Where to replace rear end ratio?

Have you considered taller tires? If currently running lo pros, maybe tall tires would get you halfway to the 1500 goal.Maybe 1650 @ 70 mph? Top of Max torque range. Don’t really know if the # s are accurate. Just a thought if needing tires soon anyway. Economically cheaper

 

@MM71
Did you change ratios or kept them the same?

 

It would not have changed the price if I did, but I did not. The price I quoted should be a worst case scenario, unless labor rate is higher. Full on warrantied Eaton Reman 3rd members. My old ones were used as cores. Removing a 3rd member and sending it to a gear shop should be much less expensive, I did not have the luxury of time, so went with drop in replacements.

 

If 1500 rpm is optimum, then that should be the priority reason to change out gears. Your fuel savings will be minimal as you are driving a barn into a hurricane.
Any shop that services class 8 rigs should be able to do the swap. Google your locals. If you not familiar with this sort of repair/maintenance procedure, I would suggest either having a friend that is do the question asking, or get some recent reviews of the shop's work and honesty.
The procedure is very simple and straight forward for a properly equipped shop. The wheels are not even removed. The drive shaft, and any air lines to the gearhead are removed while the lube is draining, the axle shafts are pulled, and the gearhead is supported with appropriate tooling while the banjo bolts are removed. Gearhead is removed from housing, then either a new or reman gearhead installed, or you gearhead taken to the bench for gear swap. Your bus however, may present some extra work depending on component orientation. A rear engine pusher might get a bit tight for gearhead removal.
You would not believe the price differences between brands and models of drive axle parts. Get the info from your axle when pricing You do not want to get a quote for a common eaton or meritor, then find out you have some special reverse rotation system. You might even find limited ratio ranges for your axle. It all depends on brand and model as well as rotation of pinion.
IMO.

 

Look under the rig, or better yet, roll under it on a creeper and have a look see at the driveline length. They'll need, and this too is reliant on the gearhead shape, but about a minimum of 18" of free space between the output yoke of the trans, and the pinion yoke on the gearhead. The more room, the better, but if properly equipped, it is amazing how tight a good gear man can get things done in. I dont know, but assume, your pusher axle is rearward of the drive axle? This is good as it should ad space to work in.
A 26K Meritor is good. That's a good heavy housing, in conjunction with the pusher, can handle a lot of weight, and be stable on the road.

You may see some small mp incease, I dont know for sure. But as a "rule of thumb" so to speak, this is a math and physics question, as it takes X amount of energy to push your weight and wind resistance forward. That is a constant that does not change with gear ratios. So how efficient your engine is at that particular power consumption, is about all you can do for mpg gain. So if 1800 rpm is 300 over that engine's max efficiency, then whatever the percentage efficiency you gain there will be the majority of your mpg gain. IMO of course.
When you get right down to it, a direct trans is the most efficient. We're talking very minor gains, but gains none the less. OD transmissions have the same parasitic losses when in OD as lower gears. Direct is straight through from the crankshaft to the pinion. No real losses other than mass rotation and lube slinging. So, IMO, you're on the right track by looking at rear gear ratios to accommodate the direct drive trans.

Tires?.......that is a hornets nest when asking for opinions. If someone can bring hard data to the table on a brand size that proves increased mpg etc., then good. All other comment is opinion and should be taken as such. That said, and there are articles by reputable authors online, that show street tread as being better for mpg than traction. I can not bring proof, so.......however, at one time when I was young, (Fred Flintstone was my neighbor at the time) I worked at a tire shop for several months. When putting over size traction tires on a pickup, the heavier the lug and open pattern, the harder it was to get the balancer up to speed. One set was directional tread and we could only machine balance 2 of the four as the 2 that ran backward on the balance machine, would catch so much air it would not allow it get to speed. That noise on the road traction tires make, is power consumption. Believe it or not, it takes power to make that noise.
So, again IMO, Stick with what's common and available at your chosen tire shop and those that you go by on the road.

I imagine your detroit is already electronically tuned for the bus application. But I have a 12.7L in a freightliner FLD120 that is one of my class8 rvs. When asking around, because it was built for commercial use, it was tuned for that app. If I want, and I may, I can send the ecm off to have it tuned for it's new use as an rv. This would get better mpg, but at the cost of other aspects, which I dont care about not being in commercial transport.

 

Have you figured out how much of a gain in MPG it would take for this to be a profitable swap? If you gain .5mpg and only drive 50,000 miles a year will that be cost effective? You haven't said what you do so we can't answer that question. Just make sure you aren't spending dollars to save dimes.