• Thank you for visiting HeavyEquipmentForums.com! Our objective is to provide industry professionals a place to gather to exchange questions, answers and ideas. We welcome you to register using the "Register" icon at the top of the page. We'd appreciate any help you can offer in spreading the word of our new site. The more members that join, the bigger resource for all to enjoy. Thank you!

73' Kenworth in the slow lane!

roadrunner81

roadrunner81

Senior Member
Joined
Dec 22, 2009
Messages
275
Location
Tacoma Washington
Occupation
Managing member KSR Excavating, LLC
Picked up a 1973 Kenworth tractor not to long ago with a fresh big cam cummins and a 13 speed rto 1213. Problem is that I,m cranking along at 18-1900 rpm at around 55mph. I tried looking up tranny specs for ratios but seems to me that rpm and road speed combined with 24.5r11 and 4.33 would give me a 1.00 ratio. If I try a .85 It shows 65 at same engine speed? I really would like to find another transmission that would allow me to do 65 or so at same or less engine speed and looking for pointers? Found a rebuilt rto 14613 on local cl just want to avoid being in the same boat.
 
W

wilko

Senior Member
Joined
Mar 4, 2005
Messages
362
Location
Oregon
Sounds more like a 5.38 rear gear. I don't know which transmissions had an OD top gear, so I'm no help at all, but I'm guessing that truck was built for logging or as a dump truck in mountainous ground.
 
S

Super S

Active Member
Joined
Apr 29, 2012
Messages
28
Location
United States
If the truck pulls good enough that you don't need the low hole you could always turn it around to give you a higher final gear. Don't know exactly how it's done but I've driven before and after and it would probably get you where you want to be on the final ratio.
 
0

02Dmax

Senior Member
Joined
Dec 4, 2007
Messages
687
Location
MO
That rto is an overdrive tranny. The "o" in the model means overdrive. It must have some crazy low gears in it.

Sent from my SPH-L710 using Tapatalk 2
 
powerjoke

powerjoke

Senior Member
Joined
Aug 2, 2009
Messages
1,125
Location
Missouri
Occupation
owner/operator/estimator/mechanic/grunt/ditchdigge
I don't think that is what the "o" means...... If it was a overdrive it would have a B at the end ?

Pj
 
willie59

willie59

Administrator
Joined
Dec 21, 2008
Messages
13,386
Location
Knoxville TN
Occupation
Service Manager
Roadranger nomenclature:

RT: Roadranger, Twin countershaft
RTO: Roadranger, Twin countershaft, Overdrive
RTLO: Roadranger, Twin countershaft, Low inertia, Overdrive
RTAO: Roadranger, Twin countershaft, Automated, Overdrive.
 
td25c

td25c

Senior Member
Joined
Feb 14, 2009
Messages
5,250
Location
indiana
roadrunner81 said:
Picked up a 1973 Kenworth tractor not to long ago with a fresh big cam cummins and a 13 speed rto 1213. Problem is that I,m cranking along at 18-1900 rpm at around 55mph. I tried looking up tranny specs for ratios but seems to me that rpm and road speed combined with 24.5r11 and 4.33 would give me a 1.00 ratio. If I try a .85 It shows 65 at same engine speed? I really would like to find another transmission that would allow me to do 65 or so at same or less engine speed and looking for pointers? Found a rebuilt rto 14613 on local cl just want to avoid being in the same boat.
Click to expand...

Almost sounds like you have a Direct 13 speed transmission or the rear end ratio on the tag is wrong.Like other membors have stated the"O" in RTO means overdrive. I had a customer with an 1982 Frieghtliner with RTO9513 and 4.88 rears on 24.5 rubber and the governor bumped out at 60MPH.Thats pretty close to where your at roadrunner81. Might double check rear end ratios. A helpful formula I have used to figure engine rpm.............

MPH X axil ratio X 336 and divide by tire diameter= RPM'S . That formula works for a direct drive transmission .
 
W

wilko

Senior Member
Joined
Mar 4, 2005
Messages
362
Location
Oregon
Jack up one corner and count number of turns on the driveline to make one turn of the wheel.
 
0

02Dmax

Senior Member
Joined
Dec 4, 2007
Messages
687
Location
MO
You need a whole axle jacked up and wheels turning evenly to figure it like that. One wheel up will spin the spiders.

Sent from my SPH-L710 using Tapatalk 2
 
0

02Dmax

Senior Member
Joined
Dec 4, 2007
Messages
687
Location
MO
R is for roadranger, T is for twin countershafts, O is for overdrive.

Sent from my SPH-L710 using Tapatalk 2
 
Last edited:
D

Dualie

Senior Member
Joined
Feb 23, 2007
Messages
1,371
Location
Nor Cal
If you want more road speed i would find a nice air ride cut off from a late model truck with better gearing. dont know what your using the truck for but a set of 4.10's or 3.73's would be good for normal every day running down the highway use
 
B

Brandt

Well-Known Member
Joined
Apr 20, 2010
Messages
197
Location
Wyoming
I'm running 4.42 gears with a .78 OD and 11R-22.5 tires and get the following:
55mph=1600rpm
60mph=1740rpm
65mph=1875rpm

I think it is your transmission - having a .87 or .89 OD and not a "double" OD
 
mike in idaho

mike in idaho

Well-Known Member
Joined
Nov 22, 2012
Messages
58
Location
north idaho
02Dmax said:
You need a whole axle jacked up and wheels turning evenly to figure it like that. One wheel up will spin the spiders.

Sent from my SPH-L710 using Tapatalk 2
Click to expand...
With one wheel jacked up, turn the wheel TWO complete revolutions and count the driveshaft revs. We have a truck in our fleet with 4.56 rears and an 18 speed, runs about 1950 rpms at 65 mph.
 
Last edited:
You must log in or register to reply here.
Top