StuSegal

I just took my 2013 GS Coupe on an 800 mi trip; it got considerably worse gas mileage on the highway than my '06 Coupe.

I'm wondering if the rear axle ratio is different than the '06. Is there any way I can know the rear ratio from the VIN or other info on the car?

JR_VETTE

Glove box should have the RPO sticker in there. It's kind of hard to see if you don't have a flashlight. It should have the RPO printed on it (Gxx). Post it here, and we can tell you what you have. Assuming you have an Auto. All manuals I believe are 3.42's.

How many miles? Maybe not fully broken in?

StuSegal

Yes, automatic. The RPO says GU2.

Um, broken in? When I left on the trip, 3k miles. Mitigating factors . . . a good stretch of the PA Turnpike has a posted 70 limit, so I was driving 80. Probably did 300-400 miles of the 800 mile trip at 80 mph, which may not be the most economical speed.

Justin-Sane

You have 2:73's

subfloor@centurytrans

There's always a Getrag sticker on the differential with the gear ratio listed on it. If you climb underneath the car it's pretty easy to see.

PDsVettes

Get on a flat road road, put the DIC on instantaneous gas mileage and try different speeds. I tried it on my C6 at 75 and 80 and it was 3-4 mpg lower at 80.

It's got to be on a flat road or this idea won't work.

Justin is correct, GU2 is 2:73 gears.

hisvett

Relax, it isn't your gearing. Put the same tires (Skinny) on as your early coupe had and strip off the fat finders for base ones and your mileage will return.

The drag coefficient numbers for a Base C6 is .29 where as the GS is .349 Be thankful that you don't have a M6, they have lower gearing in the trans making the engine spin another couple of hundred more "R's".

HOXXOH

Assuming your '06 was also an A6 it would have been 2.56. All indications show the LS2 to have slightly better MPG, but not by much.
For the most part hisvett is correct. Especially the fat tire part. The extra weight is a factor when accelerating, but not at constant speeds on flat roads.
Try what Patdown suggested on your next outing. The 5 MPH difference for 350 miles was less than 20 minutes and cost about 1.5 gallons. You'd have more fun at a slower pace by avoiding Interstate highways and leaving an hour earlier.

gbrtng

Bigger engine, gooey tires and a poorer drag coefficient = worse fuel mileage. My 13 base coupe gets 2-3 MPG better than a friend's 13 GS. He was not pleased either ...

Kent1999

At these speeds (80 or less), the small increase in drag from base fenders to the GS/Z06 slightly flared fenders probably isnt going to amount to much. However, the wider GS/Z06 tires certainly can drop you down a mpg or two.

Hard to know what you mean when you say the mileage is "considerably worse"... does that mean 1-2 mpg less or like 6 mpg less? If its more than 1-2 mpg -- you've got something else going on thats knocking your mpg down. Bad alignment? Under inflated tires? Different tires? Higher load (suitcases, etc for the trip)? Your last C6 was a manual and this one is an auto?

My real world MPG difference from my "2006 M6 LS2 C6" coupe to my "2012 M6 LS3 C6 GS coupe" is about 1-2 MPG city or highway -- and that was BEFORE I did the widebooty on the 2006. Post widebooty/Z06 tires & wheels, its about 1 MPG worse on the GS than the Coupe.

My 2012 GS is 100% stock (so far!) while my 2006 also had a Borla x-pipe and cat-back... don't really know if that affected my mileage one way or the other.

otisfooter

08 auto coupe gets between 28 and 29 consistantly going to Florida and back, running between 70 and 80. It has been tuned by Chuck (COW) and also he changed gears to 3:42 the car gained MPG after tune and gears.

0Chuck CoW

Hard to believe, but gears and my CoW BOOSTER! will put you right be back where you need to be.

I can make the car into a beast with just my TUNE and CoW BOOSTER PACKAGE and not only will it get better

mileage but it will be the best money you ever spent on the car!

Call any time 914-332-0049

Chuck CoW

HOXXOH

That's really hard to believe. It's not downhill both ways.

Would you care to explain how a 33% increase in engine revolutions resulted in a reduction of fuel being used?

Gearhead Jim

IIRC, the 2006-07 A6 cars all had the same 2.56 rear axle, regardless of Z51 or not. Only when the LS3 came out in 2008 did the A6 get two different axle ratios.

HOXXOH

You remembered correctly. Obviously, I didn't.

Fixing my post now.

AORoads

While drag coefficient (Coefficient of Drag) might not mean a lot, there is something to different speeds and changing the CoD. My suv is obviously lousy at CoD. However, at more moderate, consistent, highway speeds of 65-70 it easily gets 1-4 mpg more than 70-80 or 85. And the real number is closer to 4 mpg on long trips.

On a C6 w. a 20 gal. tank (approx.), I suspect I'd notice 70 miles less per fill up (about 18 gal. fill up x 4 mpg less) if my "old" car got 350 miles before I filled up, and 280 in my newer car. Not real numbers but close enough.

Gearhead Jim

First time in 7 years...