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Old 10-27-2013, 07:55 PM   #1
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gear ratio and fuel economy

I have a Salem cruise lite 261BHXL and tow with a 2009 Ram 1500 5.7 hemi with a 3.21 gear ratio. This weekend I got 7.5 miles per gallon hilly terrain and a head wind going, coming back I got 10.5 miles per gallon same terrain no head wind but a/c going. My question is if I switch the gear ratio out to 3.92 or 3.55 will my miles per gallon improve significantly. I'm hopping for 10 mpg into wind and 14 with no/little wind.

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Old 10-27-2013, 08:16 PM   #2
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It depend on many variables such as torque curve and tow weight but probably not.

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Old 10-27-2013, 08:40 PM   #3
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usually, going to a lower gear ratio, will lower your mpg but improve towing perfomance.
but your truck may be overmatched with such a high rear end ratio of 3.21.
did that ratio come in the Dodge factory tow package? that would be hard to believe.

forget about ever getting a 14mpg towing a travel trailer, unless it's a diesel or you're going downhill for a long time.

average range for a gasser towing a travel trailer, is 9-11mpg, no matter the size or weight of trailer.
it's the frontal "barn door" area that affects gas mileage more.
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Old 10-27-2013, 08:50 PM   #4
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My Tundra has 4.30 gears and gets 10-11mpg pulling out 261BHXL. The above are correct, not necessarily better mpg's, but better performance.

My mileage is the same regardless of AC, wind, or hills.
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Old 10-27-2013, 08:51 PM   #5
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that trailer is prob around 5500 loaded.... doubt you will ever see 14mpg....

lowering the rear ratio will make your cruising rpm higher. .. significantly higher @390ish compared to what you have now. .. pulling power will be better tho..gas mileage not so much..

nothing in towing is free. ...I was in a stiff headwind last week(26+) and was getting trailer is a lil bigger but I'm also in a diesel with a 3.55 rear....on a GREAT day I get 11...but the trade is I have power for days..!!
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Old 10-27-2013, 08:57 PM   #6
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Thanks guys, I didn't have a problem with pulling, it accelerates fine, what I did notice is the down shifting quite a bit. My average rpm's were 2,100 and it seemed to kick down to 2,600 to get over the top of the hills. I kept my speed at 60-65, and 70 going down hill sometimes.
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Old 10-27-2013, 09:02 PM   #7
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As the other respondents said, there are many variables, but changing to a lower gear ratio won't be aiding your mileage.

Think of it this way; the lower your gear ratio, the greater the RPM's from your engine to maintain an equal driving speed. The higher the RPM's, the more the engine is working, and down goes the MPG.

I go back to my muscle car days for an example, too. My Dodge Charger Daytona 500 had a 440" engine, and I commonly ran 3.91 gears, though the car came with 4.10's in it's Trak Pak format. For Saturday night bragging rights, I'd change the gear set over to 4.56's -- huge torque and extreme acceleration capabilities.

But (and I don't remember the exact details -- it was a few years back), at 55 mph, driving normal and not abusing the accelerator, I lost 2-3 MPG with the 4.56's in the Dana axle, as opposed to the 3.91's.

And, as you note, the effect of a headwind can be mighty humbling!

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Old 10-28-2013, 04:53 AM   #8
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So I guess my next question would be, which gear ration would y'all pick? 3.55 or 3.92, leaning towards 3.55. Also would a camper shell on the pick up help with the wind resistance?
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Old 10-28-2013, 08:01 AM   #9
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With the double overdrive gears in those 545RFE's (4th and 5th are OD gears) I would put as much gear as you can in it. It will not make a lot of difference in your mileage especially if you do much driving around town as the lower gears get you moving easier. Join a good forum like as there are al lot of people who can give you lots of advise. BTW I had 3.73's in my 2500 megacab and if I kept it I would have gone to 4.56's like the power wagon has std as I felt even the 3.73's were not enough with the poor gear ratios and spacing the 545 has.

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Old 10-28-2013, 08:26 AM   #10
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Before you change it try downshifting before it does it on its own. Is it searching for gears on the flat or only when climbing?
Also 65 - 70? Try 60~63 and you'll probably get closer to the mileage you want. If you really want to figure it out, get a scanguage-type reader and you can get a very accurate real time mileage figure.
A 3.55 would probably be enough if you decide to change, unless you plan on getting a heavier trailer in the future, then I'd go 3.92. My trucks were good for just about anything at/near that ratio.
Do you have a auxiliary tranny cooler? The scanguage can also track tranny temps.
A shell would have a negligible effect (if any) because of the gap between the truck and the trailer.

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