Transmision ratios

[poppi]

 There seems to be a lot of talk about changing Differential gearing when one changes from a standard to an
automatic....................sounded like no fun to me  so I looked it up

 
Standard Tranny Ratios:            ZF               Allison HT 740

  1          4.30                            3.43              3.69
  2           2.33                           2.01              2.02
  3           1.36                           1.42              1.38
  4           1.0                             1.0                1.0
   5                                             .83

Rev         6.0                              4.84              6.0

Now I'm no math wiz but I don't see that much difference.
Maybe a little slower off the line but the rest seems in line.

   Open for comments..

Thanks
Skip

edited to make the columns look right

[blue_goose]

The best reason to change the rearend is to get better fuel milage with the auto.  Most of our buses came with 3.73 or close to that with the standard transmission.  With the auto most went to the 3.36.  You don't need the lower rear to get going and the 3.36 uses less rpm at speed.
Jack

[rv_safetyman]

Skip has an MC 8 with a Cummins L10.  Thus he probably needs to get a taller gear.

Skip, we need to know a bit more about your application.  What rear end ratio do you have and what transmission?  If you run about 70 -73 MPH at 2100, you probably have a 3.73.

With a standard transmission, you need a fairly deep first gear, even with the torque of the L10.  With an automatic, the torque converter gives you torque multiplication and you can get by with a taller (smaller number) rear end.

If you have a standard transmission, it would be fairly simple to change to a 9 or 10 speed transmission that gives you both a deep low gear and .73 overdrive (depending on the model).  Folks that have done that swap love it and don't mind the shifting.

Jim

[poppi]

What started this was I supposedley have a standard 3.73 diff. When I see the suggestions to others
that they change the gearing in their diff then it leaves me to wonder. Especially when 4th is the
same in all the transmissions I mentioned, one of which was the standard trans. I am running the ZF
1st isn't that zippy but to change the diff when the transmision ratios are close (subjective) between the different
types just seems a little un-necessary. I've been clocked at speeds greater than I care to go (yep I fixed the
speedo . So it really comes down to ROI and trying to figure if it is worth the bother. Not something
that seems to be covered in the other threads on tranny changes.

  YMMV

Thanks
Skip

[DaveG]

I think maybe the reason folks change trans or rear end ratios is when they make the switch from a two stroke to a four stroke. The two stroke likes to run at a much higher rpm than the four stroke, and the four stroke gets much better fuel mileage at that lower rpm (around 1500 give or take) where the two stroke runs about 500 rpm more.

[buswarrior]

As noted, differential gear changes are made to change the cruising RPM of the engine, if the transmission won't do it alone.

For instance, Series60 Detroit mated to an older HT740 with the same 3.7 gear spinning up at 2100 rpm that the 8V71 was turning will burn almost the same fuel. Slow that S60 down to 1500 RPM at the same road speed, and it will give you a couple more mpg easily.

On the older coaches, the stock gearing was carefully selected to deal with the limited power of engines of those days gone by. Any taller a final drive ratio, and the coach would not climb a rise in the road or even battle a headwind.

To do a straight period swap, 4 or 5 speed direct geared Spicer to HT740, you may leave your differential alone, if you choose. My MC8 has 3.7 with an auto from the factory.

happy coaching!
buswarrior

[viento1]

do you know the ratio of the Alison 644? is it the same as the 740 just a lighter duty?

[HB of CJ]

Best guess based upon a failing memory....MT... 3.58 1st, 2nd around 2.04 or soss, about a 1.37--1.38 3rd and a 1.00 4th, but don't hold me to it.  Now if you want RT/RTO/RTOO 910 Roadranger ratios...he he he.. HB of CJ

[TomC]

All of the 4 spd Allisons had very close to the same ratios.  Even the close ratio World transmission has the same ratios for the first 4 with the last 2 being overdrives.  I changed out my 13 spd overdrive to a HT740 direct drive 4th gear.  With 3.55 and 11R-24.5's (476rpmile) this gives me 55 @ 1550rpm, 60 @ 1670rpm, 65 @ 1830 and a 2300rpm top spd of 81mph-all of which is quite acceptable for a 4 stroke mechanical engine.  Remember virtually all big bus/truck engines over 10 liter have a continuous rating of 1800rpm.  2 stroke engines are perfectly happy at 2100rpm all day, but at 1800 is better for fuel mileage.  Electronic 4 strokes should be cruised in the 14-1600rpm range for best mileage.  Good Luck, TomC

[rv_safetyman]

Hi Skip.  OK, we now have the most of the pieces of the puzzle.  You have the ZF with the .83 top gear.  If you have a 3.73 rear end (likely) the overdrive gives you a final ratio of 3.09 in 5th.  That should be good gearing for your combination.  If you had a 3.36 rear end, that would give you a 2.78 final ratio.  That also would be good for top end.  However, I really don't think you would be able to do a start on a steep hill. 

Probably the best way to tell what your rear end ratio is, it to drive in fourth (1.0) and tell us what the RPM and speed is.  It should be about 72 mph at 2100 engine RPM with a 3.73.

Sounds like you are in good shape. 

If you do not have a tach, you should invest in one.  Pretty important.  You can get the kind that works off the alternator or the one where you glue magnets on the crank pulley.  I prefer the latter.  I switched mine over from the 6V92 engine and it reads very closely with the electronic RPM sensor read directly by the SilverLeaf.

Jim

[poppi]

 Ok I think there have been some good explanations. Lots of pieces to the puzzle.

 So to put it in my simple mind.............

   If you are changing standard to automatic and running 270 to 318 HP leave the Diff gearing alone (if factory standard)
   If by chance your also including a more powerful engine  say 350 to 500 HP then changing the diff gearing will help in MPG.

  Jim,
     Getting a tach is on my list of items. magnet or optic I'm not sure which yet How many magnets did you go with?
     one unit I had on my combine I think I could have up to six magnets (supposedly better accuracy)

 Thanks
  Skip

[Lin]

Jim,

Just wanted to point out that your RPM:Speed estimate is also dependent on tire size.  My 11R22.5's give 500 revs per mile.  a 12R24.5 will give about 470.  For me, 2000 rpm with a 3.36 rear end comes to around 71 mph is fourth.

[JackConrad]

     Getting a tach is on my list of items. magnet or optic I'm not sure which yet How many magnets did you go with?
     one unit I had on my combine I think I could have up to six magnets (supposedly better accuracy)
Thanks
  Skip

Skip,
    A couple other options for a tach sensor is the R terminal on you alternator or a sensor that counts the teeth on the starter ring rear (flywheel or torque converter)  Jack