3:55 to 3:08, opinions?

im not sure about any of the other questions u have but i do know that the ecm takes the speed from the drive shaft.

 

You will have to recalc your speedo. Your truck will be going faster at any given current rpm. Same as putting a larger tire on the truck. There are little tabs on the back of your speedo that you change around to get it right.

I do not use the formula that comes with the speedo. The best way I have found is to find where you are on the graph. This graph shows your tab settings. Once you find it use a gps and begin making jumps up and down the graph until you are right on. This may take a few attempts but I have found this to be far more accurate than the formula they give you. I have done this many times and it always works. Use your current setting as a reference. Then move up or down the graph...whichever way you need to go. There is NO ryhme or reason to the tab settings. The graph is probably available online.

You can change configurations for max and min shift points at varying rpms. Or you can disable them in ET. We have ours fixed to where drivers have to do more progressive shifting. Yes, they get mad...but it saves on fuel and keeps their trucks in there respective powerbands...many drivers think you need to get the rpms way up before shifting and this is not the case with newer engine tech. You can also change the drive ratios...they will change. I really do not know if this a mandatory thing for what you are doing...With an 09 maybe so.

 

3:55 are good all around gears. If you go with 3:08 you will have to change speedo calibration but that is no big deal. I really would keep the 3:55 and maybe think about going to tall rubber when you go for your next tire change. You won't lose any pulling power and you can pick up your fuel mpg there.

 

I would have no problem going to a 3.08 with an 18 speed (or 13 for that matter). What you got in 18th with the 3.55's would now be the same in 17th. Same rpm to speed, just one gear lower. That being said, when you had to pull a hill, the next one down is 16th (direct), the most efficient way to pull a hill. If you were light, or on flat ground, or had a good wind to your back, and wanted to step up the pace, you could move into the top hole and run. 1450 rpm on low pro 22.5 rubber would give you 75 mph in the top hole. 1400 on tall rubber would give you 75 mph. There are quite a few owners that are doing similar thing with 3.08.

I considered 3.08 when I recently ordered a new truck, but went with 2.64 instead. It works fine for what I do. But 3.08 was running a close second as an option.

You would have to set the parameter in the ECM to show that you had the new rear end ratio. That is all you would have to do. It would calculate the road speed from that. This is why you need the tire rotation per mile, the rear end ratio, and the transmission all specified in the ECM. You do not have to calculate anything. It does the work for you.

If you are tearing everything apart, then it making the conversion would not be a bad idea. If you are not already tearing everything apart, the maybe the tall rubber option may work for you.

 

3.55 is pretty close to 3.08 in 17th gear you should be runnin the same rpm only 1/2 gear down. I'm going to do the same thing. I'm just going to have to get used to the idea of having to throw it up against the dash instead of just sliding the selector when pulling a hill.

 

If you have an 18 speed I would install 2.64 as Cowpie1 did on his new truck and run it in 16th which is direct. In this case if you need to go 1/2 gear down you do not have to "throw it against the dash".....Just slide the selector.

 

If you are going to spend much time in California, or Oregon and Wash., you may want to think about their stupid lower speed limits for trucks and where those ratios will put you.