cat c-15 18 speed 3.08 ratio???

You must work for Pittsburg Power, High flow mufflers.... HA!!!

 

I know people will disagree with me on this, but science backs up my post.

3 things factor in for your ratios. tire size, transmission, and rear end. changing any one can make up for opposite changes in another. So, on a cat, ideal cruise rpmis between 1200 and 1400. over 1400 it starts to chug fuel. To figure out what rear end you need, I need to know what rpm your at for 70 mpg is now. We then would want to reconfigure your rear to maintain that speed at an rpm of 1400. Depending on how everything is that should put 65 at 1200ish. You could also go a lot higher on your rear and drive in direct. Basically you cruise in 16th and then have 2 extra gears you dont normally use. The key to all the math is what rpm your at now for 70 mph.

Now, changing to taller rubber will add rolling resistance which is not good, but it will have similar effect to changing the rear end. Contrary to what others believe, what part of the drive train you make the correction makes no difference since taller rubber with a lower rear end cancel each other out.

 

Someone can correct me if I am wrong, but with 3:58's I don't think you can just swap the ring and pinion to go to a 3:08 or 3:21, I think you would need to change the whole center section. Again I might be wrong on that, just trying to recall a conversation with a mechanic I had last month concerning re-gearing my rears. With mine I have 4:10's in them now, I have the original 3:58's ring and pinions that the truck was built with, and I can swap them using the same center sections. But if I wanted to go to a taller gear than the 3:58's then I would have to buy new center sections as the ring and pinions from say a set of 3:42's or 3:36's would not work with what I have.


My last truck had 3:36's, and those gears are nice if you want to get out and put some miles behind you in a hurry, but they are not that great for pulling heavy over mountain passes all the time.

 

NO, I bought the Donaldson's for about $50 a piece less from Ryder,(Which ARE the same thing), and picked up .5 mpg. I can also tell when the difference in fuel mileage when I run 62-65 than 70. It's not rocket science there Einstein.

 

It all has to do with where you are applying the torqe. Changing the rear end will change which parts get the most tourqe.

 

O really, So did I years ago, just had one single, didn't do a dam thing..................................................

 

Your not going to see a black and white difference, but it might save .1 or .2 mpg. Not enough to measure but enough to make a difference in you bottom Line over a year.

 

When we installed a high flow muffler on my 05 ISX Volvo with a single stack there was not much of a difference. When I change the mufflers on my dual stacked W900 with an ACERT there was. Both trucks at the time ran the same route all the time so any difference was easy to see. I think because the ACERT moves so much more air that air flow becomes that much more important. Another thing I noticed with a muffler change on the W9 was how much cooler they were. It's a day cab so the mufflers are up the side and the grab handle did not get hot anymore.

 

I agree with the last two post totally, I don't see an improvement of .05mpg on a single turbo stock truck, unless you have potatoes stuck in your pipe...

 

Cat recommends a lot of crap, AND they are the ones that are going to sell you the parts to rebuild it when it fails too, I'm just sayin'.... But with the PDI stage 1 NOTHING Cat recommends applies any more. With the Stage 1 it should be perfectly happy at 14-1500. Mr. Haney is the resident expert maybe he will chime in here. The short rubber is what is killing your MPG's, unfortunately there is no cheap/quick fix for your situation, in my opinion (and just my opinion not looking for a debate), putting 11/R's on it will fix most of your problems, doing it now or waiting until you need drives is up to you, and I don't think you will see enough difference between the 3.21's and the 3.58's to make it worth the investment. If you decide to go with the 3.08's or even 2.64's to over come the short rubber, be prepared to "drive" it there will be a lot more shifting to keep the engine happy.
http://big-rig-road-speed-calculator.blogspot.com/2011/03/semi-truck-road-speed-calculator.html
That link I put up is a road speed calculator, you can change tire size and/or rear end ratio and trans final drive, play around with it and figure out what will work best for you.