Rear End Gear Ratio

@Cowpie1

 

2013 FL Columbia glider with pre-EGR DDEC-IV 12.7 Series 60.

 

Will I struggle in performance with 1998 fl 12.7 Detroit 60 series 10 sp with 2.79 ratio on rear ends?

 

Not at all for performance, assuming a direct drive trans. To get decent fuel economy though, keeping speed 65 mph or less is advised. Engine turning above 1600 RPM a fuel mileage killer. The 12.7 pre-EGR does fantastic around 1500 in performance and tolerable on fuel economy. With low pro 22.5 rubber, 1500 is roughly 60-62 mph with 2.79.

 

You could use taller 11R 22.5 rubber and have engine RPM's closer to 1500 at 65 mph. My setup is with 2.64 and low pro 22.5 rubber and I am turning 1500 in direct at 65 mph. I have 30,000 lb in the box today, running west at 65 mph with 15-20 mph N/NE cross wind, and pegging 8 mpg. ECM says 8.4 mpg.

 

@Cowpie1 how can I tell if it's a direct drive tranny?

 

Because that is a fleet spec. If it was an overdrive transmission it would go much faster. And who builds a dollar truck with a 10 speed?
You can also get the transmission model number. Will look something like RTLO16610A just look for RTLO
Enter that in Google it will tell you what every gear ratio is.

 

@gokiddogo dollar truck??? Lol what u meant by a dollar truck

 

I have a 12.7L Detroit with 13 speed tranny and 3.55 rear end gears and low pro 22.5. Final drive ratio on tranny is .87 and 12th gear is 1 to 1. I run I-20 from Dallas, Tx to Louisiana speed limit is 75mph. At 65mph I’m running 1700rpm. 70 mph is 1800rpm. If I changed my rear end ratio to 2.93 would that lower my rpm? Will I still have enough pull power to pull a few hills?

 

something does not sound right there. but yes it would drop your rpm. to go to a numerically lower gear ratio.

i run a 13 speed with 3:70's and ill run somthing like 84mph at 1800 rpm 65 is somewhere around 1400

that almost sound more like a 4.xx rear end? are you sure someone has not changed the gears in that truck at some point.