M923 Gear Ratios

[nf6x]

I would like to know the transmission, transfer and differential gear ratios in a stock M923 (M939 series) truck. I haven't managed to find that info in the TMs so far, and I'm not positive that the generic info I found online for MT654 transmissions is correct for whatever is in the M923. I gather that the differential ratio is 6.44?

Can anybody help me find these numbers?

 

[nf6x]

I found the transfer case gear ratios in the wiki. Still looking for the other ratios, though...

 

[NDT]

You have the differential correct, and the transmission is direct.

 

[nf6x]

Thank you! I'm still curious about the other gear ratios, but the top gear was what I most wanted to verify. If I have all of the numbers right now, I come up with a top speed of 57 MPH at 2100 RPM with 43" diameter 11.00R20 tires, which looks about right to me.

 

[Beerslayer]

Your top speed is about right. Mine runs 59 MPH at 2150 RPM. You still need a hard top?

 

[nf6x]

Your top speed is about right. Mine runs 59 MPH at 2150 RPM. You still need a hard top?

Thanks!

Yes, I am still looking for a hard top.

 

[disco]

Bumping an old thread in case anybody else is curious like me.

I found the following ratios. I don't know if these are the actual ratios. ( MT654 vs MT654CR?)
http://www.vibratesoftware.com/html_help/2011/Allison/allison_5speed_auto_trans.htm#MT 654CR

1st - 4.17
2nd - 2.21
3rd - 1.66
4th - 1.27
5th - 1.00
Rev - 10.76

Using this link http://www.apexgarage.com/tech/gear_ratios.shtml and the data plate info on a M923 for top speed in each gear, here's what I've come up with. If outside links are filtered out, just google "speed in gears calculator" and find one that works for you.

I'm making PLENTY of assumptions here.

14x20 = 49" tall
2750 (!!!) RPM redline (took this RPM in the calculator to make the speeds on data plate to match the gear ratios I found. No idea if this is correct)

So plugging in those numbers ^^^ I get:

15 mph - First
28 mph - Second
37 mph - Third
49 mph - Fourth
62 mph - Fifth
6 mph - Reverse

However, these numbers don't match what yall have above at all so I don't know how accurate this is. My numbers are 1 mph slow in all but first gear.

(That granny reverse gear makes an insane crawl ratio in low. 10.76 rev x 2.21 tcase x 6.44 diff = 153:1)

 

[nf6x]

I think that redline is 2100 RPM. Did you account for both the transfer case and differential ratios?

 

[quickfarms]

I think that redline is 2100 RPM. Did you account for both the transfer case and differential ratios?

2100 is the RPM for the rated HP

The redline is above what the governor is set at which is provably 2800 RPM

The 800 series 5 ton has a tag on the dash that warns the operator to nit exceed 2800 RPM

 

[disco]

I think that redline is 2100 RPM. Did you account for both the transfer case and differential ratios?

Okay, I did NOT account for the transfer case ratios. I assumed it was 1:1 high. I found an old thread by mangus580 where he posts the t-case ratios.

http://www.steelsoldiers.com/showthread.php?28620-5-Ton&highlight=5+ton

Transmission
All M939 series trucks were equipped with an Allison automatic transmission

Transfer Case
High Range: .732, Rated Input Torque- 7350 Foot Pounds
Low Range: 1.79, Rated input Torque- 650 Foot Pounds
Oil Capacity: 13.5 Pints
Weight: 460 Pounds

This brings the RPMs down to more believable levels. I was having a hard time seeing this 6" stroke swinging 2800 RPM for hours on end.

6.44 axle x .732 high range is a effective final drive ratio of 4.71

This puts 63 mph at about 2000 RPM. *whew

Hopefully I'm not here but I haven't seen all the info in one thread, this is for future nerds like me to search and find.

 

[M813A1]

Check your diamentions of your 1100x20 tires they should be closer to 41" for the 1100 tire not 43 " . That might be confusing you .

 

[disco]

Check your diamentions of your 1100x20 tires they should be closer to 41" for the 1100 tire not 43 " . That might be confusing you .

I know what you mean. I thought about commenting that the calculator is based on a true 49" tire diameter (14.00x20) but that doesn't take into account tire squat. Then I figured I might be splitting hairs.

[EDIT: I could subtract a couple of inches off the numbers to compensate but it's close enough for gov't work. 2000 rpm is much more agreeable than the 2800 I came up with originally.]

I don't actually own a 5 ton yet. I'm looking at a M923A1 right now. And drooling....

Now I'm wondering, will a 11x20 pull harder but less top end than a 14x20? Seems it would to me, the shorter tire would act like more effective gear.