[Skizzo]

So for a while I was really curious about swapping to a T56 with my 3.5 5 cylinder. I knew about issues with possible driveshaft length, bellhousing, and that but was curious with those problems solved why not?

Then someone informed me of the gearing differences between our transmission and the T56 and I realized that it was a huge difference and would make the truck a dog with the i5.

Until it occurred to me: what is the difference between my 3.42 gear AR5 and a 4.10 gear T56? So I did the math and while first and second are going to be a little longer gears and slightly slower...I think it could still be a very viable swap. Here are the numbers, please provide opinions:

3.75 2.26 1.37 1.00 0.73 3.67
x3.42
____________________________________________
12.83 7.73 4.69 3.42 2.50 12.55

2.66 1.78 1.30 1.00 0.74 0.50 2.90
x4.10
____________________________________________________
10.96 7.298 5.33 4.10 3.03 2.05 11.89

Final gear comparison:
80mph 27" tire 2500rpm 2.5 (AR5 3.42s)
80mph 27" tire ????rpm 2.05 (T56 4.10s)

2.05/x = 2.5/2500 --> 2500(2.05) = 2.5x --> x= 100(2.05) --> x=2050 82% of engine rpm in final gear

First gear comparison:
10.96/y = 12.83/2500 --> 2500(10.96) = 12.83y --> y= 2500(10.96)/12.83 --> y=~2136rpm or about 85% of engine rpm in first


So assuming all issues with fitment/connection are solved, why not t56 swap with 4.10s?


Or am I going about comparing these transmissions all wrong?

 

[1BADI5]

I have wondered the same.

With a 6 spd the mileage you could get on the highway would be insane.....35mpgs easy. Yeah youll be slow, but its a give and take.

 

[Skizzo]

I have wondered the same.

With a 6 spd the mileage you could get on the highway would be insane.....35mpgs easy. Yeah youll be slow, but its a give and take.

I used to always think about how it would be nice to have a 6th gear for the highway. But I also began thinking about how nice it would be to have closer ratios...thus this

 

[1BADI5]

With the I5 though, 4,5,6 gears would pretty much all be over drives.

 

[Skizzo]

With the I5 though, 4,5,6 gears would pretty much all be over drives.

4th is the same between transmissions but t56 with 4.10s is almost the same as AR5 3rd with 3.42s so i don't think it would be too bad... but yeah

 

[FixxxerKH202]

I think it would be a worthwhile swap withthe right rear ratio to suit it. You probably wont use 6th too much but its much more reliable with tons of aftermarket support.

Sent from AutoGuide.com Free App

 

[bk2life]

id go to a at least a 4:56 gearing

 

[Karlen]

You're not going to get 35mpg. You're going to be lugging that motor and it's still in a brick.

 

[Skizzo]

id go to a at least a 4:56 gearing

bk2life: 4.56 gears would be perfect with this combo actually
Is that even an option for our trucks without swapping axles? (Then again if you are swapping to this transmission...what is swapping axles lol)

Karlen: I don't know about 35mpg but I get about 23/24 right now in my truck at 80 so I could see 27 or so depending on how much it lugs or not.

 

[gway]

4.56 isn't available in our diffs.

 

[1BADI5]

Well I can get 30 on the highway driving like an old man.........I have other members that can validate this.

Yeah i have 3.42s right now, but have been considering 3.73s for sometime now.

 

[zomby woof]

You're not going to get 35mpg. You're going to be lugging that motor and it's still in a brick.

This is true.

Don't equate lower RPM with higher fuel economy. It almost never works like that, and definitely won't in this truck with that motor. Load is far more important than RPM.

 

[1BADI5]

This is true.

Don't equate lower RPM with higher fuel economy. It almost never works like that, and definitely won't in this truck with that motor. Load is far more important than RPM.

Ok :drinking28:

 

[zomby woof]

We've done several hundred thousand km's of testing to prove that. It's nothing new, but a lot of people, including about 75% of my customers, don't want to believe it. To me it's obvious, but for whatever reason, a lot of people equate RPM with fuel economy without consideration of load, which is FAR more important.

 

[xtremels6]

I am running a T56 with a .50 6th gear(4th gear is 1 to1 in all of them I believe) and 80mph with 3.73s is 1800 rpm and around 24mpg.i think at 65 30mpg would be easy but the key is I have enough torque that it is not bogging down with the LS3. I just can't drive 65 so I am not sure of the mileage at that speed but the rpm is like1450 and it is okay as long as I am not in the mountains. Oh yeah I just remembered I ran 4.10's for a while and 80mph was 2100 rpms. This is all with 27" tires.

 

[limequat]

This is true.

Don't equate lower RPM with higher fuel economy. It almost never works like that, and definitely won't in this truck with that motor. Load is far more important than RPM.

The relationship between frictional losses and engine speed is linear. The higher the RPM the higher the losses.

Load is just a measure of how much air is making it into the cylinders. Regardless of engine speed it's gonna take X amount of power to overcome wind resistance and rolling resistance. X is proportional to airflow. That airflow can come at virtually any RPM or any load. But...

In general, you'll get better MPG by having lower RPM and higher load due to having less frictional losses and less pumping losses.

The final piece of the puzzle is the engine's efficiency at each speed. The atlas's have an efficiency peak around 2200 RPM and 4400 RPM. So likely anything above 2200 rpm, you're losing economy.

 

[1BADI5]

We've done several hundred thousand km's of testing to prove that. It's nothing new, but a lot of people, including about 75% of my customers, don't want to believe it. To me it's obvious, but for whatever reason, a lot of people equate RPM with fuel economy without consideration of load, which is FAR more important.

I commute about 20-25,000 miles a year for work, so I have plenty of wheel time and I do all of the work on the truck myself.

I have an 87 S10 thats my daily and test pig for some of my thought and theories. The EPA rating for my truck as equipped below, was 25mpg in stock form.

So far, with normal 87 octane(not the winter shit) I can get 32mpg; mixed driving 80h/20c

That's with a 2.5L, 700-R4 and 3.73s keeping the rpms between 1850-2000 on the highway; which accounts for about 80% of my commute.

Above 2000(2100-2300) it falls to 25mpg on the highway

This weekend I'm going to start rebuilding another rear end with 3.42s. I firmly believe this will get my truck closer to the magical 35mpg. If not I also have another trick.

Regardless, there is a correlation between a vehicles operating RPMs and efficiency. Less load better efficiency, less load, less RPMs.

 

[limequat]

One more bit...if your throttle angles are high enough, it's possible to enter enrichment mode and kill your mileage. Stock 355s don't need to worry about this, as the PCM will target 14.7:1 as long as the engine is warm.

 

[1BADI5]

One more bit...if your throttle angles are high enough, it's possible to enter enrichment mode and kill your mileage. Stock 355s don't need to worry about this, as the PCM will target 14.7:1 as long as the engine is warm.

Yeah the old TBI IronDuke is not that smart. Either keep the RPMs down or pay lol.

 

[05_quick4door]

One more bit...if your throttle angles are high enough, it's possible to enter enrichment mode and kill your mileage. Stock 355s don't need to worry about this, as the PCM will target 14.7:1 as long as the engine is warm.

Piston protection will alter that