gear ratio and axle specs

Then go this way 12/21, but there's more that concern me than this,.
why so a low gear, go with the ISC and 300 hp or more and probably get you money back on fuel economy.

I helped a friend spec his T300, he upgraded to ISC 315 hp for only a few hundred more, but the salesman wanted to put 5.56 rears, so I ask them why would you want 330 hp and such a low gear? we end up with 3.90 and never looked back.
remember not many sales people have driven trucks and not all that drive trucks have a clue, that's why you come to this site, to get a more wide spread opinion and you can tell right away who has the knowledge and who doesn't.

 

If you're getting that truck set up like that, you'll be turning roughly 2000 rpm's @ 55 mph (give or take depending upon final gearing in the transmission)...which is ridiculous for any vehicle expected to spend any time on the road. If you're going smaller than a LP22.5, the rpm's will be higher. If the dealer sells you that set-up knowing you are going to put it on the road, the salesman should be fired.

Second, as others have stated, it is absolutely pointless to spec more than a 20,000 axle on the drives ESPECIALLY if it takes away from what you can carry on the steers. A 10K steer axle has your LEGAL gross weight at 30K. With a 20/12 set-up, you can legally run 32K, but you'd be better served seeing if they have axles rated slightly higher than that. The steer axle in my Mack is 12,800 rated...which with a 20K rear would still put you under the 33K you're looking for but allow you carry more weight on the steers for better handling. Have you thought about what it's going to feel like driving a truck with 20K on the drives and 10K on the steers on a slick rainy/snowy day? You'll be wishing you had that extra weight on your steers.

As for the SFA/SBA difference, the SBA is going to shorten the wheelbase by tucking that axle further back under the driver. It won't ride as nice, but it'll turn better. It also puts more weight on the steer axle...which makes no sense if you are going with a lighter capacity axle.

Like I said, the salesman trying to sell you that truck ought to be fired. If you haven't finalized the deal yet, I'd back out of it and find someone that knows what the heck they are doing to try helping you with the specs.

 

It's amazing how crappy some medium duty trucks are spec'd. The salesmen for most are clueless.

 

Fred why don't you tell us where your going to be running the truck, what type of load and how heavy, what make and model? and between all of us can help you put together a better truck.

I would get the dealer to give you a few more days to place the order, it seemed to me like they're rushing you to get your money before the end of the year.

Another thing to consider is the resale value of the truck you spec here. you're basically specking an F450 or a 4500 Dodge, but will end up paying a lot more,

 

Looking at a .pdf from International on the Allison 3500 series transmission, 6th gear will have a .65:1 ratio. It is also showing that you have a maximum output shaft speed of 3600 rpm.

Now for the math.

A 295/75R22.5 tire turns 512 times per mile. With a 5.63 gear set in the rears, the drive shaft (output shaft of the transmission) is spinning around 2883 times per mile...or at 60 mph, that many times per minute. 75 mph is going to have that output shaft turning 3604 rpms....exceeding it's maximum rating by 4 rpm.

That also means, when taking into consideration the transmission's .65:1 overdrive, that the engine is turning 2343 rpms at that speed, which while within the "input" specs of the transmission, are well above your peak torque which Cummins publishes being 750 ft lbs @ 1800 rpm.

You have several options, though.

First off would be to spec a torque converter that is better suited to your application. If your current truck is a dog, it might just be that the stall speed on the converter is far past the power curve of the engine, so that you aren't getting to use the power the engine is making. A lower stall speed converter (towards the start of the power band, rather than after the power falls off) would make a world of difference in that regard.

Next, select a rear gear that places your "cruising" speed within the engine's "optimal" range. I'm not seeing anything in that publication saying where that is, but if you look the information is out there. Based upon where the peak torque is made, I'd try to keep it in the 1600-2000 rpm range. If you take that 2000 rpm as a max, and put it at 75 mph, it would put you right around a 4.88 gear ratio in the rears. If you'd like to see a little better fuel mileage, you might opt for something a little less "aggressive" like a 4.35, which would put you running 1780 rpm @ 75 mph...right where your engine is making it's peak torque.

As I said before, put a 12 or 13K axle under the steer, and a 20K drive axle to keep your specs below 33K.

11R22.5 tires will be slightly taller, which will increase speed or decrease engine RPM's...but it takes more torque to turn them. If you get the gearing wrong, it is cheaper to change tire size (especially if you can keep the same size rims and do the swap when you're due for tires anyway) than it is to re-gear the truck's drive axle. If ride height is important for overhead clearance or bumping docks, tire size options may be limited due to those differences.

 

It would be better than the 5.63 for sure. I'd still opt for something in the 4.30-4.50 range though. Your best bet is to try and find a dyno graph showing the power curve of the engine. Specing a gear set that puts you past the power band at the speed you're wanting to run is going to waste fuel and won't pull a hill like you're wanting because the engine has to reduce speed to get to it's peak performance range.

With an automatic, you really don't need as low of a rear end gear, because the torque converter has a multiplying effect. Rather than having to "lug" from a start at idle like a manual transmission with a clutch, the engine can spin up to the stall speed (into it's power band, with a properly spec'd torque converter) as the transmission is engaged....which is why the stall speed of the converter is so important. Too high of a stall speed and you're past the power band with nowhere to go but down as you're trying to get rolling.