Lousy mileage
Thread Starter
3rd Gear
Joined: Jul 2008
Posts: 3
Likes: 0
Lousy mileage
I have a 97 C5, which I had a 3.42 rear end and an after market torque converter installed in. Before these were installed I averaged somewhere in the area of 25-27 MPG. Now I am lucky to get 18 MPG on a trip. Any ideas? Would these modifications change the MPG this much? How can I improve the MPG?
Pro
Joined: Mar 2007
Posts: 600
Likes: 11
From: Wilmington NC
Switching the gears will drop the mileage significantly in an automatic. I went from around 28 to 20 switching from a 3.15 to 3.73. I did make a mistake and descreen my MAF which cause issues. I replaced the MAF with a new factory piece and milage jumped back up to 24.
Bottom line, gears will affect mileage. Think; what RPM were you at before and what are you at now? Mine increased 450 RPM (that means more fuel).
Bottom line, gears will affect mileage. Think; what RPM were you at before and what are you at now? Mine increased 450 RPM (that means more fuel).
Burning Brakes
Joined: Feb 2005
Posts: 752
Likes: 1
From: Tulsa OK
I went from a 3:15 to a 3:42 and it didn't effect my mileage that much. I still average about 28 on the highway. RPMs at 70 mph cruise went from 1950 to about 2100.
When you did the stall converter, did you reprogram the computer for lock up speeds. Once the converter locks up it should drive exactly the same as a lower stall converter when cruising. Maybe the converter is not locking up properly - that will effect the mileage big time.
When you did the stall converter, did you reprogram the computer for lock up speeds. Once the converter locks up it should drive exactly the same as a lower stall converter when cruising. Maybe the converter is not locking up properly - that will effect the mileage big time.
Le Mans Master
Joined: Sep 2007
Posts: 7,604
Likes: 8
From: Belleville Il
I went from a 3:15 to a 3:42 and it didn't effect my mileage that much. I still average about 28 on the highway. RPMs at 70 mph cruise went from 1950 to about 2100.
When you did the stall converter, did you reprogram the computer for lock up speeds. Once the converter locks up it should drive exactly the same as a lower stall converter when cruising. Maybe the converter is not locking up properly - that will effect the mileage big time.
When you did the stall converter, did you reprogram the computer for lock up speeds. Once the converter locks up it should drive exactly the same as a lower stall converter when cruising. Maybe the converter is not locking up properly - that will effect the mileage big time.
I would suspect the converter too and depending on how "loose" it is,it will have an effect on MPG.
Last edited by itzza427; Jul 22, 2008 at 12:34 AM.
Burning Brakes
Joined: Apr 2003
Posts: 908
Likes: 4
From: TROY MI
I am running 3.73 gears with a 3600 Stall TC and I'm getting 27/28 mpg on the highway.I am also running 11.2 CR and this is making up for the 3.73 gears.I also have my TC locking up at 34 mph.
Burning Brakes
Joined: Feb 2005
Posts: 752
Likes: 1
From: Tulsa OK
I have the stock converter and 3:42 gears and the car feels strong but it is probably not as good as if it had a higher stall converter.
Le Mans Master
Joined: Sep 2007
Posts: 7,604
Likes: 8
From: Belleville Il
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Tech Contributor
Joined: Dec 1999
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From: Anthony TX
CI 6,7,8,9,11 Vet
St. Jude Donor '08
97 C5!! Whats the mileage? What maintenance has been done to your C5 over the years? Stuff like primary O2 sensors, plugs & wires, air filter etc.....
I also agree that if the converter is not locked up in OD, your fuel mileage would suffer. What RPMs are you pulling at 30-40-50 and 60 MPH on a straigh stretch of level highway. That info will allow others with an auto to let you know if the TC is locking up correctly.
You can also find someone one in your area that has EFI Live or HP Tuners and ask them to data log your car. That will show you a LOT
You can look at O2 sensor preformance (switching), LTFT's MAP, MAF and Transmission data. All in one driving session!
BC
I also agree that if the converter is not locked up in OD, your fuel mileage would suffer. What RPMs are you pulling at 30-40-50 and 60 MPH on a straigh stretch of level highway. That info will allow others with an auto to let you know if the TC is locking up correctly.
You can also find someone one in your area that has EFI Live or HP Tuners and ask them to data log your car. That will show you a LOT
You can look at O2 sensor preformance (switching), LTFT's MAP, MAF and Transmission data. All in one driving session!
BC
Le Mans Master
Joined: Aug 2005
Posts: 5,375
Likes: 328
From: CA.
St. Jude Donor '13-'14-'15
I agree with Bill---sounds like you have other issues at play here--You need to get it over to a tuner and have them run some data logging to check the lock-up, 02's etc.----Installing a stall converter takes specific tuning in order for it to work properly--If for example you don't max out the misfire tables, you'll get full line pressure when it throws codes or goes into a limp mode--horrible on mileage--I have a 2600 stall and a 3.42 on my '98 and my milage went from 28 to 26----
Le Mans Master
Joined: May 2005
Posts: 6,366
Likes: 249
From: Fort Lauderdale, Florida
I have a 97 C5, which I had a 3.42 rear end and an after market torque converter installed in. Before these were installed I averaged somewhere in the area of 25-27 MPG. Now I am lucky to get 18 MPG on a trip. Any ideas? Would these modifications change the MPG this much? How can I improve the MPG?
Racer
Joined: Aug 2007
Posts: 497
Likes: 0
From: Monrovia California
The TC will not reduce MPG at highway speeds.Once the TC locks there is no difference in any converter. Make sure your TC is locking up at 34/36 mph in OD.My guess is that it is not locking up and this is the cause of your low milage.
I am running 3.73 gears with a 3600 Stall TC and I'm getting 27/28 mpg on the highway.I am also running 11.2 CR and this is making up for the 3.73 gears.I also have my TC locking up at 34 mph.
I am running 3.73 gears with a 3600 Stall TC and I'm getting 27/28 mpg on the highway.I am also running 11.2 CR and this is making up for the 3.73 gears.I also have my TC locking up at 34 mph.
