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Old 07-19-2004, 01:07 PM   #1
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fuel consumption...

It seems I am getting the short end of the stick when it is coming to fuel economy. I averaged around 5.5 mpg and many of you seem to be getting 7-10. Looks like I need to do some fiddlin around and see what the culprit is. Ill start with the timing, since I suppose if it is off a bit it would maybe even be the culprit in my power problem.

Also, how am I going to be able to tell what ration my rear end is ?? I want to mass as much info as I can before I go too crazy with starting on projects.

Thanks in advance.

TonyC in Iowa


my 366 V8/Holley 4bbl drives my rig at appx. 8.9 mpg (60mph @
2500rpm). Before the "extreme makeover" she got a portly 6.5
mpg....but your results may vary. Consult your doctor/mechanic for
more information. [19,500 GVW]

and I only have a 2bbl....
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Old 07-19-2004, 01:59 PM   #2
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Re: fuel consumption...

Quote:
Originally Posted by abdabbs
Also, how am I going to be able to tell what ration my rear end is ?? I want to mass as much info as I can before I go too crazy with starting on projects.
Two ways I can think of if you can't find the info on your bus.

1) Put a mark on the tire and a mark on the drive shaft. Have someone drive it in the lowest gear (or push it in neutral) and count how many times the drive shaft turns per turn of the wheel

2) If you have a tach then knowing your speed in mph and the engine speed at said speed while in 4th gear (1:1 ratio of the transmission) then you can back calculate the ratio using the engine speed, ground speed, and size of the tires.
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Old 07-20-2004, 02:40 AM   #3
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On all the differentials I've worked on there's been a number stamped on the ring gear that indicates the number of teeth on the ring and pinion set with a colon between them. Devide the two numbers and you get your ratio. I have never worked with rear-ends that size before (at least not on vehicles ) so check with a mechanic at your local bus barn and he'll be able to tell you if the ring is marked. Better yet, I work for a city and I can ask my friend at the city shop tomorrow (Tuesday) if he knows if there's a ratio stamp on the ring gear for heavy truck rear-ends. Our city owns about 40 buses so he should know.
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Old 07-20-2004, 09:21 AM   #4
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Re: fuel consumption...

Quote:
Originally Posted by Steve
Quote:
Originally Posted by abdabbs
Also, how am I going to be able to tell what ration my rear end is ?? I want to mass as much info as I can before I go too crazy with starting on projects.
1) Put a mark on the tire and a mark on the drive shaft. Have someone drive it in the lowest gear (or push it in neutral) and count how many times the drive shaft turns per turn of the wheel
a similiar but alternate method:

shut the bus off, chock your front wheels release the parking brake put the tranny in nuetral, then jack up the duals on one side. Mark the tire and driveshaft. count the number of driveshaft revolutions it takes to make one complete revolution of the tires......then multiply by 2.

If for instance it takes about 2 turns of the driveshaft to = 1 rev of the rear tires......multiplying 2x2 = 4. Your gear ratio would be a 4.10:1......that really means that the driveshaft has to turn 4.1 times to make the wheels travel one compete revolution.

Like most of our rigs, your bus probably has more like a 5.56 (or even higher) rear. I used 4.1 in the example because it was an easy number to work with.
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Old 07-21-2004, 01:28 AM   #5
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I spoke with one of our bus mechanics at our city shop today and he gave me a couple different possibilities, depending on the make of the bus. He said that the ratio is probably stamped on the ring gear, but this means opening the diff. Not a big deal, but extra work and money. He said that some brands have a small tin tag on one of the diff cover bolts with a factory code on it. The dealer can use this number to look up your bus and tell you what gears it has, along with other info. The third was the same idea in the last post here. He said you could jack one side up and turn one wheel and count the rotations on the drive-shaft as the wheel turns. This is great since it's free (no new rear-end grease and gaskets involved) and relatively safe since the bus doesn't move. The only down side (if it is one) is that you get a ball-park figure on the fraction of a turn since you guess how far the last part of the turn was.

Anyway, hope that helps a little.
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