Gear Ratio Confusion

10 overdrive with 3.55 will put you at 1625rpm at 75mph.

You'll probably be using in the ballpark of ~250hp at that speed, so a 12.7 is giving up ~1% efficiency compared to being geared for 1500rpm.



You're giving up another 2-3% efficiency being in an overdrive.


So if you got 6.5mpg running 75 with that 10speed OD/3.55, you might get 6.75mpg with a 13-speed direct & 2.31's...

 

Any particular reason you feel the need to run 75 mph. Load/service requirements? Timing, etc.

 

Are you saying you wanna turn the truck up to 75 or are you mainly operating out west and wanna run with the big boys. Cheap, fast, and reliable. Pick two. (Kinda like building a race car)

 

I must be missing something. The transmission gearing is important in this. But differential gearing is the part I don't get. 2.64 means fewer rpms per axle rotation, right? and 3.74 (my differential ratio) means engine revolutions per axle revolution, right? So a 3:74-1 differential will cruise slower at 1,500rpm than a 2:64?

I say this because my truck, 60 series 14L with 3:74 gearing is not a fast cruising truck. It'll pull a house, but not at 70mph. I do get 7-8mpg at 65, which is a good thing, but it would be (maybe) more profitable to get there a little quicker and hook up to another load. Drop and hook, pre-loads, if they're ready when I'd show up quicker, anyway.

 

Generally trucks with direct drive transmission use rears under 3.00, overdrive trannies use rears higher.
In top gear a direct drive transmission does no multiplication or division. Overdrive transmissions in top gear divide the engine rpm by 0.73 on average.
Then there's the issue of startability. But you're not inquiring about that.

 

Same thoughts here.

I got wrapped up a bit too much in the gearing when looking for a truck - it limited the choices considerably.

It's rare that I need to travel faster than 70, monitoring the scan guage and tank to tank I found I was getting good MPG at 64mph, my 90 day average is 6.7 winter /7 summer pulling open deck.

I tend to cruise along in an open space, and just pass slower traffic when needed.

70 mph for 10 hours gets you 50 miles further than 65 does. My schedule is normally not that tight, and I feel higher speeds are detrimental to maintenance costs and fuel.

After I realized this, I set about increasing revenue, and decreasing costs.

 

I dont no about all these numbers and ratio's. I do no if you have like 4:10(like garbage trucks do) it will pull anything but isnt fast. If you have 2:10 (like my old burlington truck did) it will run its ### off. But if you even say hill, you will start down shifting. Now my last truck a 1997 ftl century had a ser 60 475hp 13sp eaton fuller and 3:55rears. Could run great (not the fastest maybe 3rd place) climb up hill well (pass most but not all) and give me 6.5-7 mpg driving 72 mph regardless of east coast or west coast all over the country. So for me i say 13sp trans and 3:55 is just right. (Others will say 3:70 but there almost equal

 

75 mph pulling containers? You'll learn soon enough why that isn't practical. 3.36-3.58 is the area you'll want to be in with an overdrive transmission. The flatter the operating area, the lower you can go, numerically.