8.1L engine 4L80E trans 4.10 vs 3.73

[1984dually]

Recently finished up the swap into my 84 crewcab dually. Currently has D70 posi rear with 3.73 gears. Recent trip to Vegas(1600 miles round trip) worked out to right at 10 mpg(70+ mph) Total vehicle weight was around 16,000 lbs(pulling a enclosed car trailer). Pulling hills was good but at times it seemed to lug more than it should. It has been brought up that a switch to 4.10 would increase the pulling power as well as have an increase in MPG. Truck is only used to pull the trailer and haul loads. Not a daily driver. I know a couple have swapped to these engines and was hoping to get some observations on what they think(anybody else too) on this subject.

 

[bucket]

What's your current RPM at 65-70? I don't know about mileage, but for power at cruising speeds while towing, I'd shoot for something that puts you in the 2500-2800 rpm range.

 

[Driver4r]

Tuning and cam have a big playing role.

 

[1984dually]

At 70 mph rpm is at 2150. Engine is bone stock and run by a Howell efi harness and computer reflash. No emissions and torque management removed. Speedo is run off the vss sensor from the trans and is dead on.

 

[Driver4r]

Yea, pulling, 2150 is toooo low, shoot, with my 3.08's i turn 2500(70) with no OD. When i had 4.11's i was turning 3500(65ish) again, no OD. And those were both on 32" tires.

I reccamend you go to 4.11's or 4.56's.

 

[oneluckypops]

Yea, pulling, 2150 is toooo low, shoot, with my 3.08's i turn 2500(70) with no OD. When i had 4.11's i was turning 3500(65ish) again, no OD. And those were both on 32" tires.

I reccamend you go to 4.11's or 4.56's.

+1
3.73's are to high for a 4L80E. IMO 4.10's are bare minimum. 4.56's work very well for a small block, but given your running an 8.1L I would stay with a 4.10 ratio....

 

[kleedus]

I agree with the 4.10's

 

[1984dually]

Have another peg leg dana 70 with 4.10's in it. Might go ahead and swap to disc's before I install it. Thanks for the info.

 

[1984dually]

4.10 vs 4.56

Been crunching some numbers on what gear I want to go to. Using a formula I found online I was able to determine rpm vs speed for different ratios. tire diameter divided by 2 divided by 168 X rpm divided by(gear ratio X trans ratio) In my case it is 31.5/2/168 X 2000/(4.10x.75) Works out to 55mph with 4.56
61mph with 4.10
67mph with 3.73 at 2000 rpm

Using same formula at 70 mph
4.56=2800
4.10=2300
3.73=2100

Most of the time traveling will be in the 55-60 mph range. Going to Vegas is the only time it will see 70+. Looking at torque tables for the 8.1 shows a flat torque curve from 2500-3500 and starts to fall off after that.(a definite low rpm engine)
Not sure on how accurate this formula is but the numbers are very close based on what it runs now. Thinking the 4.56 may be the way to go as it puts it into the middle of the torque curve in od at 70. 55 would be 2000 rpm(very bottom of the max torque curve)
Using the 4.10 it would put it at 2400 at 55 in direct( pulling hills this is the gear it would most likely be in).

Any and all opinions/recommendations are welcome.

 

[68post]

Not a big jump from 3.73 to 4.10, do you need just a small difference ? Or do you feel that a bigger diff would work better for you (4.56) ?

I've had all three , but in vastly different trucks and none had OD. My TBI/454 V3500 3+3 w/4.10's could pull a backhoe, but needs a fuel tanker for a bed.

 

[marks86]

4.10 is a great gear and 4.56 is also a nice gear (for towing)

but I have to doubt your going to increase MPG with going to a lower gear, unless your truck truly only is driven with a 10k trailer behind it ALL the time

 

[77 K20]

I had also heard that the Chevy Avalanche with the 3.73 gears wasn't the greatest for pulling. 4.10's were optional. GM gave I think another 1,800 lbs of tow rating with that gearing change.

On my Silverado I have the 8.1, but with the ZF6. At 75 mph I'm around 2500 if I remember correctly (haven't driven the truck since last summer). When towing I weigh in at around 13,000 lbs total, and hills have never been an issue. But I have a choice of using 6th, 5th, or 4th for going up hills at highway speeds.

I also have a Blackbear tune with all TM removed and it setup for at least 91 octane.

What do you have for exhaust on it? Factory exhaust on them is huge and well made.

I've logged almost 12,000 miles of fuel logs with it also. I lose 1-2 mpg when using unleaded with ethanol in it (Montana often has super unleaded that is ethanol free). But on average when pulling I get around 12 mpg. 14-15 when running empty.