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Old 05-13-2010, 01:24 PM   #1
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diesel mpg

I'm on the road right now and I'm averaging about 7.6mpg on a 2006 discovery with a cat c7 330hp the problem with it seems to be gearing because if I cruise @ 70mph the rpm is @ 2000 so I keep it at 65mph on cruise. is there a way to change the gears in the transmission or on the rear dif. It may be a dumb question but I could use another gear to lower the rpm. What MPG are you guys getting Out there?
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Old 05-13-2010, 01:57 PM   #2
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I cruise at 1800 rpm's which is about 62 mph w/ a Cummins 325 HP and Allison 6 speed tranny. I average 8.9. If your running 70, 7.6 is probably about right.

I'm enjoying the retired life so I don't have to hurry so.
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Old 05-13-2010, 02:04 PM   #3
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No no I'm cruising at 65 not 70 if I go to 70 then the rpm goes to 2000rpm and I'm assuming the MPG will bee worst
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Old 05-13-2010, 02:43 PM   #4
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Your biggest problem with MPG is pushing a square box down the road. It is just not aerodynamic. The rear gear ratio is selected to give you the best all around performance in accelaration climbing hills and pushing that box against the air. I would not change the rear gear ratio. If you change it to less gear you can get below the power curve and the MPG will drop. The other thing is that your MH weight, transmission gear ratios, converter slip percentages, tire diameter are slected to match that rear gear ratio. Your will upset that design if you change the gear ratio.

Try reading this and see if it helps you increase your MPH. It is a real eye opener. Cummins also put one out and it says the same thing.
https://ohe.cat.com/cda/files/287140/7/LEGT5364.pdf
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Old 05-13-2010, 02:46 PM   #5
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My CAT C7 lifetime average after 44,809 miles is 7.52 mpg. I generally drive at 62-63 mph and tow a 4200 lb Jeep.
Any gear modifications, imho, would cost you more than you would gain.
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Old 05-13-2010, 03:43 PM   #6
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I'm averaging between 8-9 mpg with a tow and 9-10 without a tow. Speed between 65 and 70 mph. Cummins 8.3 ISC, 360 hp... Weight 28k lbs. Tire pressure up between 115 and 120 all around. I'm easy on the pedal

Remember, it's a brick going down the road

Might want to check the air filter, etc since several things can affect MPG.. One more thing that may help with mpg is a cetane booster.
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Old 05-13-2010, 05:01 PM   #7
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Check your air filter, tire pressure and back off on the speed. I run 63MPH and see 10MPG without my toad, 9MPG pulling it.
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Old 05-13-2010, 05:39 PM   #8
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The big thing I am seeing here on the difference in what the box get in MPG is the weight of the MH. My previous National Dolphin diesel with a Cummins B5.9 with the Banks kit pushing my 21000 lb MH without the Jeep Wrangler got 11.2 MPG and with the Jeep Wrangler gets 10.8 MPH all day long. My 2004 Signature with a Detroit Diesel 60 weighing in at 50,000 lbs gets 7.5 mpg when pulling the Wrangler but it is not broken in yet so I have to give it a chance. Only has 24,000 miles on it. Also need more accurate measurements. I drive both MHs at 62 mph. On my Sig I don't think that Detroit Diesel even knows that Wrangler is behind it (lol).
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Old 05-13-2010, 05:41 PM   #9
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CAT RV Document

Quote:
Originally Posted by Mike Canter View Post
Your biggest problem with MPG is pushing a square box down the road. It is just not aerodynamic. The rear gear ratio is selected to give you the best all around performance in accelaration climbing hills and pushing that box against the air. I would not change the rear gear ratio. If you change it to less gear you can get below the power curve and the MPG will drop. The other thing is that your MH weight, transmission gear ratios, converter slip percentages, tire diameter are slected to match that rear gear ratio. Your will upset that design if you change the gear ratio.

Try reading this and see if it helps you increase your MPH. It is a real eye opener. Cummins also put one out and it says the same thing.
https://ohe.cat.com/cda/files/287140/7/LEGT5364.pdf
Mike,

Thanks for the CAT reference document...very good document.
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Old 05-13-2010, 09:58 PM   #10
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Bought this MH last year. A cummings printout showed it got 10.5 avg over 30000 miles. Since last year I get right at 10 towing my 3800 lb toad. ISB275 w/ 6 speed allison.
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Old 05-13-2010, 11:30 PM   #11
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Thanks to every one for the great advice and also thank you very much for that article is very nice. My air filter is new and it is a after market that I got from brazels RV performance and also changed my muffler and I got the cumputer for the engine management the other thing is that motor home just turned 25000 miles maybe its not fully broken in. And about rear dif gears I'm not going to try to do it I was just thinking that I could use another gear. Well you guys keep the advice on caming and thanks again for all the great help. Joe.
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Old 05-14-2010, 12:24 AM   #12
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ninos ...

I have a 2004 diesel pusher with a CAT C7 engine ... the engine now has 86,000 miles ... I have been keeping fuel consumption records for the last 30,000 miles ... I have average 8.26 mpg ... we tow a 4200 toad ... I generally drive about 62 mph ...

I have noticed that I get better mileage when I travel with a tail wind ... going downhill or both (Cheyenne to Omaha is all downhill and the wind is generally behind you) and poorer mileage going the other direction (Omaha to Cheyenne for instance) ...

The fuel consumption declines drastically when I decide to drive near 70 mph ...
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Old 05-14-2010, 07:04 AM   #13
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what Is it that I'm doing wrong that it seems that everybody gets better MPG then me and I have some modifications done that is suppose to help with better mpg
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Old 05-14-2010, 07:27 AM   #14
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Ninos, it could be where you do the majority of your driving. Out West the Density Altitude (corrected altitude that the motor sees) is way higher than on the East Coast or in Texas. For example Arizona, Utah, Nevada Wyoming have Density Altitudes of 5000+ feet plus so motors make less power and have to be pushed harder. Texas, Georgia, Florida, Louisiana have Density Altitudes of 3000 or less so the motor make a lot more power so you can throttle back. If you live in a cooler climate the the air density is thicker so your box (MH) is harder to push down the road. Here is something else I have found out...people don't brag about their bad MPG it is normally people who have learned how to drive to get good MPG brag about it.

It maybe worth taking your MH to a CAT dealer and have them put it on a chassis dyno and evaluate the motor and see if all is good. On some diesels they can do what is called a MPG tuneup. I know it can be done on a Detroit Diesel and is recommended at 60K miles. I am not sure if CAT has anything like that.
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