What Are The Avaible Gear Raitos For A Mack Axle?

[75CF611]

I have a 75 CF-600 with the thermodyne motor and a Allison ht740d tranny. The rear axle is a 6.36 ratio and only allows the truck to go a max of 43 MPH. I was going to use this truck to pull my 36' Fithwheel camper that weighs around 14k lbs on the interstate. As you can see a top speed of 45 is not going to work. I have done the math and figured out to run 65 mph I will need 4.10 gears and to run 70 it would take 3.81s. Are either one of these raitos available and how hard is it to change. I have changed them out on Pickups before but nothing this big. If I could find just a carrier that would be the best way. The numbers on the side of my axle are,

6.36

11KHA4159CP5

117 9Z7851

84KH572B

The bottom number is the casting number and it is a Mack unit. Any help I can get would be greatly appreciated.

[vanscottbuilders]

Don't forget that you don't have an overabundance

of horsepower to work with.

Raising the final drive ratio will take away from the

low end pulling power.

A fifth wheel camper is normally not too heavy,

but a ratio with a numerical value less than about

4.11:1 is just asking for trouble.

For road speed, a ratio of about 4.28 with an overdrive

transmission would be a good choice. The truck would

still have reasonable low end power, and road speed

that would at least keep you out of everyone's way.

These trucks were not hot rods.

No matter what you do, modifying the gear ratio to

raise the top end speed will take precious pulling power

out of the bottom end. For that reason, and for keeping the

driveshaft speed down to a dull roar, an overdrive transmission

is a great choice.

An overdrive Allison should be available. Not cheap - but available.

It is really nice if you can make about 65 mph at about 1,600 to 1,800 rpm.

Don't worry about the top end, as you probably won't want to twist the engine

that high for hours on end anyway. Especially if you're buying the go juice.

To do the calculations quickly, there a number of final drive gear ratio

calculators available on the internet.

Keep us posted.

Paul Van Scott

[Maxville Truck]

I deal a little with a few of the allison tranny's in newer MR's.....

First off the following ratio's are available for your rear end 3.71*, 4.00**, 4.25, 4.62, 4.88*, 5.43, 5.78, 6.36, 6.80*, 7.48, 7.85, 8.39*, 8.83 & 9.87

second off your transmission Probably has the following ratios...

First...................................3.69:1

Second...............................2.02:1

Third...................................1.38:1

Fourth..................................1.00:1

Reverse.................................6.04:1

this does not account for your torque converter ratio which may be any of the following

Stall torque ratio.........TC 487-1.50; TC 488-1.70; TC 497-2.72;

TC 470-2.83; TC 495-2.39; TC 496-1.78;

TC 498-2.45; TC 499-1.91

I'm going to make a few assumptions...... If you have 11r 22.5 tires then you may have been turning approximately 2250 rpm to achieve 43 mph.... (41.3 inch tire diameter).

I have a little spread sheet that I worked out to help with road speed calculations.... to achieve 70 you need to use the 4.0 ratio. (69.11mph) if you use the 3.71 ratio you will get a speed of (74.5mph) (assuming my guess on tire size and RPM is correct.

Most of the MR's that come in with ht 740's have 4.42 or a little higher and will get to just about 60 mph. After a little "pump tweaking" the 300 e-7's will throw rocks when you mash them down (empty of course). I Realize you have a thermodyne. I'm not sure of the HP, "237" maybe....

My Recommendation would be the 4.62 ratio with a spicer or equivalent auxiliary box. If a box with .81 over drive is used your road speed would be about 73MPH. I use an auxiliary box manufactured from the back box off of an eaton fuller transmission, I always use the under drive models for my particular use however the person that makes these boxes does offer and over under version I'm not sure of the ratio of that particular box. (3 speed) they typically run around $2500 with out shipping they are air shifted and will include what ever yoke size you need.

any more questions you can reach me directly @ 715-279-5550

Chris

[75CF611]

I have the 10r20 tires but the difference between the 2 isn't that much. We really don't want to turn the fuel up on this truck was hoping to leave it stock. The 5th wheel camper weighs right around 13k and we figure the truck after it is completed to be around 20 giving us a GVWR of around 33k. I have found a set of 4:25 gears that will fit localy and are considering them. After talking to my dad he would be happy with a road speed of around 60-65. We arent looking to break and speed records but crusing the interstate at 45mph is not an option. I have looked at the aux transmissions and if i could fine one that could be shifted on the fly and has a final drive raito of 0.70:1 that would be perfect. Since the CF-600 is a fire truck with the transfer case that engages the pump we figure we can use that setup to shift a overdrive unit. Do they make a planitary gear style overdrive for a medium duty truck? We have also looked at the option of a 2 speed rear axle. I also have brought up the option of swapping the allison out with a Eaton 5 ot 6 speed tranny and we are not completly opposed to that but we bought this truck partly because it had the auto in it. We have three fire trucks at my dept with allison autos 2 are 5 speeds and one i think is a 4 speed. I am going to go look at them and see which tranny they have and what the rearend raito is in them. I know that both of the 5 speeds are governed at 74 mph but the other one I don'T know yet. I am going to drive it and see what the redline is and how fast it will go. It is a 89 Ford C-8000 with a 3208 cat and the aformentioned allison with the same shift pattern as the mack. We were looking at the fords but I really like the mack a whole lot more and hope we can fix this problem so we can keep the truck. We are going salvage yard hopping friday to see if there is any way around this problem. Any way thanks for the info and anything else you could provide would be very helpful.