What is a good Ratio for a 13 speed?

I assume you're talkin rear end ratio. what kind of engine? and where do you mostly run?

 

also which 13 speed are you talking about?

 

3.55 always did good for me with 11r22.5 rubber or 3.70 with 11r24.5 if you dont mind alittle less pulling power on the hills go with 3.36 ratio with low pro 24.5s for fuel mileage. thats just my opinon have a buddy with 3.36 ratio he gets good mileage but falls flat on his face on a hill.

 

I agree with superhauler, but if going with the detroit, definitely at least a 3.70. the cat you will do well with a 3.55 or 3.36. but the detroit pulls better/better fuel mileage in a higher rpm range than the cat, so a 3.55 or 3.70 with the detroit.

 

i pull doubles cat c15 13 speed. 3.55
1600 rpm 5.5mpg 72mph.
2006 century

 

Bridge cats (serial prefix MBN) do better pulling and mpg's at higher rpms. I've got 3:70's and an eaton 13sp with 11r22.5 tread. I avg 5.9mpg with less than 2% idle grossing 75-80k rolling 3500 miles a week at 70-75mph. 72mph has me at 1600 rpms. Motor pulls best at 1700rpms hitting grades.

When considering rear ratios for a Cat blanket statements about 1 ratio being right are off the mark. Different years have different camshafts, programming, etc that mean they have different sweet spots. An Accert C15 ('04 and up MXS, BXS) performs best at low rpm operation 3:36's are popular choice there. 6NZ C15 ('02 and earlier) pre-emissions do well with 3:55 or 3:70's same with MBN ('03) although I'm of the opinion 3:55 is too high for an MBN.

 

Check your axles sounds like you have 3:70 you should be about 1500 rpms at 72 if you have 3:55s. What tire size do you have?

 

I run a 3406E cat , rto 16413, 3;70 and cruise 55 # 1400 on 24.5 low pros, and avg. 6.8, 7.0 mpg.