I agree.

Also, the manual says if above 49mph you can drive in 6th gear. I always shift into 6th whenever I get to the speed I want to maintain on the highway, not shift into 6th at 49mph than accelerate to 65 mph or 75, etc.

 

i have 3.42's and 6spd.... (no ****).... i usually cruise at 70+ in sixth, anything below at 5th... but I do live in CA and traffic blows, so you mihgt have a diff. easier, less straining cruisin speed

 

Just what i do

 

I throw it into 6th when i am at or above 70mph. Below that i just keep it in 5th, at 70 i feel like the car runs real smooth in 6th.
-Joel

 

That's it in a nutshell. 6th is for maintaining speed economically, not accelerating.

 

my 6th gear bucks too much at that speed...

 

The thumping noise is the driveshaft vibrating and hopping around on the joints. It's no big deal, that just means downshift. Not enough torque is being produced to keep the shaft at a constant velocity. As for which speed down the highway, I have a '97 LT1 with the M6 with 3.42 gearing. Warm idle speed for the LT1 is 1100 RPM. Anything lower than that is bad for the motor, because you'll coat the injectors. For me, going down the road at 46 MPH in 6th is equivalent to 1100 RPM. The car pulls just fine on the highway at that speed, so naturally, from 55-62 MPH would be 6th gear. I CAN go into 5th, but the RPM's would jump to about 1700, and that's too high for cruising. I usually go through town roads in 5th at speeds of around 40-45 MPH, and that's perfectly fine. Anything higher than 50 MPH on flat roads, I'll pop it in 6th without bogging the motor. You have to remember, it's not strictly your RPM speeds that differentiate good and bad MPG. It's also how much your throttle plate is open. The more it's open, the more fuel is injected in. If you find yourself stepping on it more in 6th, rather than 5th, to maintain XX MPH, then downshifting would be more efficient. It's a fine line on deciding what is the best action for good MPG, and if you were to graph it, it would be a fine-point porabola. It's just takes a good feel for your car, that's all. As I said before, on the highway, anything faster than 46 MPG would be 6th gear range (unless you see a ricer, of course Then, 3rd gear does quite nicely haha).

 

I always use 5th gear from 45 mph to 60 mph, then I use 6th after 60. The car runs fine in 6th even at lower speeds, but I think it isn't good to tach under 1,500 at freeway speeds, I still get 28 miles per gallon from a 98 z-28 ss.

 

Hi Folks
6th gear is more than good enough for 55mph. I go into 6th at little over 45 mph - if I really want to move out, I drop down into lower gears but normally 6th after 45 mph.
Bill

 

That's not really true, at least not until you're stepping on it hard enough to go into open loop and power enrichment. Short of that, even with the throttle open wider, the engine simply doesn't use as much air (and therefore fuel) at lower RPM to maintain the same speed.

It's easy to check. Just watch your mass airflow with a scanner while maintaining speed with different gears.

I expected to find exactly what you expect, and I hoped to find it was most economical in a range that felt good, but it just doesn't work that way. FWIW, my 2001 LS1 engine is stock except for the catback and lid.

In fact, part6 of the reason the engine isn't very responsive to the throttle at low RPM is that openning the throttle more simply doesn't increase airflow much.

 

The higher gear is more efficient primarily because the pumping losses are lower. If you take any physics courses this is the first thing you learn about engine efficiency. Its easier to see on the graph than describe. In the higher gear your throttle is open more so the engine ingests higher pressure air (less vacuum), since the piston compresses by a fixed ratio the final pressure at TDC is much higher. The difference between the pressure of the compressed air and the air sucked in determines the efficiency. If the pressure at 4 is higher then compression at 1 is higher and the rise to 2 is much more. The shaded area is the energy created by one cycle. Not seen is the energy lost to friction and windage, creating more energy in a cycle and running the engine more slowly means the friction losses are proportionately less because the engine is operating more slowly. All of this works up until the point that PE mode is triggered and/or detonation occurs.

 

i'm normally going more than 55-62 on the highway, but i think my car boggs alot more in 6th at a lower speed, my car runs perfect @90 mph at 21-2200 rpm.

 

From San Antonio to Houston on IH-10 I averaged 29MPG in sixth gear. I was goin about 75MPH the whole way. But I usually shift to 6th when I go over 60MPH.

 

Join Date: Oct 2005
Location: New Jersey
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I drive mine in 5th because i have that tricky thumping noise i cant figure out at 1500rpm



The thumping noise is the driveshaft vibrating and hopping around on the joints. It's no big deal, that just means downshift. Not enough torque is being produced to keep the shaft at a constant velocity. As for which speed down the highway, I have a '97 LT1 with the M6 with 3.42 gearing. Warm idle speed for the LT1 is 1100 RPM. Anything lower than that is bad for the motor, because you'll coat the injectors. For me, going down the road at 46 MPH in 6th is equivalent to 1100 RPM. The car pulls just fine on the highway at that speed, so naturally, from 55-62 MPH would be 6th gear. I CAN go into 5th, but the RPM's would jump to about 1700, and that's too high for cruising. I usually go through town roads in 5th at speeds of around 40-45 MPH, and that's perfectly fine. Anything higher than 50 MPH on flat roads, I'll pop it in 6th without bogging the motor. You have to remember, it's not strictly your RPM speeds that differentiate good and bad MPG. It's also how much your throttle plate is open. The more it's open, the more fuel is injected in. If you find yourself stepping on it more in 6th, rather than 5th, to maintain XX MPH, then downshifting would be more efficient. It's a fine line on deciding what is the best action for good MPG, and if you were to graph it, it would be a fine-point porabola. It's just takes a good feel for your car, that's all. As I said before, on the highway, anything faster than 46 MPG would be 6th gear range (unless you see a ricer, of course Then, 3rd gear does quite nicely haha).
__________________
Almost everything you said is entirely wrong. The driveshaft speed is the same at any given mph no matter what gear you are in. Also, your engine runs more efficiently at larger throttle openings because the throttle blade is a flow restriction in the intake at any point less than 100% open. As for the comment about coating the injectors, what the **** does that mean? By the way, even though there is nothing in your post that should reference such a word, the correct spelling is parabola.

 

This is basically not correct. Stepping on it more in 6th than in 5th is precisely how you achieve your higher efficiency. Of course the injector pulse width goes up, you are firing them at a slower rate so each pulse is bigger to put out the same power. That is exactly what you want.

The second big issue is bogging, every has their own "feel" for what that is. The OEM guys tend to measure it as how variable the combustion becomes. Damage to the engine comes after the point where most people just don't like the feel and downshift. At the point where factory says you can shift into 6th it feels a bit rough since the combustion variability is quite high but unless you go crazy you will not harm the engine.

 

Ditto with 4.10's. She's not a hwy cruiser anymore... But I get a lot fewer tickets on the interstate

[EDIT] With stock gears I shifted into 6th at 65mph and used to be able to cruise at 85 mph doing just under 2k rpm and get 30 mpg; sweet! I'm twice as quick with 4.10's, but when I lead-foot it around I'm lucky to get 250 miles out of a tank, sometimes only 200.

I put it in 5th at 35 and 6th at 55, plus I skip a lot of gears now.
It'll still do 110+ in 4th though , even with a stock 6K rev limit. 5th and 6th both pull hard now, whereas with stock gears they didn't have much torque.

 

Yea...I don't think i've ever gone into 6th actually...5th is nice...But half the time im scared to shift into 5th for fear of going into reverse I've only had the car about a month and its my first manual..my other ls1s were autos...

 

There is a lockout solenoid so it is impossible to shift into reverse.

 

...what Grimes said

...plus if you put taller gears in your rear, the M6 will be more forgiving. The hardest part is getting used to Skip-Shift. Eventually you learn where to shift to avoid it, but with 4.10's it never engages now

 

55 to 60...2nd gear...

drive it like you stole it