3.73 vs 2.89 ratio with a Detroit ??

2.79 in direct, 3.25 in single over, and 3.7 in double over will all give you roughly the same road speed at a given rpm. with these ratios, 1400 would put you on 58 mph, 1500 would put you on 62 mph. All with low pro 22.5 rubber. Give or take 1 mph either way.

2.93 is a little goofy. in direct at 58 mph, rpm would be 1450. At 62, rpm would be 1550. In first over, 58 mph would be 1250 and 62 would be 1350. That rear really cries out for running 65 mph or more, or less than 57 mph all the time. In between those two ranges, the rpms are not in the best spot for a balance of power and economy with a Series 60. It would not be my first choice. Remember, when Swift got those trucks, they had their fleet turned down to 55 mph. At that speed, they did ok. In direct, the engine would be turning 1375. 1400 would put you on top of 56-57 mph. Now if you want to run that all the time, it would work. Else, you are going to have to move up a gear, step it out, and run considerably faster to keep the rpms in the 1400 range. Just not a lot of options.

In general, just on gearing alone, I would opt for the 3.73 with the choices you have. Unfortunately, choosing that gives up the larger sleeper. You might actually like the 2.93 after using it, but on paper, it sure looks like you are going to have to really play around with it and find out just where it does it's thing best. Kind of a bad deal if it doesn't work well. But then you could always change out the rear ratio. Cost effective if you are going to go with single drive axle. Just trading in the interaxle diff and the other drive axle would help defray the cost. In that case, I would opt for the 2.79 and run in direct, with next choice, 3.25 and run in single over.

 

The 2.79, 3.25, 3.7, 3.73 or2.93 are various gears in your rear end. The gears in your transmission have a ratio of .73 as the most common top gear. Direct gear would be 1.0 anything lower than that would be a overdrive gear. The two most common overdrive gears are .86 and .73. When you use rear end gearing such as 2.93 or 2.79 you are using your direct gear, and not using one of your overdrive gears unless you are going faster than 55 mph to 60 mph. Direct gear is more efficient and will give you better fuel mileage. I am not a gearhead, someone may come along that will explain it better.

 

Terry has it spot on. The concept is to keep it out of the overdrives for maximum efficiency. If one felt the "need for speed" then they could always move up a gear and still keep rpms at a tolerable level. The concept is widely used. If you notice, most Ford pickups (and others) have a "tow/haul" mode on the transmission selector lever. This is to keep the tranny from going into overdrive when towing or hauling heavy. The idea of spec'ing something like 2.79 rears is the same concept. You would primarily be running in direct drive (11th with a 13 spd or 16th with an 18 spd). This allows more engine power to actually make it to the wheels. The overdrives have a parasitic effect on power. This is why when a truck is put on the dyno at a shop, the testing is done in direct drive to measure the actual power the engine is putting out.

Now by using something like 3.25, you would get the same road speed in the first overdrive gear (12th in a 13, 17th in an 18spd) as you would by running in direct drive with 2.79. This way might be a little more practical for some who get into situations where they need that rear ratio to pull around in some challenging situations on the low side, like maybe a grain hauler. At least by running in the first overdrive is not as parasitic on power as using double overdrive is.

Now this all does work. I am running an 18spd tied up to 2.64 rears behind a DDEC IV Series 60. it does just fine. For instance, today I am pulling 46K of steel coils, up and down the rolling hills of I-80 in western Iowa, bucking 30 mph crosswinds. Truck does just fine. Obviously not as good of mpg as flat ground, calm wind, on a summer day, but still in the mid 6's. When things are more perfect, it is not uncommon for this truck to get into 8 mpg territory.

 

If the engines have been rebuilt check for a sticker on the enging that says LoNox, If one does not have it have that truck checked to confirm it. The Lonox setting will cost you 1/2 mile per gallon. If you find a truck that has not had the Lonox setting put in it that is the one you want, use a independant shop if you ever need a overhaul, the factory stores will mess up your mileage. To get the best MPG sent the ECM to a tuner, I like Pittsburg Power. (ecm tuners can not remove the lonox settings but can dial them back)

Lite load? Consider a single screw truck, more MPG, less tires.