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Old 08-03-2012, 04:05 PM   #1
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Fuel Mileage Info

Would like some input about fuel mileage comparison between Allison 5 and 6 speed behind 300 Cummins engine.
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Old 08-03-2012, 04:13 PM   #2
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Probably very little difference.
But more info on the trans models might help. That way we can look up the gear ratios.

Cummins studies show that the driver can make a 30% difference between the best and worst, but that's with manual transmissions. With an automatic there would be very little difference.

As I understand it some of the 5 speed trans are actually 6 speed with the top gear locked out and can be reprogrammed. But the overall gearing might be too high then so they just locked 6th out.
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Old 08-03-2012, 06:05 PM   #3
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MPG

Over 6k miles YTD. Just over 10.6 mpg with a six speed. I think with a little tweaking, there is another 1.5 to 1.8, maybe 2 mpg out there to be had
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Old 08-03-2012, 06:58 PM   #4
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The six speed has two over drives, 5th and 6th. Might get a bit of improvement with the extra gear, but it would not be a deal breaker not to have it.
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Old 08-03-2012, 07:27 PM   #5
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The 6 speed engine usually has a little more torque like 660 ft lb to the 5 speed 600 ft lb.
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Old 08-04-2012, 09:49 AM   #6
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Odds are the final drive ratio is the same for both the 5 & 6 speeds. The 6 speed just has an extra gear to provide more optimal power transfer. That would mean that the highway cruising mpg is the same, but the 6 speed would perform a bit better - and get slightly better mileage - on rolling hills and long grades in the 3-4% range.
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Old 08-04-2012, 10:58 AM   #7
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To a question like that all you are going to get are guesses and generalities. You will not find anyone that had experience with both under the same conditions to be able to give you a meaningful answer.

Jim
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Old 08-04-2012, 11:23 AM   #8
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To a question like that all you are going to get are guesses and generalities. You will not find anyone that had experience with both under the same conditions to be able to give you a meaningful answer.

Jim
X2, unit weight, speed/RPM, DRIVER, all bigger factors in mileage .
If you want to cruise at 70mph with a 32' box trailer, your going to use more fuel than 60mph, with a Civic in tow.
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Old 08-04-2012, 07:17 PM   #9
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I am looking at buying a Beaver patriot thunder with a 425 HP engine or a marquis with a 505 MPG engine. What can I expect to get fuel mileage wise.
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Old 08-04-2012, 08:59 PM   #10
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2007, 300 Cummins, 5 speed, I drive approx. 62 MPH towing or not I get between 10 and 11 MPG.
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Old 08-04-2012, 09:23 PM   #11
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Originally Posted by Gary RVRoamer View Post
Odds are the final drive ratio is the same for both the 5 & 6 speeds. The 6 speed just has an extra gear to provide more optimal power transfer. That would mean that the highway cruising mpg is the same, but the 6 speed would perform a bit better - and get slightly better mileage - on rolling hills and long grades in the 3-4% range.
Maybe but the usual reason for locking out 6th is that the RV is too big/heavy to easily use the 2nd overdrive and factory testing has shown that 5th is better for performance and mpg as I originally said.
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