gear ratio and fuel economy

[kwbybee]

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.

[garbonz]

It depend on many variables such as torque curve and tow weight but probably not.

[bikendan]

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.

[John4th]

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.

[ironj]

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 6.2mpg...my 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..!!

[kwbybee]

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.

[Badlands]

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!

[kwbybee]

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?

[SaskCampers]

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 dodgetalk.com 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.

[BigBaron]

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.

[garbonz]

Slow down to 60. Thats the only way to appreciably increase MPG, regardless of what else you do.

[wilem]

i concur with most all of what has already been said. but if you are having a lot of down shifting and the engine is lugging you could possibly see an increase. not much but it sure would reduce transmission heat and friction withing the torque converter (some refer to this friction as fluid shearing). the gear stated is not enough IMHO i would say you need a 3:73 or 4:10 especially if you have 2 OD ranges. it will however decrease you non towing mileage without a doubt. But i would be more concerned with transmission life than fuel mileage either way. I have installed dozens of gears and out side of towing you will not likely ever get better mileage with a higher ratio. YMMV

NOTE: you should see the things that happen when the fluid temp of a transmission goes over 200 deg/ valves stick, fluid temp runs away, fluid is vented at an alarming rate, sometimes onto the catalytic converter and then you have tons o smoke and maybe even a fire.

[wilem]

Originally Posted by garbonz

Slow down to 60. Thats the only way to appreciably increase MPG, regardless of what else you do.

this is the best way to increase mileage,

Air pressure increases exponentially with velocity so its painfully obvious that the faster you go the more it costs. I would install a roof wind deflector if they weren't so permanent... that would help a lot.

[rccktmn2]

http://www.ringpinion.com/calculators/Calc_RPM.aspx

This might help.

[bikendan]

as others have said, slowing down to 60mph would give you better gas mileage than any of the things you're thinking of doing.
and expecting a cap to improve gas mileage, has been proven many times NOT to work, just like leaving the tailgate down.

[kwbybee]

great advice everyone. so maybe I'll go for the 3.92 ratio as that is as low as I can go. There is only like $10.00 difference in price between the 3.55 and the 3.92.

[Timtation]

Do you have it in tow/haul mode? I have essentially the same mix and do not find much gear changing.
A long time ago I had a GMC 454 1 ton with a slide in, a cattle rack or most often just an empty bed. Mostly I found the mileage was limited (or maximized) by the flow through a 3/8" fuel line. Generally 8 mpg uphill with the slide-in camper and sailboat or 8 mpg downhill with an empty bed and the tailgate down.

[Prothos]

i have a 2013 Ram 1500 5.7 with the 3.92. the most i get with the ac on towing around Texas is 10mpg TT is right at 4k loaded for the weekend. and i tow at 65 on the freeway. You almost get run over in the 75mph area's

[kwbybee]

Hey Prothos, what is your fuel mileage when not towing?

[prairiecamper]

Originally Posted by garbonz

Slow down to 60. Thats the only way to appreciably increase MPG, regardless of what else you do.

I run at 55 mph, and get even better mileage! I know that probably ticks some people off, but that's what 4 lanes are for. On 2 lanes when traffic builds up behind me I pull over and let the traffic pass! Last summer on our way to Branson and back I was getting 14 + mpg with the wind in my back. 10 to 12 with no wind conditions. Safe travels all!