2nd Gen Non-Engine/Transmission Explain this one science/math guys

[Walker]

I just put 315's on my stocks and definitely notice more "drag" (resistance) from the extra weight. I've got 3. 55's and automatically thought, "Well, I've got to get the 4. 10's now. "



Then I started thinking: We're only talking about a 10%, 200 rpm difference at cruising speed (lower), and since the tq. /hp curves on the cummins is pretty flat at highway speed (and strong enough to pull the extra wt), what would be the noticeable difference of jumping to 4. 10's anyway?



I know from a stand still it's gonna help, but the reason most "other" vehicles "gear down" is to get the rpms up to get back into the "sweet spot" of higher hp numbers, right?



If we're running a pretty flat curve, what's the difference (or advantage if any) of getting the extra 200 rpms back?



I lost about 1mpg (first tank) and was wanting to get back to the original 3. 54 ratio, which 4. 10's with the 35's would give me, but don't see how this would help. Any ideas?



Ps. I'm just trying to convince or prove to myself that I don't need the 4. 10's and save a few bills.

 

[LSMITH]

I have 3. 5 gears and dropped from 285/75/16's to 235/85/16's and LOST 1. 5 MPG. Give your setup some time and make sure you actually lost economy. I should have gained 1+ MPG but I didn't, who knows why. I am going taller next time for sure.

 

[Little Bill]

Don't forget to factor in the odometer correction. If your speedo is off 10% with taller tires then your odometer reading will be understated by 10%.

 

[Groover]

The change to 315's is about 10% increase. Dropping from 3. 54 to 4. 10 is more like 15. 8% reduction in gearing or around 300+ rpm's.



My story - everybody (friends, dealer etc) said you are crazy for getting 4. 10 your mileage will suffer reduced roadspeed etc.



My answer was I am going to tow with it and eventually put bigger tires so I bought with the 4. 10 ratio. Now that I have 315's on it I cannot imagine having 3. 54 ratio as I tow pretty heavy and sometimes wish I had the stock 245's back on just for towing.



If you don't tow much then 3. 54 with 315's will be fine ( my definition of not towing much is less than 10,000 # gtw once or twice a month or so).



Towing ability will suffer but if you can pull the load satisfactorily stay with the gears you have you will never recover the cost of gear replacement in fuel.

 

[EricBu12]

Any of you guys know the Circumfrence of a 265/75 16 tire? I bought the tru-speed claibrator and one way to calibrate it is by inches on the tire. The new tire is about 33" (285/75 16) Is the 265's about 31"? or 32?

 

[jponder]

I cant see the 4:10 myself. The only advantage would be to get the load moving. If 5th is too high for you use 4th etc. You can always down **** but you cant get away from 4:10 when you are empty. I have been rolling down the highway at 40K and 4th was all I wanted. I could have went to fifth but I wanted rpms because I was so heavy. You can always downshift. I dont think the 4:10 serves a purpose if you have the Manual transmission. If you want higher rpms it easy to do

 

[Hohn]

There is a happy medium when it comes to tire size/ gear combos. When tires get tiny, effective gearing gets (numerically) higher- and your economy goes South along with it.



With very big tires, you have a lot more rolling resistance and weight-- both cause economy to go down.



If you change gears to get your RPMs corrected for larger tires, you will still get worse mileage, b/c of the higher weight and rolling resistance.



The highest mileage combo would be taller gears and smaller tires. Which, by the way, is exactly the opposite of what most people want because we need the higher load capacity of the bigger tires.



I also can't see the need for 4. 10s with a manual transmission. 1st is plenty low.



Diesels are different than gassers in some respects. Gassers have mpg influenced more by rpm. Not so in a diesel. A diesel's mpg is determined primarily byt the AVERAGE load on the engine over a distance. Basically, the lightest load you can place on the engine over the longest distance. You get the best mileage in 6th (empty) because the increase in load on the engine is more than offset by the gain in speed (distance).



The flat torque curve of the engine is the main reason that RPM doesn't have much effect on fuel economy within a certain range.



ON the highway, you best economy will come at the lowest speed that allows using the tallest gear within the rpm range of the engine. on my eth/dee that means about 55mph in 6th.



Watch your boost gauge. If you run fast enough empty to where you see any boost at all, then you are going faster than the optimum fuel economy rpm range. You can usually go about 55-62 even uphill without generating any boost.



If you are at 10psi all the time on the hwy, your mpg won't be great. If you are sustaining 15psi of boost or more, you are SERIOUSLY burning a lot of fuel, and your mpg will be bad.



HOHN

 

[Groover]

No need for 4.10?

Hohn and Jponder "I can't see 4. 10"



Guy's, First I am not trying to start a 4. 10 vs 3. 54 war here.



For a stock truck empty 3. 54 is best.

For a stock truck towing moderately 3. 54 is best.



If you buy a truck knowing that you are going to ...

A. Install larger tires.

B. Tow in excess of 10,000# trailer

C. Tow in hills

D. Tow more than 1 day a week (tow a lot)



If you can answer yes to all these there is a reason for 4. 10



4. 10 are not for everybody. I Personally would not run anything but 4. 10 as I have 35" tires and tow heavy. My effective gear ratio is like 3. 70 with the tire change.



"In my observation" 3. 54 equipped trucks pulling the same loads as my 4. 10 (when I had stock tires) had to downshift on even the smallest hills. I don't like the constant shifting myself.



Just my opinion guys but there are those of us out here that really do prefer 4. 10



I have heard the argument that if you bomb it the extra horsepower will make up for the gearing and that is partially true. You can grunt through a hill at 1400 rpms with shower head injectors if the pyro will let you but it is not good for the engine.



Bottom line when towing heavy I would rather be at 2200-2500 RPM's than under 2000. Contrary to belief the engine is happier when under load if it is well above 2000.

 

[DaveN]

Hohn's got it right

Bigger tires weigh more and provide more rolling resistance. If you don't think tire size makes that much of a difference, jack up your truck and spin the front tire. Then try to spin your kid's bicycle tire. Then try to accelerate your kids bicycle tire to 60 mph, and then your truck tire. One takes alot more energy to do because it has a larger mass moment of inertia, meaning it takes more energy to spin it, and more energy to move it forward.



The other thing to remember is that the taller tire puts more load on the drivetrain due to teh reduced gear ratio. This in turns requires you to give it more fuel to get it moving at the rate that you are accustomed to. Hence more fuel consumption, etc.



THEORETICALLY - if you had two trucks going at a constant rate of speed down the highway, the only difference being the gear ratio, they should consume the same amount of fuel. The taller gear might give you a slight advantage due to engine RPM and fuel calibration curves, etc. BUT, according to physics, they both require the same amount of fuel to move down the road. The difference is when you are trying to accelerate. One has more mechanical advantage than the other.

 

[Brand-X]

Originally posted by EricBu12

Any of you guys know the Circumfrence of a 265/75 16 tire? I bought the tru-speed claibrator and one way to calibrate it is by inches on the tire. The new tire is about 33" (285/75 16) Is the 265's about 31"? or 32?

This should give you what your looking for ... george http://www.miata.net/garage/tirecalc.html

 

[Oil Burner]

regarding the original post, the torque curves are fairly flat but the horsepower curve fluctuates with rpm. just a thought.

-Jason

 

[Big White Beast]

My experience swapping gears.......

Was almost a 600 RPM difference between 4:10's and 3:54's. Took me to the other end of the scale. I went with 3:73's and my truck sits in the sweet spot now.



With the utility bed changing tire sizes wasn't a viable option. And after buying 6 new tires it was also cheaper to swap gears.



One thing to keep in mind is that going from a 3 series gear to a 4 series you'll have to swap the carrier also. Going from 3:54 to 3:73 you only have to buy the gears.



Garrett



EDIT: I forgot to mention, between all 3 gears the mileage didn't change one bit. It still takes a lot of fuel to move my extra large brick.

 

[Walker]

Yeah, I did think the hp curves would vary a bit at different rpms and cause a little less power advantage; but with my mods and only a 2-300 rpm dif. it still didn't seem like the 4. 10 jump would be worth it.



But, I see now that the "mechanical advantage" would be a plus in accelerating (and of course from a stop), especially when hauling.



Even though I have the ponies to overcome the rolling resistance, it'd be nice to have to rely less on power and let the work of the gears do more to get (keep) things moving.



Look like more $$$ for O'l Gus.

 

[Walker]

Just read your post BWB. Good pt. about going from the 3's to 4's.



What size tires did you go up to? And how much for "just the gears" in the 3. 73's?



And why was yours a 600 rpm difference? I thought math was math.

 

[Big White Beast]

Walker

I stayed with the stock tire size, just needed new ones. I went with the Michelin XP's LT235/85. All six with mounting, balancing and road hazard were close to $1300.



The swap from 3:54 to 3:73 only cost $800 including gears. (2WD)



As far as the RPM's I never really tracked exactly how much of a jump but my guess of 300 RPM per gear change is real close. 4:10, 3:73 and 3:54.



Garrett

 

[nwilliams98]

Drag increases the higher off the road your truck is too. 315's are higher.

 

[Hohn]

My opposition to 4. 10 needs clarification. Notice I said that it is only with the NV5600 manual trans. I should also have mentioned "on an otherwise stock truck" as well. The 600 rpm difference between the two stock ratios is close (kinda) to the difference between 5th and 6th gears:



3. 54*1 (5th gear direct)=3. 54

4. 10*. 73 (6th OD) =2. 993



Notice that you are geared lower in 5th w/ the 3. 54 than you are with the 4. 10 in OD.



I would be curious as to whether towing in 5th w/ the 3. 54 was easier on the trans than towing in OD w/ the 4. 10s. It would seem to be true because the 5th gear combination would be geared lower overall, and be using a stronger gear in the transmission.



OF course, the ultimate combo would be something like a 4. 30 or 4. 56 with an aux transmission OR TWO.



Groover, I agree with every word of your post. There are definitely good reasons for the 4. 10s. I also think that the higher RPM would be very good. I am amazed how many 24V guys are afraid to wind up their engines. It doesn't make any realistic difference in engine longevity (probably helps it) compared to lower rpm. When I ran a whole tank with the rpm over 2K, the engine REALLY smoothed out and seemed a lot happier afterward (this, with 8K on the engine).



** for the record, drag does NOT increase simply by having a higher truck. Drag is a product of the shape of your truck and its frontal area (shape of silhouette when viewed from the front)-- neither of which changes significantly when you put on bigger tires. The decrease in fuel economy with larger tires is a result of weight and rolling resistance only.



BWB makes me glad I don't have training wheels!! $1300 for tires! I am not surprised that your economy didn't change-- your are doing the same work, moving the same load from point A to point B.



Hope this is helpful in some way.



HOHN

 

[nwilliams98]

Hohn

I have to disagree with you on the truck height effecting drag point. I've done tons of simulations on light trucks using CFD and have found that changes in ride height have a significant impact. The stagnation points on the axles are exacerbated with ride height increases as well as there being a larger frontal section.

Nate

 

[Oil Burner]

Nate,

I agree with what you say. You sound like a mechanical engineer also. Anyway, the tests I have seen done in wind an water tunnels suggest the same findings.

-Jason

 

[Chief USA]

I mounted 285-75R 16 tires on my truck about 7 months ago. To be honest I hardly noticed a difference from the stock 265-75R 16's. When I calculate the milage correction due to the larger tires, I am still getting between 21 and 23 mpg. That seems to be just a little bit better than with the stock tires. Maybe there is some difference in power transfer efficiency between the NV5600 and the lock up torque converter. If you went to a larger size tire and did not make system speed sensor corrections, maybe this maybe throwing off torque converter lock up and trans. shift points. Just my speculation.