3:73 or 4:10 looking for advice

[klenger]

I was unaware that the 2500 was offered with 245 tires because I was only shopping for 3500's. The 245's would produce an overall higher (3. 6% by may math) effective gear ratio than the others compared to about a 10% diff. between the 3. 73 and 4. 10's.

 

[InThinAir]

It would be higher than 3. 73 or 4. 10 assuming that those number are measured using a 265 tire. Those numbers may be spot on if that 245 was the tire that was intended to be the stock size.



I guess my point is ...



What the heck size tire is gives you the 3. 73 that was intended. I would assume that the gearing is not different for every truck, it is probably the same in every AAM rearend. I would bet that it is based on a theoritical tire diameter. But what the heck is it? I know we all get wrapped around the axle (pun intented) about the gear ratio.



So what is the actual diameter that nets you 3. 73???



Anyone know for sure?



---Doug

 

[DPelletier]

Doug,

The ratio is nothing more than the ratio of the input shaft to the ring gear. It doesn't matter what the tire size is, the rear end is still 3. 73:1. If you take one revolution of the drive shaft, the axle will turn 3. 73 times whether it is a 29" tire or a 44" tire. Of course your speed of the vehicle changes dramatically.

The real question is: what tire size did the manufacturer use when calculating that a 3. 73:1 rearend was the desired ratio? With the tire sizes mentioned in your post there is an approximate 4% variation depending on stock tire sizes. Probably not enough to make a difference from an engineering perspective.



Dave

 

[XcumminsX]

to the gusy who is gonna run 40's..... You need something like a 4. 88 or lower. . I had 42's with 4. 88 and even that wasnnt good enough



Nick

 

[klenger]

DPelletier



Well stated. I think that many people think that the axle ratio sets the final RPM vs. drive speed. It is only one of several parts of the equation, the others being the transmission gear ratios, the transfer case ratios, and the outside tire diameter (not the wheel size).