Gear Ratio MPH

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Gear Ratio MPH

Post by johncarrol » Thu Mar 15, 2018 6:39 am

Can anyone that has 2.56 gears, 4L80E with a tach, and GPS tell me what RPM they are showing at 55MPH in OD? I have 2.73 gears and I'm showing approx. 2175RPM. I've been a member of The Diesel Page for a few years and learned from them that our engines are the most efficient at 1700-1900 RPM. I also used to drive a 1991 GMC pickup with 6.2 - 4L80E and at 55MPH I would run around 1800RPM which seemed to be the best RPM area for the engine, not to mention 25MPG for a 4x4. I would love to put 2.56 gears in my HMMWV and see how much it will drop my RPM at 55MPH. 2.56 gears are cheap, but the hassle of resetting the rears correctly with new gears keeps me from going that route. As many HMMWVs that the government had scrapped, I can't believe I can't find any complete 2.56 rears for sale. I must not be looking in the right places. I can't accept the fact that there are no places with piles of older 2.56 HMMWV rears that the owners don't want to make some money selling them. 2.73 complete rears are widely available, but not 2.56 ones? Does anyone have any links to 2.56 rears for sale? Can I replace my 2.73 housing with a 2.56 one without any mods.?


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Re: Gear Ratio MPH

Post by 86humv » Thu Mar 15, 2018 7:03 am

I have extra take out 2.56's that look good inside, just I would replace seals in them.
$ 250 ea.
They have the 1330 yoke, not the early front or rear flat yoke.
Fastenal shipping.
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Re: Gear Ratio MPH

Post by Action » Thu Mar 15, 2018 11:34 am

What is the difference between front and rear besides possibly the yoke size and dust shield? You kept stating you can't find a rear 2.56. So put a front 2.56 in the rear.

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Re: Gear Ratio MPH

Post by johncarrol » Thu Mar 15, 2018 2:56 pm

I'm sorry, when I say rears, I mean both front and rear differentials.

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Re: Gear Ratio MPH

Post by dilvoy » Thu Mar 15, 2018 11:47 pm

The change to the 2.56 ratio probably won't get you the rpms that you had in your pick up truck. The difference is not that much and don't forget that the portal hubs have their gear ratio that is almost two one in reduction to help with the large tires.
Your Humvee won't drive like a pick up truck, it's more like an on road tractor, but if you built some custom spindles to replace the portal hubs and put on smaller tires, you would get just what you want.
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Re: Gear Ratio MPH

Post by Retired War Horses » Fri Mar 16, 2018 5:55 am

^^^^^^^what George said...
They went to 2:73 to get the truck off it’s fat arse a bit quicker...the latest trucks use the 3:08
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Re: Gear Ratio MPH

Post by johncarrol » Fri Mar 16, 2018 8:47 am

Can anyone with a tachometer, 3 speed automatic, 2.56 gears, and GPS tell me what their RPM is at 55MPH? Mine is 2350RPM in drive which I believe is a 1to1 ratio in the transmission. The 3 speed transmission should also have a 1 to 1 ratio in drive. The difference between the RPM readings should give me an idea of how much reduction in RPM I will have with 2.56 gears vs. 2.73 gears.

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Re: Gear Ratio MPH

Post by SinDiesel » Wed Mar 21, 2018 5:59 pm

37" Tires, 3L80 in 3rd, 2.56:1 diffs, 242 TC in high:
2480 RPM = 55 mph
3157 RPM = 70 mph

37" Tires, 4L80e in 4th (Overdrive), 2.56:1 diffs, 242 TC in high:
1860 RPM = 55 mph
2368 RPM = 70 mph

I set up a calculator years ago in excel while doing CJ-7 builds and then set up another when I started restoring HMMWV's where I could select several different tranny's common to hmmwv conversions with their respective gear ratios, and similarly the 242 Tcase whether in Hi or Lo along with selecting 2.56:1, 2.72:1, or 3:08 in the diffs. These calculation were based on 37" diameter tires which obviously aren't truly 37" and true speeds/rpms would be reflective of the actual dimension from the center of the Hub to contact with the ground in diametrical terms along with the assumption that no slippage occurs in the torque converter (i.e. locked up)

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Re: Gear Ratio MPH

Post by SinDiesel » Wed Mar 21, 2018 6:05 pm

obviously those numbers take into account the 1.92:1 ratio in the portals

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Re: Gear Ratio MPH

Post by johncarrol » Thu Mar 22, 2018 5:10 am

Thank you so much for sharing your calculations. If the difference in reality is no more than 50RPM, I will be happy investing in 2.56 rears and running down the highway around 1900RPM at 55MPH. I'll start to think I'm in a road tractor going down the highway. I might even hammer down to 60-65 in a 70MPH zone! With what I have now, I still catch myself looking down at the shifter for another gear. Right now my M1123A2 is not only my daily driver, it's my ONLY driver. I believe the gearing change will be good for me. One quick question for anyone. The research I did on a M1123A2 stated that the gear ratio was changed in the TC by AMG. Is it true that it was only changed in the low range side? And my geared hubs should still be 1.92 Ratio?

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Re: Gear Ratio MPH

Post by Action » Thu Mar 22, 2018 5:37 am

I thought the m1123s have a 4 speed trans...

Jason

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Re: Gear Ratio MPH

Post by dilvoy » Thu Mar 22, 2018 10:19 pm

Action wrote:
Thu Mar 22, 2018 5:37 am
I thought the m1123s have a 4 speed trans...

Jason
Yes they do and yes his does. He is looking for a 50 rpm drop from what it has now, I guess while it is in fourth gear with the Converter locked. Larger diameter tires would be the easiest way to get that, but the spectre of "more economical in price" may rear it's ugly head and nix that idea.
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Re: Gear Ratio MPH

Post by Action » Fri Mar 23, 2018 2:01 am

johncarrol wrote:
Fri Mar 16, 2018 8:47 am
Can anyone with a tachometer, 3 speed automatic, 2.56 gears, and GPS tell me what their RPM is at 55MPH? Mine is 2350RPM in drive which I believe is a 1to1 ratio in the transmission. The 3 speed transmission should also have a 1 to 1 ratio in drive. The difference between the RPM readings should give me an idea of how much reduction in RPM I will have with 2.56 gears vs. 2.73 gears.
I thought so too. But, by reading this, I would figure he has a 3 speed with 2.73 diffs...
The first post was looking for 4 speed and 2.73.

Jason

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Re: Gear Ratio MPH

Post by johncarrol » Fri Mar 23, 2018 4:46 am

I did not state or imply I was looking for or happy with a 50RPM drop from what I have now. Running down the road at 1900RPM at 55MPH is not a 50RPM drop from 2175RPM. The previous post calculated perfect conditions. I implied that if the difference in reality from 1860 is only 50RPM, I would be happy. THIS is what I wanted to know from my original post. What RPM I will be running at 55MPH with 2.56 gears in OD. That tells me (worst case scenario) With 2.56 gears- 1860+50= 1910RPM at 55MPH in OD. With my 2.73 gears- 2175RPM at 55MPH in OD. 265RPM drop (worst case scenario). Nowhere in my posts did I state or imply I had a 3 speed with 2.73 diffs. I stated that I had a 4 speed and 2.73 gears. I hope this clears things up for some of you.

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Re: Gear Ratio MPH

Post by Action » Fri Mar 23, 2018 5:32 am

You have me curious now. I want to get my tach installed and see what i get with 3.08 and OD.

Jason


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