Gears changed but MPG got hammered!
Thread Starter
Le Mans Master
Joined: Sep 2003
Posts: 6,386
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From: Just a little outside San Antonio, Texas
Gears changed but MPG got hammered!
Tossed in a set of 3.42 gears and a 2800 torque converter into my 2004 coupe in the past couple of weeks. Changed the PCM to reflect the new gears.
Just got back from a 1000 mile trip. Gas mileage went from 30 MPG before the new gears to less than 23 MPG afterwards!! The 23 MPG was on nearly flat interstate highway with cruise control on 75 mph or so.
Should I have expected a drop of 7 MPG from this gear change or did my tuner miss something??
Thanks for your comments.
Just got back from a 1000 mile trip. Gas mileage went from 30 MPG before the new gears to less than 23 MPG afterwards!! The 23 MPG was on nearly flat interstate highway with cruise control on 75 mph or so.
Should I have expected a drop of 7 MPG from this gear change or did my tuner miss something??
Thanks for your comments.
Last edited by ArmyVette; Jul 13, 2004 at 12:59 PM.
Melting Slicks
Joined: Dec 2000
Posts: 2,118
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That sounds about right...if you went from 2.73 to 3.42 your geared 25% lower......25% of 30mpg is 7.5mpg.....so 23mpg is correct. It's impossible to gear your car 25% lower and not have increased fuel comsumption....my motor change ...stock 346ci to a S/C 427 resulted in a drop from 32mpg to 19mpg!!!!
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
this is where the 6 speed Corvettes have an advantage. They have 2 overdrive gears - so even 4.10's are (somewhat) tame-able on the highway.
Are you certain yout TC is locking ?
My 2002 A4 has the 3.42 but I retained the stock TC. My highway MPG is well in the 26mpg range - though it's been quite awhile since I last checked it.
Are you certain yout TC is locking ?
My 2002 A4 has the 3.42 but I retained the stock TC. My highway MPG is well in the 26mpg range - though it's been quite awhile since I last checked it.
Thread Starter
Le Mans Master
Joined: Sep 2003
Posts: 6,386
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From: Just a little outside San Antonio, Texas
Originally Posted by Mike Mercury
Are you certain yout TC is locking ?
How can I tell if the TC is locking and unlocking properly?
Also, Mike, can you check my thread over in C5 Scan and Tune about A4 shift points? You might be able to point me in the right direction.
Thanks!
Jeff
Last edited by ArmyVette; Jul 13, 2004 at 01:24 PM.
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
Jeff; I don't use LS1Edit - as I don't have the time to utilize 3 or 4 pages of screens just to set shift points. So I opted for the less capable Predator.
On my Predator, I set the WOT shift points by placing my A4 in first and taking the car up to redline - noting the MPH. And then did this in "2nd". I then set the WOT shift points 3 MPH below what IU noted for those gears.
I made no changes in downshift points as my model Predator doesn't allow one to adjust downshift points. But I have had no abnormal shifting after my 3.42 upgrade.
Sorry, but I don't know enough about the LS1Edit parameters to give any useful advice.
On my Predator, I set the WOT shift points by placing my A4 in first and taking the car up to redline - noting the MPH. And then did this in "2nd". I then set the WOT shift points 3 MPH below what IU noted for those gears.
I made no changes in downshift points as my model Predator doesn't allow one to adjust downshift points. But I have had no abnormal shifting after my 3.42 upgrade.
Sorry, but I don't know enough about the LS1Edit parameters to give any useful advice.
Pro
Joined: Mar 2002
Posts: 527
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From: Colchester VT
I have 3.42 gears and a Yank 3000 TC and get 26-27 mpg on the highway. I agree that maybe your TC is not locking up. It should feel almost like a shift. You should feel 4 shifts, 1>2, 2>3, 3>4 and TC lock. Also you can check your tach, at 75 mph I believe you should be around 2200 rpms, if you are much higher your TC may not be locking up. Not sure what TCC setting is false means. In LS1 Edit there is a table where you set the speed at which the TC will lock and unlock according to throttle position and gear. I hope this has been some help, good luck! 
Melting Slicks
Joined: Oct 2003
Posts: 2,655
Likes: 5
From: Brea CA
WOW.... 25% drop in gas mileage.... man
I just drove my Coupe A4 from California to Arkansas, and back... I got between 29 and 32 miles per gallon... of course I have the 273's....
The thing I don't understand is... most people with the 342's claim no loss in gas mileage, which would kind of make sence, as the load would be less. Also most see only a small gain in their 1/4 mile time. I know the lower gear will feel like the car is lighter, but is it worth a 25% drop, and do most people get that much of a drop?
The thing I don't understand is... most people with the 342's claim no loss in gas mileage, which would kind of make sence, as the load would be less. Also most see only a small gain in their 1/4 mile time. I know the lower gear will feel like the car is lighter, but is it worth a 25% drop, and do most people get that much of a drop?
Team Owner
Joined: Aug 2003
Posts: 38,314
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Originally Posted by 98silverC5
I have 3.42 gears and a Yank 3000 TC and get 26-27 mpg on the highway. I agree that maybe your TC is not locking up. It should feel almost like a shift. You should feel 4 shifts, 1>2, 2>3, 3>4 and TC lock. Also you can check your tach, at 75 mph I believe you should be around 2200 rpms, if you are much higher your TC may not be locking up. Not sure what TCC setting is false means. In LS1 Edit there is a table where you set the speed at which the TC will lock and unlock according to throttle position and gear. I hope this has been some help, good luck!
I get 20.5mpg-21.0mpg just cruising around through the burbs. I also feel the 4 transitions described above, except when I drive in 3 of course to avoid the P0757 code when driving at very slow "neighborhood" speeds.
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Team Owner
Joined: Aug 2003
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Originally Posted by AtHomeSoda
I just drove my Coupe A4 from California to Arkansas, and back... I got between 29 and 32 miles per gallon... of course I have the 273's....
The thing I don't understand is... most people with the 342's claim no loss in gas mileage, which would kind of make sence, as the load would be less. Also most see only a small gain in their 1/4 mile time. I know the lower gear will feel like the car is lighter, but is it worth a 25% drop, and do most people get that much of a drop?
The thing I don't understand is... most people with the 342's claim no loss in gas mileage, which would kind of make sence, as the load would be less. Also most see only a small gain in their 1/4 mile time. I know the lower gear will feel like the car is lighter, but is it worth a 25% drop, and do most people get that much of a drop?
The term "small gain" is often misunderstood in 1/4 mile times. 4 tenths is not a "small" gain in a drag race. Its huge. A big reason for anyone with 3.42s seeing nominal improvement in 1/4 mile times is lack of traction, plain and simple. I actually ran worse 1/4 mile times on my runflats with 3.42s and my 2800 stall converter than I did with 2.73s and a few other non agressive mods. The reason, traction. 3.42s should be good for at least .4 in the quarter over 2.73s assuming good traction.
Also worthy of mention when comparing before and after 1/4 mile times, it helps to compare the average ET before the mod, with the average ET after the mod.
Too many times its the BEST ET before the mod, compared to the first, worst, or the average, ET after. This can lead to thinking that the modification didn't amount to much.
For example, my "best" with 2.73s was 13.14. I don't believe anyone here could "average" 13.14 with 2.73s unless they limited themselves to one run of 13.14.
Team Owner
Joined: Apr 2001
Posts: 54,204
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From: S.W. Ohio. . . . . . NRA Life Member
The term "small gain" is often misunderstood in 1/4 mile times. 4 tenths is not a "small" gain in a drag race. Its huge. A big reason for anyone with 3.42s seeing nominal improvement in 1/4 mile times is lack of traction, plain and simple. I actually ran worse 1/4 mile times on my runflats with 3.42s and my 2800 stall converter than I did with 2.73s and a few other non agressive mods. The reason, traction. 3.42s should be good for at least .4 in the quarter over 2.73s assuming good traction.
And we have just one person reporting a big MPG drop - with all the others that have done this mod reporting no appreciable drop in MPG. So how can anyone seriously ask "is it worth a 25% drop..." when it's just one person having a 25% MPG problem?
Also; IMO anyone that worries about the MPG of their Corvette really shouldn't be performance modding their car. If MPG is important to a person, then they should leave it totally stock - as that config will give the best MPG.
Drifting
Joined: Oct 2000
Posts: 1,396
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From: Sacramento CA
Mike,
What you sat is somewhat true for modding cars. With engine mods that make the car breath(heads and header combo) better you actually improve MPG because the car runs more efficently. That is the true test of if what you are doing is right.
Jeffy'
What you sat is somewhat true for modding cars. With engine mods that make the car breath(heads and header combo) better you actually improve MPG because the car runs more efficently. That is the true test of if what you are doing is right.
Jeffy'
Melting Slicks
Joined: Oct 2003
Posts: 2,655
Likes: 5
From: Brea CA
Gas Mileage.... Corvette
Well, some of us own more than one Corvette.... My other 72 4 speed with a ZZ4 and Nos... is a ball to drive... not a reason to worry about mileage... but my question was... "Is it normal to see that much drop" as I am aware the RPM's are higher... BUT there is a thing called LOAD that is involved...if the car is pushing "X" load at "X" RPM or it is pushing "X-" load at a higher RPM... it will balance out... as there is no such thing as a free lunch... so ... is there such thing as "Wasted HP"... as in my first post... my thought was... you should not see a 25% drop in mialage... as that would mean you were not gettin your bang for your buck... I also have a 69 Camaro with 383 stroker, and 355's in the rear... it gets great mileage..as it runs higher RPM, but it is has less strain on it... it is not that bad... Come on... on the freeway... it should just idle at 2200 RPM... not have to work.... 
Thread Starter
Le Mans Master
Joined: Sep 2003
Posts: 6,386
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From: Just a little outside San Antonio, Texas
Originally Posted by 98silverC5
I have 3.42 gears and a Yank 3000 TC and get 26-27 mpg on the highway. I agree that maybe your TC is not locking up. It should feel almost like a shift. You should feel 4 shifts, 1>2, 2>3, 3>4 and TC lock.
Originally Posted by 98silverC5
Also you can check your tach, at 75 mph I believe you should be around 2200 rpms, if you are much higher your TC may not be locking up.
From what I've read in the previous posts, the MPG is still a little less than what I should be expecting. I wonder if my air-fuel ratio may not be set properly. I'll check my latest PCM read and post those values here later today.
Mike Mercury, can you post your A4 shift point values for me, for both the "normal" and the "WOT" settings? I believe they have MPH and TSP values, and I think they also have RPM values.
Thanks for all the help, gang.
Melting Slicks
Joined: Oct 2002
Posts: 2,855
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With the 6speed when I went to 390s I think my mileage stayed about the same as it took less pedal and effort to take off and get to speed and on the highway at 2000 I was at 90 with the 342s and 80 with the 390s---one thing I did learn is the AF has a lot to do with it---for instance when I had a 346 blower motor my normal driving (town) was about 13mpg as the AF was kept rich--I have a 436 stroker with a FAST 90mm setup now when the AF was at 12 to 1 I was getting about 13 again same as blower when we tuned it out to 13 to 1 I am getting 15 or 16 under the same conditions and better highway economy than you are and this putting down way over 500 of each with 390s---I would check the AF--
Burning Brakes
Joined: Apr 2003
Posts: 908
Likes: 4
From: TROY MI
Originally Posted by ArmyVette
Tossed in a set of 3.42 gears and a 2800 torque converter into my 2004 coupe in the past couple of weeks. Changed the PCM to reflect the new gears.
Just got back from a 1000 mile trip. Gas mileage went from 30 MPG before the new gears to less than 23 MPG afterwards!! The 23 MPG was on nearly flat interstate highway with cruise control on 75 mph or so.
Should I have expected a drop of 7 MPG from this gear change or did my tuner miss something??
Thanks for your comments.
Just got back from a 1000 mile trip. Gas mileage went from 30 MPG before the new gears to less than 23 MPG afterwards!! The 23 MPG was on nearly flat interstate highway with cruise control on 75 mph or so.
Should I have expected a drop of 7 MPG from this gear change or did my tuner miss something??
Thanks for your comments.
Team Owner
Joined: Apr 2001
Posts: 54,204
Likes: 180
From: S.W. Ohio. . . . . . NRA Life Member
[better you actually improve MPG because the car runs more efficently. That is the true test of if what you are doing is right.
Thread Starter
Le Mans Master
Joined: Sep 2003
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From: Just a little outside San Antonio, Texas
Originally Posted by yankeevetteconvert
Your post states that you lost the mpg at 75 mph cruise...IMHO I think that your 30mpg was a bit optimistic for a A4.I would say that 23mpg is within 1-2 mpg of most cars with this setup.
I was getting about 29 MPG prior to the Vararam install; got over 30 afterwards (after the PCM-relearn period, of course). And, yes, the driving conditions to determine the MPG ratings were almost identical including that they were over the same 60 mile stretch of Interstate highway.
After all that has been said, it appears this MPG reduction is normal and that I was just getting EXCEPTIONAL gas mileage prior to the gear and TC installation.
Thanks for all of the good input, guys!!
