Doh. I thought this thread was about a z06. No wonder, better aero and taller gearing = better mpg for sure in the coupe.

 

It's ok. I still love my Z, even after adding a cam to further reduce the mileage.

 

It was. It just devolved quickly when the 30+ mpg was brought up. That's coupe mpg; I doubt a stock Z06 like the OP's could get that kind of mileage.

 

Yeah I was still surprised to see that I can't really break 27 at 70 mph without drafting anyone though, I was thinking more like 28-30 since coupes can do 30-35 depending on circumstances.

 

"The Z06 utilizes the specially built M12 6-speed manual transmission. While outwardly similar to the base-model MM6 manual transmission, the M12 transmission is unique to the Z06, and not available on other Corvettes, i.e., coupés and convertibles. The M12 has more aggressive gearing to increase torque multiplication in most forward gears, allowing for more rapid acceleration and more usable torque at higher speeds. A transmission temperature sensor was added to protect the M12 from higher thermal stresses. The sensor warns the driver, via the Driver Information Center, with a "TRANS OVER TEMP" message if transmission temperatures approach their operating limits, and which could lead to damage if the transmission were not allowed to cool down. As with all C5 Corvettes, the transmission is located toward the rear of the vehicle, as a rear-mounted transaxle assembly. This allows a better overall weight distribution for the vehicle, as the weight of the transmission offsets some of the weight of the forward-sitting engine.
The M12 6-speed causes the Z06 model of C5 Corvette to have a lower top speed than a normal C5 despite having a higher RPM limit and more power, this is due to its gearing.
Gear ratio comparisons of MM6 (base-model C5) vs. M12 (Z06):

Gear LS1/MM6 LS6/M12
1st Gear 2.66:1 2.97:1
2nd Gear 1.78:1 2.07:1
3rd Gear 1.30:1 1.43:1
4th Gear 1.00:1 1.00:1
5th Gear 0.74:1 0.84:1
6th Gear 0.50:1 0.56:1
Reverse 2.90:1 3.28:1
Axle Ratio varies 3.42:1
Final Drive Ratio varies 1.91:1"

From Wikipedia.
So, the Z06 turns more RPM than a non Z06 MM6 car, the Z06 should be a 44% overdrive in 6th, vs 50% overdrive of the non Z06.

My 1999 vert, with MM6 (ha I did'nt know it was called that, I just said "6 speed") 3.42 rear gears, no mods, I have gotten as high as 33 MPG with cruise control on, and going 75-83 MPH on I40 where the speed limits are 75 MPH.

So, lets say a non Z06, 6 speed car, like mine can get 30 MPG with cruise set, in 6th gear.

Now, the Z06 with 6% less overdrive, doing the math you would lose 1.8 MPG in 6th gear, lowering it down to 28.2.

That is just the MPG lost from the engine spinning more RPM, now maybe the LS6 engine takes more fuel to run than a LS1, even if both are run at the same exact RPM, the LS6 heads, cam, intake, and possibly exhaust flow more than the LS1, so the "tune" has to run that engine richer than a LS1 to make up for the extra flow.

Then last, someone said the Z06, and I suppose FRC are less aerodynamic than the coupe, not sure how a convertible compares to the others.

I guess in the end, you lose some MPG vs the base Corvettes, however your car makes more power, so you have to trade that off.

Still, for a car that makes 385-405 HP (did not see what year you have), you are still getting pretty damned good MPG.

 

If your engine is idling fine and doesn't have hesitation, you probably don't have a vacuum leak. The old school way is to systematically spray carb cleaner on vacuum lines. If the idle improves, you located the vicinity of a leak because the carb cleaner momentarily plugs the leak.

edit: forgot to include spraying around the intake manifold.

 

Come on now ED, you know there are hills there in Dallas

What about FlagPole Hill ?? See there you forgot about that one...

Roland

 

FWIW I have a '00 FRC with a built 402, pretty big cam, etc. Average right about 30mpg on premium (91-93 octane). I am in N TX and very few hills here too... In light traffic, where I can draft a bit, it'll go up to 35ish!
To improve mpg, look at Tire pressure, alignment, spark plugs, wires, clean & functioning MAF and MAP sensors. Run standard weight oils in engine, trans, diff (from past experience, some folks run unnecessarily thick 20W/50 oil in their car and wonder why mpg goes down)
If the engine is actually running rich only a wideband will tell you I think...
The O2 sensor may be shot and running it rich?
If the exhaust pipes are coated on the inside with a lot of soot it's an indication of perhaps a rich-running engine.
One more thing I can think of: slightly clogged or dirty injectors can hit mpg. I don't believe in snake oil but the Redline Sl-1 gas additive has worked wonders for cleaning various engines of mine (4 & 2 stroke). Not a bad investment for $12.

EDIT: just noticed from your first post that you did most of the above... hmmm....
best of luck!

 

New C5s got better mpg on pure gasoline that was sold then, a 10% alcohol blend is common now.

 

The question is what transmission and rear end gear ratios do you have? That makes most of the difference it seems. Z06 trans with 3.42 gears is going to be worse than your stock FRC gearing unless you've changed it.

 

Summer 1997, I bought a used car, for 1600 bucks, it was a 1985 Nissan Stanza, automatic, 4 cylinder and it had Multiport fuel injection, 2 spark plug per cylinder, made 105 HP.

I did the MPG test by the gallons vs the miles, it got 23 MPG, and that was driving it careful, to conserve fuel.

When I got my Corvette in 2003, I was blown away that a car with such power could make better MPG than my older "economy car"

 

Which direction? Flagstaff to Kingman is a 3600-foot drop, with another 2800 feet down to Needles.

 

You bought a Z06 and are worried about gas mileage........?

 

The weekend after I bought my '01 Coupe I put over 1500 miles on it and averaged 28ish. That included spirited driving along the coast as well as flat nothing highway. I can get 32mpg on the highway at 68mph.

But I am only averaging 22mog on my morning commute and was hoping for more like 28. But it's still 10x funner than my Silverado on Skyline Blvd.

 

good point... I think it's been changed to 3.42 but now I gotta check just bought it recently!

 

Well, I started out from the Bay Area, went to L.A. spent the night, then went to Phoenix where my bother lived.

Stayed a few days, then drove north I17 to Flagstaff then east to Amarillo, to see other brothers and mom, etc.

Are you saying I would get better mileage going towards California?

 

That's how it worked every time I've gone to the Grand Canyon, to the tune of 3-4MPG.
(Haven't done it in the 'Vette yet though.)
(EDIT: Amarillo is about 3500 feet lower than Flagstaff, so you were going downhill on that stretch too.)

 

I had a 6 speed in my '00 coupe (3.42 rear gears) and got 30+ mpg out on the highway many times. A lot will depend on the road conditions (hilly or flat), do you use cruise control, variations in driving speed due to traffic conditions, etc.

The Z06 does not have as good of aerodynamics as a coupe and the tranny ratios are not as good for mileage purposes. Still, I would think you would get better than 24 mpg on the highway from the car.

 

LOL. That is about spot on with what I am seeing as well (lightly modded with long tubes, and a tune). 55 is about the sweet spot for the more aggressively geared, and less aerodynamic ZO6 as far as gas mileage is concerned.

Also,
no one mentioned that you get less mileage in colder weather, partially due to winter gas. Not knowing where the OP is from, this may be partly the cause.

 

less restrictive catalytic converters will up your mileage. Depending on your year, you may have the pup cats in addition to the main cats. if you get an 02 z06 h-pipe, it comes without pup cats, and is less restrictive. You might be best off just getting an h-pipe without pup cats and get some high flow cats. be aware that the 01+ h-pipes have a different flange angle than earlier years.

the stock air box is extremely restrictive. going with a high flow aftermarket intake will also boost your mileage.

Also check your tire pressures. They should be 32psi all around. lower pressure will reduce mileage.

Change your differential and transmission fluid if they are due. more friction in the gearbox means more parasitic loss.

Reducing weight also can help. make sure you are using non runflat tires. you can remove the air system which will also reduce drag on the alternator. next time you do a clutch job, opt for a light weight setup with an aluminum flywheel. stay away from heavy aftermarket wheels.

An underdrive pulley will reduce parasitic loss due to less rotation of the serpentine accessories. I wouldn't suggest any more than a 10% underdrive ratio.

Last but not least, TECHRON fuel system cleaner.