If U were spec'ng a truck for fuel mileage...

Same HERE had a RTLOO Double over 13 that had a .5 over turning 3.55 rears on 24.5 Big rubber with a Big Cam 4 Truck needed 1600 For Best Fuel Mileage so 55MPH was 11th GEAR. 65 I barely hit 12th. I never saw 13th til minumum 75mph.

 

Doesn't Pittsburgh Power have a "Signature" truck running 2.64 rears? If memory serves, the one I read about is a 2011 389 Pete running a fully decked out Pittsburgh Power 6NZ Cat, 18 speed, 2.64 rears, on 22.5 singles. They claim 8.5 mpg at 70 mph in 16th, and it's throwing down over 800 horsepower. If I had the money to spend, I would DEFINITELY consider this setup, but the truck is between $150-180K.

 

I heard the same thing about the PP glider. I'm considering putting the 2:64s in my 379 Pete with my 13 speed and my PDI Stage 1 tuned and properly set up ACERT. Even pushing 750hp more or less my tranny never runs over 150 - 170 except in long hard pulls. With the 3:36s I turn 1300 running 62mph. I think I can do the same in 11th and the only difference will be starting out in 1st instead of second.

 

reverse will be alot faster

 

I have STOCK 2000 Pete with 3406 and super 10 transmission and 3,55 rears .I get average 6.5 mile per gal. I WILL NOT CHANGE IT TO anything else. Try it you will love it.Yes I am an Old Fa.t but it works.

 

my company truck has a 13 and a post04 475 cat. Its a TMC truck. I average 6-6.25 depending on the load and that 475 has the perimeters pulled way back. Down shifting before 1100 RPM from 13th does not happen. It has 3.36 gears. It does pretty good though and is very reliable so far. 563K miles on it and it is running great!

 

You're right, I never thought of that and pulling a RGN double drop when hauling a heavy load it might make backing under and running the trailer up the frame ramps to the 5th wheel difficult to impossible!

Does an 18 speed have a lower reverse gear than a 13 speed?

 

Yes 13 has a 13.22 rev ratio 18 has a 15.06 rev. and a 12.82.

 

Why would running a 2.64 ratio in a lower gear gain anybody fuel mileage vs. running a 3.34 in top gear with rpm being the same? You've still got to match the tire size, gear ratio and rpm to maintain mph. In essence your still running the same final drive ratio, just in a different gear. Whats the point? Were not talking about 1000 RPM here, were talking about a few hundred RPM spread max where the truck is in its fuel efficient range.

 

The theory is based on running the transmission in the direct ratio gear(1:1). Running it in that gear is more efficient for the tranny than running it in the final overdrive(.73-.75:1 ratio) due to turning one less shaft. Running the direct tranny gear with a 2.64 rear end gear gives you the same final output ratio(rpms) as you would get running in top gear with a 3.55 rear end gear. Granted this philosophy doesn't achieve a huge fuel savings, but it does help some. Probably .2-.3mpg. Not enough to retrofit a current truck with new rear gears, but possibly worth specing a new truck that way.