2017 CXU - 2.64 DIRECT M-DRIVE

[CXU613R9]

Currently running CXU-613 w/MP8, M-
Drive, and 3.21 -3.23 rear ends.  I am looking at a 2017 CXU 613 MP8 445E with the direct M-Drive 2.64 ratio w/ 24.5 tires and transmission ratio 14.94.  Hauling 25 tons 85% of the time, on flat grade.  Will this truck be weak, and/or a fuel saver?

Thanks for any advice.

 

[wiserfrombud]

I wouldnt think it'd be a whole lot different.  I dont know much about the M drives but im guessing the one you have now is a single overdrive???   If you are after ultimate fuel economy, just curious why you're opting for 24's?  Do you require the clearance for some reason?

[CXU613R9]

No. Would rather 22.5's but the truck has 24.5's on it that's for sale. 

[wiserfrombud]

Well 22's should be able to be negotiated into the deal if you pushed for it.   I dont anticipate it being a night and day difference from what you have now but every bit helps.  22 low pro tires with low rolling resistance, direct transmission....it all adds up.  What hp is the truck you have now?  And as i asked earlier....that is a single overdrive M drive?

[CXU613R9]

The trucks I have now are single overdrive gear with MP8 445HP.  

[MackMann85]

You have to look at your overall drive ratio.  Take top gear multiplied by the axle ratio and you will find your overdrive transmission with 3.21 rears is actually geared faster than the direct drive trans,  Meaning you may like how the direct pulls slightly better but the cruise rpms may be higher.  So the advantage you get with direct drive may be a wash with the higher cruise rpm.  But it wont be weak like you are worried about.  This is assuming tires are the same between trucks so you'll have to take that into account if not too

[kt_Engineer]

The Mack super Econodyne (445SE) is spec'd with OverDrive mdrive (top gear ratio of 0.78:1) and rear axle ratio of 2.64:1 (Meritor 14x axle). This will be best fuel economy package with 22.5 tires. You will be cruising @ 65MPH at 1150 Engine RPM in OD gear and when you downshift the transmission will be in Direct Drive 11th gear (1:1) with no losses in transmission gear mesh even though engine will jump up to 1400 RPM to maintain ~ 65MPH. You will get double whammy fuel savings with this combination.

If you want to stick with 24.5 tires and Direct drive transmission, I highly recommend you consider switching rear axle ratio to 2.28:1 Meritor 14x (or 2.26:1 Dana Advantek). You will be at 1150 Engine RPM @ 65MPH with this combination and get better fuel economy.

Edited July 13, 2017 by kt_Engineer
sentence correction

[Lmackattack]

correct me if im wrong but wont a trans with a over drive final gear driving very fast rears give you very poor pulling ability. I can see how it might be better on fuel if the engine can maintain such low RPM on cruise but once you hit any slight grade you will need to downshift to get the RPMS back up?  

[kt_Engineer]

13 hours ago, Lmackattack said:

correct me if im wrong but wont a trans with a over drive final gear driving very fast rears give you very poor pulling ability. I can see how it might be better on fuel if the engine can maintain such low RPM on cruise but once you hit any slight grade you will need to downshift to get the RPMS back up?  

it depends....if you are speaking about gradeability in OD gear, it matters how much HP and how your torque/HP curves look like. Most of the new diesel engines can produce constant power/HP over a wide band (~ 1000-1500RPM) and normal 3% grades will not cause to down-shift to maintain vehicle speed. When you encounter steeper grades is when it will be hard to drive in top gear. Being in Direct-Drive or under-drive will allow you more flexibility in terms of maintaining those gears for longer periods of time when compared to being in Over-Drive gear. If you are in a linehaul application, OD transmission definitely has advantage as of now but faster axle ratios being introduced will be a game changer going into future and Direct-Drive will be more beneficial (OD with faster axle ratios will push you into <1000 Engine RPM @ 65MPH and vibration issues will become predominant in this RPM Range).