Info on Changing rear end
 

I have a2000 freightliner fld det engine./ 12.7 I have 10 spd eaton transmission with 390 rears. I am considering changing to 355 rears.What is the cost of rears and would I need to have my speedometer calibrated due to the change in my rears , Any info will be greatly apprecited.

Thx in Advance

Wow I want to do the exact opposite change!

i would recommend going to the 3.55's. I am running a factory rebuilt pre-egr 12.7 series 60 in my new FL glider, and am running 2.64's behind an 18 speed. The idea is to run primarily in 16th (direct drive) for maximum power to rears and max efficiency. It is working real good in every situation I have put it in since August. Now, that being said, 3.55's on a 10 speed OD is virtually the same road speed at the same RPM as my 2.64's running in direct drive.

The cost, depending on where you go, from what I have heard from others that have done it is around $3K to do both rears. But don't begin to hold me to that, since I have not done it myself. But, by getting the engine RPM's down from running 3.90's, you could theoretically pick up enough mpg gain to counter the cost in a year. With my setup, running similar to what you are wanting to do, I am averaging a calculated pump to pump mpg in the mid 7's. With lighter loads and smaller rolling hill running, I can get into solid 8 mpg territory on a fuel fill. I filled up yesterday and got 8.02 mpg calculated. Don't know what the ECM is showing as I do not have a mpg display to tell me.

At 65 mph, your RPM's would be near 1475 RPM with low pro rubber. Subtract 50 RPM, more or less, if you are using tall rubber. Mine seems to do real exceptional between 1350 and 1450 RPM (60 - 64 mph) for normal cruising. Very nice balance of economy and power. Yes, you would need your ECM changed to reflect the new rear ratio for your speedometer to be accurate.

wow.. i always was told detroits liked more rpm. good to know! great advice copperhead.

When I had to replace my front rear it was $2100 on the counter, oil another $150, then there's the labor, I did mine myself.
That was a NEW not exchange Eaton rear with a 3 year warranty.

Back rear is around $15 to $1800, plus oil, I think with labor and everything else, you are looking at an easy 5 grand minimum, got to be a significant improvement to justify the cost IMO

I do step it up on the RPM's under a hard, long pull. But for regular cruising, Series 60's do exceptional in the range I mentioned. You just have to figure out what rear will give you the road speed you are wanting to run and keep the RPM's in that range. Once mine starts to move below 1300 on a hill, I usually split down and get the RPM's back up. If I am close to the top of the hill and about ready to go over, I will sometimes just let it pull at 1250 - 1300. Just have to use a little common sense. I hardly ever take the engine over 1600 for anything except maybe engine braking on a downgrade. And since i have the splits of an 18, most of my shifts up occur at the 1500 - 1550 range. Sometimes a tad higher on a real hard pull.

The earlier Series 60 engines did do quite well running a little higher. But the DDEC IV ('98) and newer engines do quite well running below 1500. Whether it be the 12.7 or the 14L. Now, I would also recommend to take the time and consider a higher flowing exhaust manifold, turbo, and mufflers to make this all come together. Before I dropped in the engine in this glider, we put on a Bully Dog ported/polished/coated exhaust manifold and got rid of the wastegate turbo and put on a larger non-wastegated Borg Warner turbo. We also removed the OEM mufflers off the stacks and put on Walker Megaflow's. I believe these mods did much to improve the overall performance of the engine at the RPM range I have been running it at.

this is a bit off topic but.. shifting at 1550 is wayyy too high.. i love my cat! haha only time i bring my truck over 1500 on a shift is when i'm pulling super super heavy (80k+)

are the mufflers loud? being i got a 04 freightliner with the MBN, i was told that the factory freighliner mufflers have cat converters in them. mine are pretty well blown out and i HATE loud. just waiting for a hole in one of them so i can replace them. just do not want the cat converters.

With low RPM it just can't handle the road...No offense, just kidding....

lol i drove a 500 detroit with a double over 13 in it.. my god was it a dog! again.. no offence.

My first question to that would be, "what was the rear end ratio and tire size?"

Well, to be honest here, the condition of the engine means as much as brand name. My 500 HP DDEC IV Series 60, in front of an 18 speed which is hooked to 2.64 rears, does as well as anyone else out here. I can walk right along with everyone on a hill pulling heavy. And consistently get into mid 7 mpg territory with several bumps into 8 mpg territory. Now to be fair, each engine is different in what RPM it does it's thing best. Detroits will need a little more RPM than the typical Cat or Cummins engine. But how well they do side by side has more to do with gearing than anything else. Just as in real estate it is all about location, location, location, with engine performance it is all about gearing, gearing, gearing. You can't take typical office mentality fleet specs and make a solid determination on which engine is best. They each have their certain "quirks" and you have to know how to use them properly. There is not one engine, Cat, Cummins, or Detroit that I have not been able to pull good mpg with and it worked fine under all loads I have put it thru, at least since the mid 90's and before all this emissions stuff came about. I have gotten 7 mpg averages out of Cats, Cummins, and Detroits on a regular basis. If things are spec'd properly, all of these brands of engines will do very well. You can't just strap the same drivetrain on each of them and expect the same result.

Now also to be fair, I did replace the stock exhaust manifold with a Bully Dog ported/polished/coated one, and I dumped the stock waste gated turbo for a Borg Warner 171702. Complete with high flow mufflers and Fleet Air Filter. I am sure these have something to do with the performance I am getting out of that Detroit.