Driving for economy 12.7

Mammoth Transport

Well-Known Member
i have a 12.7 Detroit with a EF 13 speed. 24.5 low pro. At 62mph tachometer reads 1650 rpm. How does one drive this truck for economy at 80gross.
 

Mike

Well-Known Member
Staff member
1400-1450 rpm at cruise. With lighter loads, you can easily run it in the 1300-1400 range.
 

Blood

Driveler Emeritus
Supporter
1400-1450 rpm at cruise. With lighter loads, you can easily run it in the 1300-1400 range.
+1

I have a 12.7 with 2:93 rears and a 13 [double OD].
I run in the mid-sixties in 12th gear an right around 1450 yielding just either side of 7 mpg [until recently].

This truck was born with a 10 speed direct so running 65 mpg would be tach'd out at around 2000 R's as I recall.
Couldn't break 5 mpg without getting under 60 mph.
:coocoo:

I rarely use 13th [unless I'm bob-tailing with a tail wind] but this SOB will run 74 mph at 1400 R's in 13.
:)
 

Mammoth Transport

Well-Known Member
Great! That means I will be stuck at 55 in 13th. I've got no height room for bigger tires. I wonder if I could get closer to direct by changing the sprockets in my rear end. (I know sometimes the jokes write themselves)it seems like running a reefer I am always running as fast as I can to make on time delivery. I wonder if I could change the differential so I could at least get a good rpm at both 65 and 55?
 

Blood

Driveler Emeritus
Supporter
What is the gear ratio of your drive axles?

What is the ratio of your trans in 13th gear?
 

Mammoth Transport

Well-Known Member
I don't know how to tell except it must be wacky because I am slightly above 1700rpm at 65 with 24.5 lo pros. 1800 gets me to 70mph in 13gear.
 

mndriver

curmudgeon extraordinare
Supporter
LP 24.5 is 492 rev/mile (Eaton Fuller says 501)
Tall 24.5 is 470 rev / mile
LP 22.5 is 507 rev / mile
Tall 22.5 is 492 rev/ mile.


Typically what is programmed into ecm's any way. Mine is showing off about 10%. After 6200 miles on the odometer since 12/1, I have a 10% error compared to my GPS.
 
Last edited:

mndriver

curmudgeon extraordinare
Supporter
i have a 12.7 Detroit with a EF 13 speed. 24.5 low pro. At 62mph tachometer reads 1650 rpm. How does one drive this truck for economy at 80gross.
13 speed tranny has a 0.73 and 0.86 over drives for direct 12th gear and 13th gear. 7 OD (11th) is really your direct drive. Why the Eaton Fuller 13 spd is also called a 13 Double Over. Because both of the big hole gears are overdrive.

So either you are driving a truck set up for dirt work or heavy haul.

I'm guessing you'll be in the 4.33 or 4.56 gear area.
http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

roadspeed.jpg


If you want to CRUISE at 65 MPH at 1450 RPM, you'll need to swap in gears like 3.58 to 3.90.

roadspeed_1400.jpg
 
Last edited:

Blood

Driveler Emeritus
Supporter
13 speed tranny has a 0.73 and 0.86 over drives for direct 12th gear and 13th gear. 7 OD (11th) is really your direct drive. Why the Eaton Fuller 13 spd is also called a 13 Double Over. Because both of the big hole gears are overdrive.
They also make a single overdrive 13 with a final ratio of around .86.
That's why I asked.
He either has a single OD or [like you said] really low gears.

I don't know how to tell except it must be wacky because I am slightly above 1700rpm at 65 with 24.5 lo pros. 1800 gets me to 70mph in 13gear.
What make and year is your truck?
There should be a tag on the door or in the glovebox that tells you what the ratio of your drive axles.
Failing that there should be a metal tag on the axle.
 

mndriver

curmudgeon extraordinare
Supporter
yeah, they made the single over 13 spd.....Back in the 70's and 80's wasn't it.

So the likelihood that it's in anything on the road today mainstream is rather low.

You used to be able to get the product specifications without filling out an online spam emailer for Roadranger/eaton fuller. But you need to sign up for their literature spewer now to get it.
 

Blood

Driveler Emeritus
Supporter
yeah, they made the single over 13 spd.....Back in the 70's and 80's wasn't it.
I think they came back for a period there.
The bean counters are always coming up with fantastic ideas to save pennies.
A lot of them don't make sense from the start [except on paper].
Like 20 gallon fuel tanks on reefers and 70 gallon tanks on OTR trucks.
That was big for a while [with the mega's].

Save a couple of hundred pounds [max] on weight in a reefer...
then pay a guy to come by and fuel it every day if it's a drop trailer or sitting over a weekend.
:coocoo:

It's hard to feature a [road] truck spec'd with around 4:50 ratio.
 

Injun

Rabid Squaw
Staff member
Supporter
13 speed tranny has a 0.73 and 0.86 over drives for direct 12th gear and 13th gear. 7 OD (11th) is really your direct drive. Why the Eaton Fuller 13 spd is also called a 13 Double Over. Because both of the big hole gears are overdrive.

So either you are driving a truck set up for dirt work or heavy haul.

I'm guessing you'll be in the 4.33 or 4.56 gear area.
http://www.roadranger.com/ecm/groups/public/@pub/@eaton/@roadranger/documents/content/ct_062746.swf

View attachment 32723


If you want to CRUISE at 65 MPH at 1450 RPM, you'll need to swap in gears like 3.58 to 3.90.

View attachment 32724
I have 3.58 Rockwells and E-F 13 (a converted 9). At 1,400 rpm in 13, I'm running 62 mph. Cummins 14.7 ISX 435.
 

mndriver

curmudgeon extraordinare
Supporter
you forgot your tire size.....

LP 22.5 and LP 24.5 are two entirely different settings.
 

Mammoth Transport

Well-Known Member
They also make a single overdrive 13 with a final ratio of around .86.
That's why I asked.
He either has a single OD or [like you said] really low gears.



What make and year is your truck?
There should be a tag on the door or in the glovebox that tells you what the ratio of your drive axles.
Failing that there should be a metal tag on the axle.
Tag on the door says axle ratio 411.
99kw t600

Tire size Yokohama RY023 285/75r24.5
 
Last edited:

Latest posts

Top