So if I have a 3.55 10spd .73. And swap in a 18spd .74 it will be the same. Good to know.
264 Rear Axle Ratio???
Discussion in 'Trucks [ Eighteen Wheelers ]' started by whoopNride, Jan 26, 2013.
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I though most of the 10 speeds were .74, and the 13s and 18s were .73, but it has been a while since I have looked at the numbers.

Hi Cowpie 1.Thanks for your input for switching to 2.64 .I wont pass up the 3.o8,they seem like a good middle ground. I ll search a little more for the 2.64s.Would switching to 1122.5 rears after I wear my 285s out help any? Also what about mods to the c16 ? I know Cat recommends running it at 1325 but it seems pretty low,almost like lugging it.Any input from you or any other members would be very much appreciated.

The 7 speed can be had in .74:1 or 1:1 ratios
The 8LL can be had in .73:1, .74:1, or 1:1 ratios
The 9 speed can be had in .73:1 or 1:1 ratios
The 9 speed T2 is only available in .73:1 ratio
The 10 speed can be had in .73:1, .74:1, .75:1, or 1:1 ratios
The 11 speed (?) is only available in .73:1 ratio
The 13 speed is only available in .73:1 ratio
The 15 speed is available in .79:1 or 1:1 ratios
The 18 speed is only available in .73:1 ratio
The Autoshifts....
LHP, EHP, and Autoshift 18 speed are .73:1
LST is .74:1
Autoshift 10 speed can be had in .73:1 or .74:1 ratios
But this is what they list currently, and doesn't reflect past offerings. I seem to recall that, prior to ACERT, Swift used to run Freightliner FLD120s and Columbias with a 12.7 60 Series, and they'd run either a 9 direct with 2.93s (but some of the FLDs had a 10 direct with 2.79s), and some of those trucks with the same gear ratios did have 13 speeds (the trucks running the Rocky Mountain doubles had them, and it was an option for lease operators). So it would seem that a 13 speed direct drive is possible, and I recall someone else on this forum saying that they were running 2.93s with a .87 OD transmission... while Eaton Fuller lists the ratio on their site, it says they don't currently offer anything with that ratio, but it seems likely they had in the past. 

When you were heavy did you have any startability problems? Did it ever feal hard on the clutch when pulling away on an up hill grade?


Cowpie do you have some numbers? What kind of mpg are you seeing? What rpm does the engine run at highway speeds?
Im specing 3.08s with an x15. I would go lower but then I would lose the diff lock. I was also considering going to 2050 torque instead of 1850 to make up for the low ratio, but then im afraid id lose mileage.... 
Detroit 60 500 hp 1750 torque turning 1400 at 62 mph and 1500 at 66 mph. 18 speed running in 16th (direct drive) with 2.64 diffs on low pro 22.5 rubber. Average mpg over 926,000 miles has been roughly 7.5 mpg.
3.08 would give me roughly the same engine RPM in 17th (1st OD) instead of 16th (direct) with 2.64. I would have no problem with that. Could offer a little better start off compared to 2.64.
I have a spreadsheet where one can plug in different differential ratios, different tire revolutions per mile, and the various transmission gear ratios and see what the actual road speed would be at various RPM's. Here is a screenshot from it with 3.08 diff ratio combined with a typical 13 speed trans....
Given many would run the X15 at a little lower RPM than a Detroit 60 12.7, a 3.21 ratio might work a little better, depending one what you are doing with the truck.....
Last edited: Jan 12, 2019
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