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better towing


 
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backpackjp
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Joined: 22 Dec 2006

PostPosted: Fri Dec 22, 2006 5:02 am    Post subject: better towing Reply with quote

Hey
I have recently purchased a trailer and since I am in 3rd gear in my auto trans 65 mph= is like 3000+ rpms. I was looking for a way get a more favorable rpm/speed ratio.

I was told that if you increase tire size you will decrease rpms at a particular speed. Is there anything to this? If so, at what point over stock will I even begin to notice it ?

I bought the rig pretty recently and a lift with bigger feet were definatly in my plans so this is a little more appealing than installing a better trans.
I am driving a 2000 xj sport and was looking at 3-4" lift and 31x10.50

Any thoughts are greatly appreciated
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dancerman
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Joined: 08 Jun 2006
Location: Atlanta

PostPosted: Fri Dec 22, 2006 10:21 am    Post subject: Reply with quote

What kind of trailer are you pulling? A big one? Putting bigger diameter tires on it will lower your engine RPM for a given speed but it will also be harder on the drivetrain when you are taking off from a stop. Something to consider if your pulling a heavy trailer.

The new RPM for a larger diameter tire is the original RPM times the original diameter divided by the larger diameter. For example.... starting @ 65 MPH and 3000 RPM with a 28 inch tire; switching to a 35 inch tire would be 3000 x 28 / 35 = 2400 RPM.

Hope this helps.
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ehirner
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Joined: 01 Jan 2004
Location: Eastern PA

PostPosted: Fri Dec 22, 2006 2:31 pm    Post subject: Reply with quote

I wouldn't suggest running larger tires if you're pulling a trailer often. Not only is it harder on the drivetrain when starting out, pulling hills, or accelerating, but it also creates less effective braking from the larger diameter tires. Couple that with a 1000lbs or more in a trailer and you surely see the problem.
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backpackjp
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Joined: 22 Dec 2006

PostPosted: Fri Dec 22, 2006 10:13 pm    Post subject: Reply with quote

Hey guys, I appreciate the replies.

In reference to the trailer; it is a pop up camper and weighs 1500-2000 lbs. So since bigger tires seem not to be the best route would regearing the rear diff or simply installing a heavier duty trans be worth it?
It pulls well and the temp stays pegged as it would be under normal conditions. So I see the merits of leaving it as is. I obviously won't tow it daily but fairly often. Only thing will be that it will be for realitvely long distances:15,000-20,000+ miles on occasion. Would simply installing a trans cooler and temp gauge as a safety system be more sensible? Or are there more important things to think about like brakes and cooling?

Thanks so much for all of your help.
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ehirner
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Joined: 01 Jan 2004
Location: Eastern PA

PostPosted: Sat Dec 23, 2006 6:36 am    Post subject: Reply with quote

Tranny cooler and a temp guage are good ideas. I wouldn't regear the diffs and certainly wouldn't do a tranny swap.
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Jim Mesthene
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Joined: 23 Dec 2006

PostPosted: Sat Dec 23, 2006 10:37 am    Post subject: Reply with quote

All your problems could be solved, and you could put a buck or two in your pocket, by cruising at 60 mph instead of 65.
You'll notice that even the truckers have slowed down.
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ThePhantum
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Joined: 23 Jan 2004
Location: I knew it...I'm surrounded by Assholes!

PostPosted: Sat Dec 23, 2006 10:38 am    Post subject: Reply with quote

Towing that much weight, you actually want your RPM's to be in that range. First of all, the engine is making maximum torque at 3000 RPM, which very helpfull when towing. In addition, when you come off the gas, engine braking will help you slow down.

I would install a trans cooler and upgrade to a 3 row radiator as well.

http://jeephorizons.com/tech/xj_cooling.html
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PNWXJ
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Joined: 20 Dec 2006
Location: PNW

PostPosted: Thu Dec 28, 2006 9:42 am    Post subject: Reply with quote

65 is a bit fast with a tralier behind you, light or not. My 16 ft John boat sways from left to right above 60MPH.
Wants to wander over to the neighbors lane and say "HI" if I go too fast.
Recieved a well deserved salute for that one Finger .

Found if we load the front of the boat up with coolers and gear it doesn't wander as bad but still sways so I keep it down to 55-60 to be safe.

I noticed even the small jump from stock 28" to 30" affected my braking distance. 3.5" lift didn't affect it, it was after I put the tires on that I noticed. gas milage is still 240 miles per tank. I drive mostly in the city. 300 per tank on the highway. Don't think the bigger tires made much of a difference. Sits at 2,000 RPM at 60MPH all day long. Speedo might be off a tiny bit with the bigger tires but not enough to worry about.

Slotted rotors and green stuff pads all around helped with the braking distance but it definitly takes more foot power to stop than it did before.

Ideally I'd like to do the ZJ disc conversion for the rear but airlocker is on the list before that.
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