get around 28 at 75mph. In town around 19-20. I love the instant mpg. I play with it all the time, even local, to see how high I can get the reading!

 

While the Vette is fuel efficient even at 768hp (28mpg hwy), my right foots kills the mpg but isn't that why we bought this car. If I need fuel economy, I'll driver my 95 Accord DD.

 

Hell I got close to 40 doing 60mph on the interstate in Fla. all flat land with cruise on ,the thing is only showing 1100 RPM,s

 

No PeterPan, I think it surges because of your right foot.

Yes, because we are talking about miles per gallon. While you certainly will use less fuel in a give time at lower RPM, you will not have gone correspondingly as far (hence the miles part). I will guarantee you that you will use more fuel driving to work at 25mph in 5th, than at 60mph in 6th.

Well, yes and no. Where the confusing comes in is operating RPM and coasting RPM. When traveling on a flat road, the driver modulates throttle to maintain constant speed. Sometimes you give it a little more gas, sometimes you don't. As you said, when you hit an incline, you have to apply more throttle to do the same speed, but that would be true for holding any rpm or speed, so not a valid counter argument. This engine "cycling" to maintain constant speed happens most efficiently at around 1500RPM according to the LS1 data that I saw. Coincientally, what top gear ratios did GM tune the cars to operate most efficiently for the EPA 60mph highway test? You guessed it. ~1500RPM.

Yes, it is very complicated. Once again, there's a difference between operating and coasting RPM's. Technically, I can drive my car off a cliff, and run 6000 RPMs with no thottle strait into the ground and get 1000mpg. But when trying to maintain a constant speed on a flat stretch, the engine cycles throttle to acomplish it. This cycling happens most efficiently when the engine operates most efficiently.

My only point was to say that it was more engine dependant and that the choice of auto vs manual, and gear ratios would affect the answer.

 

That wasn't the question. At the same speed, choosing a lower gear in order to make the engine turn at 1500 rpm will not improve mpg.

There certainly is some speed where the C5 gets the highest mpg. And that does not necessarily correspond with the speed where the engine is most efficient. I guarantee you I would get better mpg at 1000 rpm in top gear than at 1500 rpm in top gear.

And I'm not even sure about 25 mph in 5th vs 60 mph in 6th. I'll run some experiments on my lunch hour since I'm really bored here at work.

 

man we got some penny pinching jews on here. how much could this possibly save you on gas. Just like when people freak out over gas going up 10 cents a gallon thats like 1.90 per tank of gas. i could probably find that on the ground while im filling up.

 

This is exactly what I'm disputing. You will absolutely get better fuel rate, but better fuel rate doesn't translate into mpg. To resolve the fuel rates gallons/hour units, you have to consider running it for a period of time, and then calculate the distance you traveled over that time. The vettes computer measures fuel rate and then interpolates estimated mpg.

I certainly understand your confusion, and yes, you will certainly use less gas in top gear at 1000rpm than at 1500rpm or 3000rpm. Perfectly makes sense. Here's where the other part comes in. In top gear at 1000rpm, your going Xmph. At 1500rpm, your going 1.5Xmph. Effectively, your going 50%faster. (using a gear calculator for real numbers, I believe that comes out to 46 and 70mph respectively for M6). So in a given amount of time (lets say 1hr), you've also traveled 50% further. If fuel rates were constant (which we know they are not) you'd actually gain 50%mpg. Obviously, we know it takes more fuel at higher rpm's. The question though, is where is the fuel rate curve, most complimentary to the speed curve.

Do you use 50% more fuel at 1500rpm than at 1000rpm. I doubt it, and the report that I saw (I believe it was in the ASME journal) had all this data in it.

Back to my original comment. I didn't want to turn this into a scientific debate. I just wanted everyone to know that the optimal economy speed depended on thier own cars configuration and gave a guideline for "about" where it was most efficient. Clearly GM agrees and provided gear ratios to optimize the epa tests at around that engine rpm range.

Certainly, you can't be saying that there is some optimal speed for all corvettes, reguardless of transmission type and gearing?

Let us know how you make out, I'm curious myself.

 

No, of course not, all that stuff matters.

I didn't get good numbers in my experiment, the DIC was bouncing all over the place even with the cruise control on. At 1000 rpm (cruise set at 46 mph) it was between 32-36 IIRC. At 1500 rpm it was roughly the same but I didn't get a chance to measure it for long enough to get a good number.

I might try again in a few days if I remember to.

Fascinating enough already!

 

 

Unless this is a mis-print, I'd like to buy my gas at his price!!! At 5 MPG 20 miles will eat 4 gallons (@$3.60 per = $14.40) in 20 minutes??? Maybe he meant 4 gallons, not $4.00......

 

I know this discussion is supposed to be a friendly and informative discussion. My opinion of this posters statement is that it is dirogatory, prejudicial, and anti semitic. While I'll defend and have defended his right to speak openly in a free society, I find it distasteful, and hope it does not indicate his underlying true feelings. Maybe I'm wrong and it is all in fun, but it is inappropriate.
Luck To All

 

Set my cruise at 80 and slipped it into 6th... got 32.9 from San Angelo, Texas to Las Vegas, Nevada. Got a bit less than that (around 31 as I recall) from LV to SLC, UT.

 

If you are really that concerned about what the most fuel efficient speed for a c5 is in my opinion you probably have the wrong car sitting in your garage. I spent an another $20.000 on my c5 for race car handeling and performance and when it comes out of the garage i focus on how much fun i can have with the car. This i just my opion though. Life is way to short to worry about things like fuel efficiency with a car like a corvette.

 

And remember GM cars computer usually runs about 10% higher than actual MPG. Just verify with actual miles run by actual gallons pumped in. It makes people feel better but it is not true...

 

i agree with this statement

my computer said i got 30mpg on my last tank and i really got like 28 according to fill up/miles driven

 

wow after reading this thread I wonder if there is something up with my car.....I have an A4 with 2.73 and I only get like 26-27 mpg on the interstate at 74 mph.

Any ideas on what I need to do?

 

It sounds like your car is fine. I have the performance axle ratio in my A4 (3.15, right?) and the DIC will read 30 mpg at about 60 mph and 27 or 28 mpg at 70 mph. My real mpg is probably lower than that as a few others have stated. Terrain also has a major impact on gas mileage. In my area we have small mountains and rolling hills, which means I'm almost never on a flat road.

 

must agree with that one

 

Jimmy Carter said 55mph was the best remember? He is the one who gave us the national 55 limit when there was an "Oil shortage" and gas was pushing $1.00 per gallon. I remember people saying it wouldn't go over a dollar because the pumps only would go to $0.99.9. Now they are set to go to $99.99.9.

 

Agreed, but relative numbers are believable. That is, if the DIC shows a higher number at one speed than another, then it's safe to assume that MPG really is higher at that speed, even if you don't believe the actual number itself.