changed injectors (ID1050X) Said I was getting almost 40MPG. There is a fuel density value that is only used by the trip computer.
I adjusted it by what the injector scaling was and it got me close. I monitored the mileage over a few tanks and compared the trip computer with my actual calculated MPG, and changed the value by the percentage difference I had. It's pretty damn close now. Filled up today and calc'd out to 17.6MPG, dash said 17.5

View attachment 502569
good to know there, I'd like to make my car read somewhat accurate lol
 

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GR-RRR!
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Discussion Starter #18
I have stock injectors. Tuner is going to look at the cruise fueling next Saturday. He is thinking perhaps the cruise fueling mapping or whatever may be cutting off too early (too low in the RPM range).
 

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The last time I even bothered to calculate or watch the DIC MPG’s was in 2005. Believe I got roughly 28, cruising in 6th gear, in my then bone stock 04.
 

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GR-RRR!
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Discussion Starter #20
The last time I even bothered to calculate or watch the DIC MPG’s was in 2005. Believe I got roughly 28, cruising in 6th gear, in my then bone stock 04.
Well, here's the sum total. On my trip last Saturday with real math, not the DIC, I got 18 MPG over 250 miles. Now that included about 8 hard charging laps on a 2.15 mile race course. The rest was highway miles, about 250 of 'em. On my way there I probably averaged between 75-80 MPH, on the way home probably averaged around 75. In my mind, that just seems like a pretty low average for the whole trip even keeping the "racing" portion and my modest mods in mind especially given I've been able to pull down 30mpg in the past with a not drastically different version of the car.

Anyway, the tuner will be able to tell me if the cruise values in the tune are out of whack a bit. If not, it just is what it is.
 
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