3.09:1 Loading Drivetrain, or?

[DA BIG ONE]

I'm trying to better understand the pros/cons of gear changes for better highway mpg.

Someone told me because of the changes I have made (going from 4.10's to 3.42's then 33" tires) the drivetrain has to work harder like I'm towing a load, explaining that's why my egt's and iat's are higher. Anyone think this is true, or?

[rjschoolcraft]

There is some truth to it.

To understand this, consider what makes the vehicle move in the first place.

Tractive effort (or force) at the interface between the tire and the road surface is what pushes the vehicle down the road. How is this tractive effort generated? A torque applied to the wheel hub is changed to a traction force by the radius of the wheel and traction coefficient. The traction force will be F = C * (T / R) where C is the traction coefficient, T is the torque and R is the wheel radius. As you can see, the force goes down as the wheel radius goes up so for a given applied torque, you get less tractive effort at the road surface with bigger wheels and tires.

Torque at the wheel hub comes from torque at the transmission output shaft through the final drive gearing. The final drive multiplies, or steps up, the torque from the transmission output. A higher numerical ratio provides more "step-up" or multiplication than a lower numerical ratio.

As an example, assume you originally have 4.10 gears and wheels that are 25" in diameter. To run 60 mph requires a certain tractive effort to overcome rolling resistance, wind drag and other losses. That tractive effort requires a certain torque at the output of the transmission to generate it. If you change to 3.42 gears and 33" diameter tires/wheels, the torque at the transmission output must increase by a factor of 1.58 to provide the same tractive effort to go 60 mph.

So, yes, the drivetrain is loaded more when you decrease the final drive ratio and increase wheel size.

Other factors such as increased ride height and aggressive tread patterns further increase drag and require more torque yet.

[Dvldog 8793]

Howdy
My .02.....
Every engine has two optimal RPMs. (1) - for best efficiency, ie MPGs. (2)- For best power, ie most horses. If you drop your engines rpms too low then it can't generate enough power to move/keep moving your vehicle. So your engine strugles and has problems keeping up. Your Trans probably also heats up and causes problems. It also takes more power to move/keep moving those big tires and your vehicle being up in the air changes the airodynamics. I've found that my truck gets the best mpg at about 1850 rpm and around 65 mph. This would be in drive(3rd) with my Gearvendor engaged. Hope this helps...
L8r
Conley

[DA BIG ONE]

Thanks guys!

Have a better understanding now of what not to do.

[moondoggie]

Good Day!

My 95 pickup (extended cab long box, 4L80-E, the rest below) has 4.10 gears, Gear Vendors Aux OD, & 4% oversize tires. This gives ~ 3.08 overall ratio. This give ~ 40 mph/1000 rpm, or 1500 rpm @ 60 mph.

My best guess, based on having records of every tank of fuel I've run through this truck, is mpg increased ~ 2 mpg with the Gear Vendors.

I have no idea what grade this rig will pull without downshifting. It's pretty flat here, but even here there's some rolling hills. At 65 mph or above in double overdrive (tranny in OD, Gear Vendors engaged), my truck will hold a pretty good grade without downshifting. The only difficulty it has is if the cruise gets behind the hill; adding a little footfeed just before the hill gets the turbo going & up I go without a downshift.

I've pulled our US Cargo SM625TA2 trailer (looks like the US Cargo SM727TA2 (http://www.uscargo.com/uscargo/products/enclosed/snowmate.asp) [Click in colored text] except it's 25' long instead of 27') in double overdrive at 65-70 mph & total trailer weight ~ 4000#. I've gotta be a little more careful to make sure to add footfeed before the hill; I don't think it'd pull much of a grade.

Blessings!

[DA BIG ONE]

Originally posted by moondoggie:
Good Day!

My 95 pickup (extended cab long box, 4L80-E, the rest below) has 4.10 gears, Gear Vendors Aux OD, & 4% oversize tires. This gives ~ 3.08 overall ratio. This give ~ 40 mph/1000 rpm, or 1500 rpm @ 60 mph.

My best guess, based on having records of every tank of fuel I've run through this truck, is mpg increased ~ 2 mpg with the Gear Vendors.

I have no idea what grade this rig will pull without downshifting. It's pretty flat here, but even here there's some rolling hills. At 65 mph or above in double overdrive (tranny in OD, Gear Vendors engaged), my truck will hold a pretty good grade without downshifting. The only difficulty it has is if the cruise gets behind the hill; adding a little footfeed just before the hill gets the turbo going & up I go without a downshift.

I've pulled our US Cargo SM625TA2 trailer (looks like the US Cargo SM727TA2 (http://www.uscargo.com/uscargo/products/enclosed/snowmate.asp) [Click in colored text] except it's 25' long instead of 27') in double overdrive at 65-70 mph & total trailer weight ~ 4000#. I've gotta be a little more careful to make sure to add footfeed before the hill; I don't think it'd pull much of a grade.

Blessings! Surely, you are better off w/gearvenders splitting gears than I w/straightup 4.L80e. My AutoTrac t-case/ABS (NP246) pervents me for usung gearvenders. It's the getting up to speed that is an issue, around 65/70 everything is nice and sweet w/o gears searching.

It appears my whole problem w/excessive iat's/egt's is the combo I'm using. Changed back to the origional fitment tires to see if it made a difference in temp's and it does.

Drove across country several times since installing the 3.42 gear sets & 245/74/16 tires, had no issues on all types of terrain loaded and towing 2,000 lbs (2 motorcycles).

I'll drive somemore, and see about a larger exhaust to help cool down things when at WOT.

[moondoggie]

Good Day!

[i]

[DA BIG ONE]

[quote]Originally posted by moondoggie:
[b] Good Day!

[i]