Have had the car almost a week..learn something new about it each day…..like Corvettes limited to number of remote starts per day. Btw, just LOVE this car.

Good & bad on MPG: So I noticed how impressive—awesome— my highway MPG was..even at speeds above where MPG typically peaks. Most cars I’ve driven optimize in low/mid 60’s…looks to me like the sweet spot is around 72.

The bad news is..l was disappointed at the crappy mileage it gets driving conservatively even coasting around town. Sooo..I noticed the car switches out of 4 cylinder mode INTO 8 cylinder mode—as your speeds varies higher OR LOWER….like when your speed DROPS from 35 mph to 10 mph to stop. Gee, I REALLY needed those 8 cylinders to kick In for that powerful ride from 35 mph to 10 mph. It DOES.

So Mr. Engineering idiot me suggests GM change: DCT, when operating in 4 cylinder mode…STAYS in 4 cylinder mode when speed declines. It returns to
8 cylinder mode when..it’s needed to accelerate at all.

Might this be the algo change reportedly coming in MY 2022. GM says a change is coming to increase MPG. This would seem like common sense unless I’m missing something or doing something wrong which is highly likely

 

🤷‍♂️

If the milage is your chief concern.......

 

If decelerating what does your mileage day, mine often says 99. The computer can cut fuel entirely when slowing in gear. So V4 or V8 doesn’t matter if no fuel is delivered.

 

I knew some one would post that. It’s not my “chief concern”. If interior comfort was a chief concern you’d by a sofa, right?

I totally get why you might not want DCT to EVER deactivate 4 cylinders. But if we’re going to play that game…why re-activate those cylinders when the car slows down?

 

Good question…I just noticed the MPG on “current trip” decelerates in anything less than smooth consistent driving and there’s no benefit to coasting imo.
I didn’t know the fuel use can be “cut off”….unless there’s a lag I noticed a simultaneous drop in “current trip” mpg. If you drive the car consistently at ANY speed it gets great MPG. Any variation at all plus/minus eats into it considerably. Much MORE so than any car I’ve driven in 40 years.

Again this is NOT a priority for me, it’s just the latest in a myriad of unique things I’ve learned each day of usage..most of which is pleasantly surprising.

 

Yep your right foot controls when the car goes into V4 Mode and your gas mileage. GM probably determined that when your right foot is demanding more hp to go up a slight incline that more throttle in V4 Mode was poorer mpg that switching to V8. Having those "resting cylinder" piston rings, bearings etc doing nothing but creating friction is not always the most efficient drive mode. V4 mode in a 6.2 Liter engine isn't magic! It's not a low cid, efficient 4 cylinder

If you want to get max mpg drive like you have a raw egg between your foot and the gas pedal. (A trick from the old Mobil Economy Run that operated from 1936 to 1968- wonder why it stopped?) Plus like I did when prices were sky high in the early 1980s and we could only buy gas every other day based on the last number on your license plate. The 15 gallon tank on CJ5 V8 Jeep could my not make it to LaGuardia from CT (where I lived and where Corporate Office was located) and back on 1 tank of gas! Since I traveled about every week bought a 1.4 liter Dodge Colt so I didn't have to stay out of town to come home on a day I could buy gas. (Looks like we're heading there again!) Recall making a bet that I could get 50 mph! Won, but that required driving using another Mobil Economy Run trick I had on my 1st car a throttle cable! Although the Colt didn't have one or cruise control, needed to let it slow down going up hills keeping the throttle fixed. Cruise control is NOT the best for gas mileage. Want to keep it in V4 don't use more throttle when going up a slight incline- let it slow down. Foolish way to drive a C8 but will help!

Now I have had my car a year! Never track mpg when I'm driving. Not what I bought a Corvette to do. In fact do keep track of gas mileage on an Excel spread sheet when I buy gas. I often drive in Z Mode Power set to Track. Never goes in V4 and will only shift to 6th over 73 mph. Never in 7th or 8th.

It drives like a Corvette should in my rural area and they way I drove my two manual trans C7's. See this video: https://www.youtube.com/watch?v=RI9M...annel=jrock702

For fun to see how much mpg I was losing I used MY MODE set to Sport for a tank full where it uses all gears and goes into V4. For that tankful I got 2 mpg better mileage. Now that is zero Interstate driving just my normal back roads 20 miles into town and 20 back. BUT not driving like an old man (heck I'm only 78!) For 2 to 3 mpg I'll drive where the C8 does its best as a sports car, Z MODE Power set to Track!

 

ALL modern manual transmission cars cut the fuel completely when decelerating in gear IF you take your foot completely off the gas. If you just back off the throttle a bit it will keep supplying fuel. GM has years of experience with the algorithm for AFM system, and it exists solely to improve MPG. If it switches to V8 mode while decelerating (and you're correct, it does), I think there's a good reason for it.

 

I've find that the shift from V4 back to V8 occurs for the 'Right" reasons, "ENGINE BRAKING" for the most part,.as well as not to LUG it, with that 3600+ weight it trying to move.
Easy way to see that, is on a hilly 40-50 MPH road, with Cruise on. Part of that is when engine is braking going down hill, V8 engine braking then V4 in the Flat, then V8 up the next hill. Best Gas mile, Best for not Lugging Engine, and Dam smooth doing it.
Without the V4/V8 showing on the Hud I wouldn't know it. as I can't Butt feel the Swaps....

 

Very interesting. So what your saying is..if I’m cruising along a side road at, say 45 mph with 4 activated cylinders…then I get to a hill and coast down (we both agree it switches to 8 cylinders ironically) that it will not use gas with the 8 cylinders activated as long as I don’t touch the gas. I’m technically getting, briefly, unlimited MPG with 8 “activated” cylinders but no gas? In my case… I must have just been gently depressing the gas pedal when I THOUGHT I was literally “cruising” during a recent trip when I monitored my driving numbers.

Thanks SO much for your help on this. And thanks for the many other things I’ve learned from you on this board. Your generosity is much appreciated.It will be interesting to see how GM tweaks the algorithms for MY 2022. They’ve said it will result in better MPG. My assumption is more use of 4 cylinders but perhaps there’s a magical way to get more out of “8” in certain situations….

 

Makes perfect sense, thanks. The reality of a powerful machine rolling down a hill. A little different than a bike…or a Corolla lol.

 

The DCT doesn't shut down 4 cylinders. It's the transmission, not the fuel and spark management. Do some research and learn a few things before making stupid comments.

 

And how is the fuel and spark management directed? Where does it take it’s cue?
And apparently DSO is also an idiot like ME: “ALL modern manual transmission cars cut the fuel completely when decelerating in gear”,

Please enlighten Mr. condescending jerk. TIA.

 

You have a brand new Corvette that can do dozens off exhilarating things and you are monitoring gas mileage and algorithms !!! What did you call yourself " idiot me ". naled it !

 

I drove it on highway and did pretty much everything a new owner would do including a fantastic 75 mile drive from Nashua. It was the mpg part that got my attention raising questions. It was one of MANY things I observed about the car.

You want me to be a FANBOY to pass your litmus test? Ok, it flies, iit’s refined, its interior is way beyond Corvette standards, it takes off like a rocket. It’s easily best sports car ever built in USA. And a better bargain.

There. Do you feel better? Is that the context you need for minor issues?

 

MPG around town is severely impacted by sitting at stop lights, stopping at stop signs, yield signs, people turning, etc.

 

Absolutely correct. As is the case with every gas powered vehicle on the road. Agree. C8, for reasons explained in previous posts on this thread…perhaps occasionally MORE so than typical. If you cut through the insults and condescending comments of a few posts here —several people have shared very generous and quite helpful responses to this issue. Learned a lot. Sometimes even more about human nature than C8 around here. But it’s worth it.

 

You're turned around on what is controlling what. The DCT gets it's orders from the ECM and that gets its info from throttle position sensor, vehicle speed sensors, engine temperature, and about two dozen other inputs. The DCT just takes orders and does what it's told. What was left out of your statement was, it isn't limited to modern manual transmission cars, but automatics too. Fuel and spark are OTHER things like a DCT that get marching orders from the engine controller.

 

The AFM is using the computer to manage the coils (remember the C-8 engines has one coil per cylinder) and the fuel flow to the unused cylinders is shut down by the computer too. the inactive cylinders have modified valves also. the crankshaft continues to rotate and all the pistons still travel up and down, the vales open and close but no combustion occurs in the deactivated cylinders.

 

I don't believe that's correct, the AFM lifters are split and when called for AFM activation they don't open those valves at all and those cylinders work as air springs.

 

If you are concerned about gas mileage, may I suggest buying a Chevrolet Cruze instead? This is a sports car, or a somewhat entry level super car. You gotta measure it in smiles per gallon and if you can’t afford gas you can’t afford the car.