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Old 03-12-2014, 01:51 PM   #1
paxmanz06
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Default Suboptimal Highway Mileage

I bought a stock z06 several months ago and I've never come close to getting 30 mpg on the highway. For 100% highway driving, I get 24mpg. I've done the following so far:
- replaced air filter
- changed ALL fluids
- replaced spark plugs and wires
- replaced both primary oxygen sensors
- new rotors and pads
- changed serpentine and accessory belts
- alignment
- new tires

I also have a bluetooth OBD2 adapter but don't remember seeing anything out of the ordinary. There are no codes. What are some other things I can check?

Thanks!
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Old 03-12-2014, 02:02 PM   #2
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First of all, what rear axle do you have? I have the 3.15:1 Performance Axle and the best I can eke out is 25 mpg pure highway.


UPDATE: I just looked at corvetteactioncenter.com and it looks like the '03 Z06 came standard with 3.42 axle. I doubt one can get 30 mpg with such an axle. I believe you need the 2.73 axle or mods to expect 30 mpg.
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Last edited by s'noJob; 03-12-2014 at 02:09 PM.
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Old 03-12-2014, 02:04 PM   #3
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Originally Posted by s'noJob View Post
First of all, what rear axle do you have? I have the 3.15:1 Performance Axle and the best I can eke out is 25 mpg pure highway.


.
I have 3:42's and get 32 mpg highway with a manual trans.
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Old 03-12-2014, 02:10 PM   #4
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This past weekend...70 miles @78mph 2001 m6 3.42 gear with only black wing and bb bullets for mods i got 29.8, also used cruise control. Exxon 91 for petro. There are so many variables ...
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Old 03-12-2014, 02:12 PM   #5
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I have 3:42's and get 32 mpg highway with a manual trans.
Well, sure, but what's the fun of owning a 'Vette if you never take it over 50 mph!
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Old 03-12-2014, 02:14 PM   #6
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As far as I know, it's stock but I'll double check on the ride home tonight. Came across this on wikipedia:
Gear Distance per engine revolution Speed per 1000 RPM
1st gear 8.1 in (210 mm) 7.7 mph (12.4 km/h)
2nd gear 11.6 in (290 mm) 11.0 mph (17.7 km/h)
3rd gear 16.8 in (430 mm) 15.9 mph (25.6 km/h)
4th gear 24.0 in (610 mm) 22.7 mph (36.5 km/h)
5th gear 28.6 in (730 mm) 27.1 mph (43.6 km/h)
6th gear 42.9 in (1,090 mm) 40.6 mph (65.3 km/h)
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Old 03-12-2014, 02:14 PM   #7
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I get 26 on cruise control at 72 mph in a stock 04 Z06. I also expected to get 30+ based on the tales told on this forum... either some cars get unicorn mpg or some people just like telling tall tales.
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Old 03-12-2014, 02:18 PM   #8
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Just to let you know, a Z06 will not do as well on the highway as the coupes / convertibles. This is due to the transmission and rear end gearing. The manufacturer sticker shows the Z is rated for 28 highway. Your mileage isn't really bad. I did a 5,000 mile trip last year and averaged 26 (calculated not DIC indicated). In Michigan, I can get up to 28 (at 73 MPH) because of the very flat land I travel.

What speeds are you travelling? What is the terrain and elevation like? You didn't mention if you cleaned the MAF sensor. How does the engine seem to be running? Possible air or vacuum leak or nothing wrong at all.

IIRC the final gear in a Z is 0.56 and the coupe manual is 0.50. Only 4th gear is the same in both manual transmissions.

Last edited by 3sACROWD; 03-12-2014 at 02:39 PM.
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Old 03-12-2014, 02:23 PM   #9
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Coups also have the better drag coefficient. And adding a spoiler could hurt it a bit also.
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Old 03-12-2014, 02:29 PM   #10
paxmanz06
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Quote:
Originally Posted by 3sACROWD View Post
Just to let you know, a Z06 will not do as well on the highway as the coupes / convertibles. This is due to the transmission and rear end gearing. The manufacturer sticker shows the Z is rated for 28 highway. Your mileage isn't really bad. I did a 5,000 mile trip last year and averaged 26. In Michigan, I can get up to 28 (at 73 MPH) because of the very flat land I travel.

What speeds are you travelling? What is the terrain and elevation like? You didn't mention if you cleaned the MAF sensor. How does the engine seem to be running? Possible air or vacuum leak or nothing wrong at all.
Engine seems to run fine. I've varied my speed on trips to drive constantly at the following speeds: 65, 70, and 75 mph for at least 100 miles. The times I logged my pure highway mileage were during my trips in TX between Austin and Houston.

I have cleaned my MAF sensor, forgot to mention that. When I first got the car and was replacing the air filter, I removed the intake system and cleaned everything since it was dusty.

How can I detect a vacuum leak?

Thanks for the tips!
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Old 03-12-2014, 02:48 PM   #11
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Quote:
Originally Posted by s'noJob View Post
Well, sure, but what's the fun of owning a 'Vette if you never take it over 50 mph!
.
That mpg is obtained going an average of 75-80 mph.
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Old 03-12-2014, 02:51 PM   #12
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Quote:
Originally Posted by troyguitar View Post
I get 26 on cruise control at 72 mph in a stock 04 Z06. I also expected to get 30+ based on the tales told on this forum... either some cars get unicorn mpg or some people just like telling tall tales.
The non Z06 6 speeds usually get over 30 on the highway (mostly level road cruise).

It is not a tall tale or some "unicorn" mpg. I took my hatchback '99 on a 1700 mile trip to the Corvette Museum and then to the Tail of the Dragon last year. There was a mix of 75mph highways and quite a few miles "playing" in lower gears too. At the end of the trip the DIC said 32.0 mpg although the calculated was just over 30 mpg.

Quote:
Originally Posted by 3sACROWD View Post
Just to let you know, a Z06 will not do as well on the highway as the coupes / convertibles. This is due to the transmission and rear end gearing......IIRC the final gear in a Z is 0.56 and the coupe manual is 0.50. ....
The Z06 gearing and the stronger LS6 motor both contribute to a little less mpg. I suspect that the 28 mpg mentioned here might be about tops for a Z06, and there have been a few posts that it has been done.

If you are trying to optimize it, try letting your tuner know what you are trying to do. They can help set up the car for maybe a few mpg's better.

Good luck.

Last edited by QCVette; 03-12-2014 at 02:53 PM.
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Old 03-12-2014, 02:51 PM   #13
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That mpg is obtained going an average of 75-80 mph.
How are you calculating it and where do you live that is all downhill?
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Old 03-12-2014, 02:52 PM   #14
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Quote:
Originally Posted by troyguitar View Post
I get 26 on cruise control at 72 mph in a stock 04 Z06. I also expected to get 30+ based on the tales told on this forum... either some cars get unicorn mpg or some people just like telling tall tales.
This is after a 3000 mile trip that included 800 miles of city driving. My DIC reports .4-.5 on the low side. Say hello to the unicorn.

Click the image to open in full size.
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Old 03-12-2014, 02:53 PM   #15
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Quote:
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How are you calculating it and where do you live that is all downhill?
I calculate it at the pump, and I live in Dallas. We don't have any hills.
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Old 03-12-2014, 02:55 PM   #16
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I calculate it at the pump, and I live in Dallas. We don't have any hills.
You have more hills than Houston and I've never gotten anywhere close to that mileage. I think my best has been 26-27 and that was with really trying on a trip to Arkansas.
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Old 03-12-2014, 02:58 PM   #17
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I also have very similar result. Unicorns are real, maybe hard to fine, but real.
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Old 03-12-2014, 03:00 PM   #18
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You have more hills than Houston and I've never gotten anywhere close to that mileage. I think my best has been 26-27 and that was with really trying on a trip to Arkansas.
Sorry?
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Old 03-12-2014, 03:00 PM   #19
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My Z06 gets 30 mpg with the cruise at 55 mph and 26-27 at 72 mph... guess my car is broken.
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Old 03-12-2014, 03:04 PM   #20
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Quote:
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My Z06 gets 30 mpg with the cruise at 55 mph and 26-27 at 72 mph... guess my car is broken.
You have a Z06, I have a coupe. There's a big difference between the two.
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