8.5+ mpg

[allan5oh]

These past few trips I've been getting fantastic fuel mileage, checked at the pump of course(although my satellite is very accurate, and dash is about 2% out). Two trips ago was 8.5 MPG.

If my satellite is correct for the last trip I just did, I got 9.1 MPG! I'll be checking it tomorrow when I fill up to go on a trip. I'll let you guys know!

A few things I'm doing right:

58-60 mph
light loads(last trip averaged 20k)
no idling(satellite shows .3%)
easy take-off, coasting coming up to a ramp
~1300 rpms at cruise speed
105 psi in the tires
exhaust manifold is insulated using insulation from summit

A few things i'm doing wrong:

39" trailer gap due to my front axle being a pig(5th wheel should be even further back)
Michelin XDN2's on the drives

[allan5oh]

Actually this whole year has been good, I got 7.6 MPG in January(in Canada) and haven't got much lower since.

I can honestly remember about 3-4 tanks I got in the 8.7-8.8 range. Some were heavy as well.

[Paul McGraw]

Very impressive MPG numbers. What is your engine, transmission and ratio?

What is special about the Michelin XDN2 tires?

[allan5oh]

volvo 425 hp, very well maintained, rebuilt 3 years ago, new air/air 1 year ago

13 speed .73 final gear

3.73 gear ratio

XDN2 24.5 lo pros

The XDN2's are probably the worst tire out there for fuel mileage. Huge grips, no shoulder to speak of.

http://www.tirecraft.com/commercial/MichXDN2.asp

[allan5oh]

I also have a straight through muffler on.

[Graymist]

volvo 425 hp, very well maintained, rebuilt 3 years ago, new air/air 1 year ago

13 speed .73 final gear

3.73 gear ratio

XDN2 24.5 lo pros

The XDN2's are probably the worst tire out there for fuel mileage. Huge grips, no shoulder to speak of.

http://www.tirecraft.com/commercial/MichXDN2.asp

As someone new to trucking, may I ask what exactly is meant by gear ratio and final gear ? What exactly do the numbers that you've posted indicate ?

[solo379]

Michelin XDN2's on the drives

XDA Energy, would get you around another 0.25mpg.
I'd get it, but unfortunately, it only comes at 22.5 size! :sad:
I'm averaging about 7mpg right now, but due to hit, my AC is always on, and idling up to about 35%. Normally, it's around 20-25%.
Oh, and my speed is around 65mph also.

[allan5oh]

As someone new to trucking, may I ask what exactly is meant by gear ratio and final gear ? What exactly do the numbers that you've posted indicate ?

First I will explain gear ratio because in order to explain final gear, I need to explain gear ratio first.

All a gear ratio is is a number explaining how many times your "input" is multiplied, or how many times it turns.

In other words, my driveshaft spins exactly 3.73 times for every time my tires turn.

Final drive ratio is the ratio of the last gear(final gear) in my transmission. it is .73 or an "overdrive" gear because it is below 1:1

so for every time my engine spins .73 times, the driveshaft turns 1 time.

As you can guess, the lower "numerical" gear in your transmission, the higher numerically it is.

For example, "low" gear is around 16:1, so the engine spins 16 times for every time the driveshaft spins.

[allan5oh]

Michelin XDN2's on the drives

XDA Energy, would get you around another 0.25mpg.
I'd get it, but unfortunately, it only comes at 22.5 size! :sad:
I'm averaging about 7mpg right now, but due to hit, my AC is always on, and idling up to about 35%. Normally, it's around 20-25%.
Oh, and my speed is around 65mph also.

You continue to impress me with that big ACERT, you must've got a monday engine or something!

I am considering switching to 22.5's, I really wish I could get super singles but they're not legal in the prairie provinces.

Right now, at 60 mph I spin 1348, and if I switched to 515 revolution per mile tires I'd be at 1400. Just about perfect. My volvo engine needs a little more RPMs

The reason I bought XDN2's, I was still on the overnight run from winnipeg to minneapolis. I really needed them in winter, because there are no salt shakers out during the night. So now I'm stuck with them until they wear out.

[Graymist]

As someone new to trucking, may I ask what exactly is meant by gear ratio and final gear ? What exactly do the numbers that you've posted indicate ?

First I will explain gear ratio because in order to explain final gear, I need to explain gear ratio first.

All a gear ratio is is a number explaining how many times your "input" is multiplied, or how many times it turns.

In other words, my driveshaft spins exactly 3.73 times for every time my tires turn.

Final drive ratio is the ratio of the last gear(final gear) in my transmission. it is .73 or an "overdrive" gear because it is below 1:1

so for every time my engine spins .73 times, the driveshaft turns 1 time.

As you can guess, the lower "numerical" gear in your transmission, the higher numerically it is.

For example, "low" gear is around 16:1, so the engine spins 16 times for every time the driveshaft spins.

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 ?

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

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

[allan5oh]

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]

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

[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]

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: !