RATIO

I just finished renting an '11 T600 day cab for a few weeks and I was surprised at how bad the fuel mileage was doing 65 - 70 mph.
I was getting ~5.3 - 5.5

 

I am looking at a T600 for otr... Any suggestions

 

Here's what needs to be kept in mind.... people always overlook what motor and power it had and that plays a huge part. I haul super b 140,000lbs and have 3 trucks.

2008 pete 389 c15 deleted 580rwhp 18 speed 3:36 24.5

2008 prostar 500hp 18 autoshift 4:11 22.5

2006 volvo 630 volvo d12 465hp 18 speed 4:30 24.5

According to many on this forum, the d12 is a dog and you can never get good mpg out of them yet due to the gearing I have it set at to compensate for low hp, it is my best mpg truck.

Next is the peterbilt with alot of power to compensate for the tall gears and heavy weight, this truck is barely behind my volvo in mpg but I am done work a hour earlier each day because I don't slow down or shift gears often whereas my volvo is always foot to the floor and mashing gears, slowing to a crawl on hills.

The prostar is $30 a day more fuel than the other 2 trucks so it adds up.

That d12 needs lower gearing to help it out. the mbe4000 and 12.7 are in the same boat. if you were specing a c15 or isx with 3:55 gears. .... the d12, mbe and 12.7 shiuld have 3:70 in order to compete.

Now looking at this from another angle.... My cat has to run higher horse to compensate, therefore that motor will not last near as long even with the fact that I never have to floor it..... it just won't. ... My volvo is always foot to the floor, balls to the walls so I assume it will not last as long either, although some may argue it will last longer running all out the way they were meant too.. (same reason marine engines last longer)..... but now my prostar has hauled 140k it's whole like and is geared right for the weight and not over powered and although it's worse on fuel, it's probably due to the fact that it is about to turn a million miles.

a million miles on a isx with dpf still intact hauling 140k is Devine intervention. And because this motors lasted so long while the other 2 were rebuilt long ago..... my properly specd prostar makes me the most money.

Alot more goes into speciING a truck proper than one might think.

 

just wondering how 3.36 gears could possibly be a good set up for b trains in a country where the speed limit is 60mph no matter how much power you have u ridding around in 16th gear?

 

110 km/h = 68 mph on the big road out west

 

I governed my truck at 105 km and when loaded I run in 17th. Look at what volvo is doing with the XE package.... detroit I'd doing the same, it's called down speeding.. cat did it in the early 2000's with the c16. run near peak torque for optimal fuel mileage...... can only do it with a engine capable of lugging.

That's why volvo built their engines with heavier bottoms now, to handle their xe13 and xe16. I had a 2014 volvo 730 xe16 with 500/2050 d16 ishift and 3:21 with 24.5 and that truck was designed by volvo specifically for the north American lcv market.

 

When my volvo salesman told me he was trying to sell.me a super b truxk with 321 and 24.5 rubber, I asked to have a new salesman and not the new guy.... but that truck would out pull a 475 cat with 3:90 and save 100 L of fuel a day.

 

Not to mention that Volvo knows how much load demand there is and will only slip into the big gear when there is low hp required, flat ground only really.

 

When specs are drawn up for a truck, there are a few ratios to take into account -
1: rear end gear ratio. This is where you'll see "3.36 rears", "3.55 rears", "4.88" rears", etc. in ads. It's the number of turns of the driveshaft to make one complete turn of the ring gear. So, 3.55 rears means that the driveshaft will make 3.55 rotations for one complete turn of the ring gears (and, typically, the axle shafts). The lower that number, the higher the vehicle is geared in that regard.
2: Transmission ratio. There are direct drive and overdrive transmissions. Overdrive simply means that the transmission's output shaft turns faster than the input shaft. A transmission with a .73 OD will have one complete rotation of the output shaft for every .73 rotation of the input shaft.
3: Tire revolutions per mile. Yes, that is also a ratio... there is a difference between running 11R24.5 tires and 295/75R22.5 tires, and that'll affect the whole equation, as well.

On top of that, there are other factors to consider...

1: Engine "sweet spot" - where is that engine running optimally? It'll vary between engines.
2: Terrain. A truck that drives mostly highway miles isn't going to be geared the same as a truck which does a lot of off-highway driving. And what works good on long stretches of highway may not be ideal for a local P&D truck which is going to do a lot of running from stop light to stop light.
3: Weight. You're not gonna gear a truck which will be grossing 250k the same as you will a truck grossing only 80k

And so on, and so forth. I'm out of time, so I've gotta cut this one short, but I'll follow up on it later.

 

you do realize your loosing alot of low end startability especially in reverse going from say 4.11 to 3.36 they I shift and M drive are running running a direct drive trans on those super tall gears there OD box has a .78Od so running 3.21 gears and there OD is like running 3.42 and a fuller .73 OD