Double over 13

The 13 double over is overdrive in the front and rear box totaling 38%? If ran in 10th direct that is the same as a standard 9 or ten speed in high. That adds three more gears with splits to maintain RPM in the best range.
Some things changed over the years. Even New Mexico has a speed limit. HP and torque on the average engine has greatly increased along with the torque band RPM range. The little engines that could needed every gear in a 9 speed all day on a regular basis if heavy. Most of them were geared around the 4:11 range and ran about 60-65 anyway. Those extra gears were for a little extra run down before the next hill or running empty. There were a few exceptions with some big engines.
The need for them is gone replaced by high torque low RPM engines and lower numerical axle ratios In my opinion. There are 18 speeds and such for heavy haul.

 

A 3:70, 11R 22.5, .62 overdrive would run about 75 mph at 1400 RPM. Mass, inertia, wind resistance and other things come into play. My wife's Kodiac likes to run at about 78 but gets two miles per gallon less than running at 68. Keeping the left door closed and the right foot action often have a larger impact on mileage than many other things.

 

The 13 speed in high range is designated 5th direct 5th over, 6th direct 6th over and so on. Not 5-6-7-8-9-10-11-12-13
The 13 double over comes from 8th over and the stick pattern of the 10 speed overdrive which means the last gear is shift toward the front of the truck. I hope this isn't confusing. I can't draw a diagram on my tablet. Maybe some one with a windows 10 can draw a diagram on this page?

 

The double overdrive 13 speed is so the newer engines can run at lower rpms with the rear diffs in the 370, 390 range. Lately, with the lower numbered rear end ratios, 274, 279, etc, the truck can be driven in 11th, (direct) when loaded, and when mty, can be driven in 12th, first overdrive, at lower rpms at higher speeds.

 

I don't want to confuse anything, but to some of you wondering about double over, it's a chance to go beyond direct and reduce your rpm's for a given speed and fuel burn per hour (Miles) provided your weight is good you will be able to have the cruising at speed without the excessive fuel burn over horse rpm. You would slow the engine to above torque where it's sweet spot is and really go.

Double under which I am more familiar with is there to save you the actual shifting. All you have to do is raise your fuel pedal and put it back down and you will gain half a gear. Which is more economical and steady compared to rev'ving to downshift across the entire RPM chart plus saving against the possibility of a missed gear.

I want to caution some of you talking the devil in the details enjoying this topic to keep it simple for many who likely will never use one of the 13 speed double anything in trucking. It's a very nice transmission and I love it a great deal. The heavier you go (Are) the more you want such as 15's and 18's And you can walk away from anyone with the basic 10 speed and save fuel doing it.

 

On most 13 speeds 12 and 13 are both overdrive gears ?? Are you perhaps thinking on triple over??single overdrive transmissions are rare and most were for lower torque motors

 

The term double over comes from overdrive in the front with an up against the dash high gear. They also have an overdrive aux box. The aux gear is about half as wide as it needs to be. The countershaft bearings in the rear section are always wore out even with the updated covers. And it's gonna howl. The only way to pull .62 at high speeds is horsepower. Horsepower creates heat which burns up any fuel savings you think you made. They actually have 3 ranges above direct.

The later RTLO 13:s are often called double over by the misinformed because they have 2 ranges above direct.

 

Single overs are rare? Every 13 speed out before the RTLO 13s were single over besides the two RTOO models eating actually built.

Torque ratings up to 1500ft-lbs, and even then I would be afraid to run an old RTO12513 behind a higher torque engine.