First let me say that I am not an engineer, but I did stay at a Holiday Inn one night last year. So here is my question for you engineers out there:
My truck (GMC, 6.0L gas) in 5th gear at 2000 rpm runs 55 mph. When towing, a slight up grade will cause it to downshift to 4th and run at 2700 rpm at 55 mph. On the level it stays in 4th and only
shifts to 5th at the next slight downgrade. It seems to me that the power required to run 55mph is almost the same in 4th or 5th. So....which is best for the engine, and which will give the best mileage? 4th or 5th?
5th gear reduces torque multiplication at the transmission because of the higher overdrive gear, which is why the trans down shifts on hills to keep the truck going 55 mph. The engine makes more torque and horsepower at 2700 rpm than it does at 2000 rpm.
Your best mileage will occur in the gear that causes you to be able to run at part throttle at a comfortable rpm. Running in a higher gear at a lower rpm is fine if you can do it at part throttle (as in on the flats), but running that higher gear and lower rpm on hills with your foot on the floor will waste gas and is not great for the engine or transmission. Let it downshift for the hills and run in OD on the flats and you will be OK.
I am not an engineer, but I did study engineering in college years ago.
What year is your truck? GM only makes one 5 speed auto, and it's an Allison. If you have a 6.0L gas engine, it's either a 6 speed auto, or a 4 speed in all but the newest (08) trucks.
That said, onto your question. Likely, if it will not run in top gear on a level surface, manually shifting it down one gear would be best. Constant shifting (usually referred to as "hunting for the right gear") causes wear and heat build up in a transmission.
As far as mpg goes, best to try both and check the mileage.
Sounds like you have a 2500HD with 4.10 and the 6 speed
At 55 mph you will have a hard time making the 6.0 pull at 2000 rpm. but with very sensitive foot throttle input it should have the power on the flats to use 5th. At 60 to 65 MPH and 2200 to 2400 it will be a little easier.
I find with my truck I have to back right out of the throttle to make it shift and be very careful to "slowly" add throttle input to keep it in the higher gear. Your truck does have a different tranny but should work similar. (Tow Haul on)
You have to expect that the trans will not let you speed up as fast while towing in the OD gear.
It will also depend somewhat on how much the TT weighs and the headwinds of the day.
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J Gerber wrote: It seems to me that the power required to run 55mph is almost the same in 4th or 5th. So....which is best for the engine, and which will give the best mileage? 4th or 5th?
The power required to run 55mph has nothing to do with the gear you're running.
Power has two components: RPM and torque. The gear you're in defines how the engine power gets distributed between those two components. Higher gears give you less rear-wheel torque with lower engine RPMs. Lower gears give you more rear-wheel torque with higher engine RPMs.
This is where it gets interesting: since power is not constant across the RPM band, the power output of the engine will vary as RPMs change. For gasoline engines, the power output will increase as RPMs go up (to a point, usually around 5000). This means that lower gears not only give you the advantage of torque multiplication to the rear wheels, they also (indirectly) give you the advantage of more engine power because of the rise in RPMs.
Which is best? Find the highest gear you can hold without excessive downshifting and use that. As for mileage, try both ways and compare.
Of course, I'm one of the little guys here as far as hauling a TT. I did an experiment last year. We go to a CG in Virginia frequently and have a 50 mile stretch where I am able to use overdrive with a fuel station at each end. I tried this 4 times. Running in OD at 55 mph 1900 rpm's got 10 mpg. Running same stretch not using OD at 55 mph 2300 rpm's got 10.6 mpg. On overdrive, I tried something, I held the gas pedal at the same spot and downshifted out of overdrive and speed went up to 60 mph. I was wasting fuel. A vacuum gauge would have verified this. I am sure my tranny prefers not being in overdrive also. If I try to run 60+ mph, forget the overdrive at all and the mileage gets poor regardless what gear I am in (wind resistance) Last winter we traveled 520 miles going avg. 52 mph not using OD and managed 12.8 mpg.
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weathershak wrote: Last winter we traveled 520 miles going avg. 52 mph not using OD and managed 12.8 mpg.
I couldn't travel 520 miles at 52 mph if I was paid.
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