[allan5oh]

Originally Posted by Graymist

Thanks for explaining the concept so well, Allan. Does that mean, therefore, that the higher the gear ratio, the more power that is generated ? Is that why heavy haulers, like the Volvo I drive, have a gear ratio of 4.11, as it means more power ?

Yes, it means the engine is turning at higher rpms, and more power is available. Not only is more power available, it is multiplied more.

As for the final gear, does a higher number result in a higher RPM at final gear ?

If everything else is the same, yes.

My truck cruises at its final gear ( 18-speed ) at around 1450-1500 rpm, at 100 km/h. Does a high final gear lead to better pulling power on hills when in the final gear ?

Yes, a higher numerical gear contributes to better pulling power in ALL gears if everything else is the same. If two trucks have the same tranny, but different gears, the truck with the higher numerical gear ratio will outpull the other truck while its in the same gear as the other truck. However, the second truck can just downshift, and it will have more multiplication.

Also, is it possible to get the gear ratios and final gear lowered to save on fuel, or raised for more power output ?

Yes it is done all the time. Costs several thousand dollars to change the gear ratio in the differentials. Final gear in the transmission can be changed by swapping the transmission. This is probably never done, as it's easier to change the gear ratio in the diffs.

Gear ratios generally range from 2.64 to 4.30.

Also, some transmissions have a direct drive(1:1) final gear. Supposedly this is more efficient because it churns less oil in the transmission. Of course, the gear ratio has to be changed as well. 2.64, 2.79, or 2.93's are found in these trucks.

If you want to fiddle around with final ratios, gear ratios, tire height and such, download "cummins powerspec". Very nice piece of software.

[solo379]

Originally Posted by allan5oh

I am considering switching to 22.5's,

I guess, you could do that. The way you ran, i mean speed, tires rolling resistance, requires the same, if not more power, than air resistance.
So, fuel efficient tires, would make a difference.
http://www.michelintruck.com/micheli...calculator.jsp
Look it up! 8)

[yoopr]

Well this means that your engine is about to let go :P
8.5 is pretty astounding

[roadranger]

I've been getting 8.25 with my old 1995 FLD120 with the bumper fairing missing! Part of that is it's only a single drive axle. 11.1L series 60 Detroit, 9 speed OD, 4.11 rear, 1R22.5's. She's turning 1400 @ 55 and 1500 @ 60. I'll stay at 55 unless I'm short on time when I'll do 60. On rare occasions I'll run at the governed speed of 66. Roof air deflector but no cab extensions. Fifth wheel back just far enough to clear the stack with the trailer in a corner. With 200 gallons I'm under 14k and I can't get 12k on the front axle without hitting the stack - might go to a weedburner to push the fifth wheel foward a bit more. Normal gross 50k (53' cubed out) but have hit 70k (40k cargo). Up here in the northeast I'm legal for up to 70.4k depending on state (12k + 22.4k + 36K) but can't get that last 400 lbs off the drive onto the steer.

[heavyhaulerss]

when i tell people that i get 7.0 7.6 mpg. they come back on the radio telling me what a lier i am. 11.1 det. with over 1 mi mi's keep it around 58-60 most of the time. no idle, even when fueling. no a/c. i use my alternate air wich is a 2-60... 2 windows down 60 mph :lol:

[Ian Williams]

How much of a fuel economy hit do you take from running the A/C in in a truck?


I'd imagine its not nearly as bad as a car given that its the same size compressor on an engine that's 3-5X as big.

[roadranger]

The A/C compressor doesn't take much - but the engine fan comes on a lot when the A/C is on and I know when mine was stuck on for a week I lost .5 MPG and can really feel the truck being held back on hills. I do run the A/C when I can't be somewhat comfortable with just the windows and I don't seem to lose more than a couple tenths. I also don't idle, use a fan clipped onto the window frame when waiting at a dock in this weather - but I only run the northeast...

[heavyhaulerss]

my engine is low enough on horsepower the fan takes about 15-25 horses away. imo. when idleing i'll hit manual fan switch to keep fan running constantly while a/c is on this way trucks stays cooler, but more important the fan selenoid will not keep clycling off/on & will last a whole lot longer. for me a rule i follow is if it not too hot & i'm not loaded heavy i may run a/c but if running it causes me to run fan a lot & temp to climb, i'll do the window down deal.

[rank]

I dream about mileage like that. I am at 6.2. 45,000 lbs through the hills of PA. 475 C-15, 410 n14...it doesn't matter. Drivers in the trucks don't much care about the mileage.

[roadranger]

Originally Posted by rank

I dream about mileage like that. I am at 6.2. 45,000 lbs through the hills of PA. 475 C-15, 410 n14...it doesn't matter. Drivers in the trucks don't much care about the mileage.

Have you thought about sharing any fuel savings with the drivers? Maybe split it 50/50? If I was't getting compensated for the extra driving time for going slower I'd have it to the floor all the time, too :shock: ! I have a rather unusual deal where I get paid per mile for the truck but am also paid by the hour and the company pays for the fuel, plates, permits, and insurance etc. Usually I'm not so busy that I can't slow down and increase my pay - works out to about a 50/50 split without even considering the lower wear and tear on the truck and tires 8) . The company truck I was driving before was at 6.25 (International Eagle w/ 500 Detroit) but the engine wasn't right - had no low end pull. Before that had an old ford w/ a Cat 3406E 410hp that got 6.75 (single drive w/ liftable pusher). If I drive the new truck like those old ones (65mph) I still get 7.75 :shock: !