Smaller tires = higher axle ratio

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pinoyfunk

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Jul 17, 2005
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Dear forum members and staff,
    I checked the weight limitations imposed by my tow vehicle to be within limits.  The van (97 Nissan Quest) towing capacity is stated as 3500 in the owner's manual.  My Jayco 165 sport has an unloaded weight of 2690.  We never bring tank water when we go to campsites, probably just 2 gallons of bottled. Our stuff consists of 2 suitcases 2 lawnchairs, 13"tv -father wife son dog-1 cooler of food.  My curb weight is much less than my gvw (tow vehicle i.e.)
    My van is in pretty good shape even though it has around 89,000 miles.  How do I know?  I keep up with the Maintenance pretty well  to include recent fuel filter and timing belt change.  The Quest also runs like a bandit - good torque engine sings on hard acceleration, no misses no smoke.  A transmission cooler is installed
      Problem:
1.  When we go on the highway with trailer,  I note that the accelerator is pressed around 1/2 to 3/4 of full range to keep around 60 - 65 on the levels and slight uphills.  I don't feel comfortable with stressing the engine this much.  With the overdrive off, the rpm Will sometimes shift into the 4000 rpm to attempt to keep up the speed.  Me and the wife find this a bit disheartening.  The result is that I drive around 50 - 55 in areas with a 65 speed limit - on the slight inclines.
2.  I checked the possibility of lack of power due to clogged catalytic converter - cats found to be o.k.
    Questions:
1.  Am I overloaded and don't see it?
2.  Is it possible that you can have a crappy tow experience like mine even though my set up is under weight limit.
3.  Will switching to a smaller tire - (Change of aspect ratio) have the effect of increasing the axle ratio
4.  Should I just forge ahead and drive the van harder throwing caution to the wind.
Thanks ahead of time for your replies people - and you better  give good engineering reasons for not using #3 not just a bunch of hot air about steering geometry.
 

John From Detroit

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Or buy a nice Chevy Silverado with an 8.1 Liter Vortec V-8 feeding an Allison Transmission and forget the words "Towing limits".

Seriously: There are many things that affect how much a vheicle can tow.  Not only is WEIGHT a facter but SIZE is too when you are trying to do 60MPH or close to it (50-70)  What is holding you back more than the weight of the trailer is it's SIZE, all that wind resistance.

In fact, they have done studies.  They find that long about 50 MPH you start using a LOT ore power per mph to overcome wind resistance.

Now.. As to put the pedal to the metal and throw caution to the wind... I don't know.. Some engines can take it, some can't and quite frankly I have not a clue as to your Nissan.

I would not mess with tire sizes, as that won't cut down on the RPM at alll, in fact for any given speed it will INCREASE the RMPs

You should give that Silverado I mentioned the opening line some thought.  nice truck, lots of power and the engine transmission combo I mentioned is used in Class A motor homes weighing over 20 thousand lbs (Like mine which is based on a Workhorse W-22 Chassis)

Heck, my MH even has a Silverado after market part on it (locking gas cap)
 

Gary RV_Wizard

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1.  Am I overloaded and don't see it?
2.  Is it possible that you can have a crappy tow experience like mine even though my set up is under weight limit.

Yes, very much so.  The towing limit makes no claim about towing performance - just that you can tow that much weight without damaging the vehicle or being grossly unsafe.  Nissan is not saying the van will not be impacted in any way by 3500 lbs of additional weight - just that it can be done.  In point of fact, performance will suffer just as you describe and engine and transmission wear will be substantially increased.


3.  Will switching to a smaller tire - (Change of aspect ratio) have the effect of increasing the axle ratio
Yes, tire diameter has an effect on the overall drive ratio. A higher overall drive ration generally increases engine RPMs and decreases fuel economy but provides more power and responsiveness under most conditions.  A lower overall ratio improves economy by lowering engine RPMs for a given speed. However, when the vehicle is heavily loaded, e.g. towing, a lower ratio may cause you to operate more of the time in a lower gear, thus negating the efect of the lower ratio.  You should also be aware that changing tire size will affect your speedometer reading, both speed and distance.  You may be able to get it recalibrated, though.

4.  Should I just forge ahead and drive the van harder throwing caution to the wind.

No. You really need a different tow vehicle if you want good performance and modest engine/transmission wear.  Your Quest is not really designed for routine hauling of 3500 lb trailers over long distances.  Taking a boat to a local lake is more within its capabilities.


You should also be aware that 3500 lbs is the Quest model's maximum towing capability but your particular vehicle may be less, depending on the weight of optional equipment. A bigger engine, for example, may actually decrease the tow capacity because it weighs more than a smaller one.  You also have to subtract the weight of all passengers and gear from the brochure's towing capacity. My guess is that you are operating right at the Quest's actual limit.
 

Lou Schneider

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If you aren't actually overloading your Quest, you're pretty close to it's limit.

The towing capacity is computed by subtracting the weight of the vehicle from the GCWR (Gross Combined Weight Rating).? ?The vehicle used is the lightest version of that model, to make the tow rating as large as possible.? ?Usually they include the weight of a 150 lb driver and a half a tank of fuel.? ?Any additional weight in the vehicle like optional equipment, other passengers, cargo, etc. subtracts a like amount from the real towing capacity.? The weight of your father-wife-son-dog probably makes your GCWR approach or exceed the limit.

The other part of the equation is wind resistance.? Usually the tow rating specifies a trailer with the frontal area of a horse trailer.? RV trailers have more frontal area, which means the tow vehicle needs more power to overcome wind resistance at highway speeds.

Being at or near the vehicle's limit means your performance will be at the minimum acceptable levels.? ?Sounds like you're about there.? A larger vehicle with more reserve capacity will give you a better towing experience.
 

pinoyfunk

Member
Joined
Jul 17, 2005
Posts
6
? ? Thank you two gentleman for answering my question.
? ? I've been convinced of the fact that I need a larger tow vehicle. ?
? ? My question now is 1)what size engine do I need for good towing performance. ?With gas prices the way they are, I wonder if an 8.1 vortec will be overkill when a 5.3litre ?could possibly do the same duty with less gas - or even a 4.8?
? ? I'll define good performance for a tow vehicle is something that will pull my jayco 16.5 foot 2700 lbs. unloaded at 60 - 65 on 90% of interstates. In a few years I may ?upgrade to a larger travel trailer - probably 25 ft but for arguements sake, I'll limit the lbs to 5000 unloaded.
? ? Quest 2) Will someone please give me a good rule of thumb for buying a tow vehilce that will give good highway performance - say for ex. - stay 30% above loaded vehicle weight for the tow vehicle capacity, or maybe even a fomula that relates the amount of torque and horsepower required to push a certain amount of frontal surface area down the road at a certain speed.
? ? I don't mind buying the 8.1 vortec if in fact it will give me the same gas mileage when under tow as a smaller v8. ?Quest 3) can someone tell me what kind of mileage they get with an 8.1 vortec when towing and when unhitched?
? ?
 

Gary RV_Wizard

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The engine size is not all that big a factor in a good towing vehicle. With an appropriate rear axle and transmission, most any of the mid-sized gas engine (or a small diesel) will be adequate.  You also want a vehicle with a sturdy frame (and a real frame rather than unit-body is a plus), large brakes, strong transmission, a large radiator and a fairly stiff suspension.  The larger SUVs, vans and pick-ups  generally meet these criteria, especialy if equipped with a "towing package".

On the other hand, a big engine (8.1L Vortec or Ford V10) is loafing while towing a smaller trailer and generally delivers decent mileage along with plenty of reserve power.
 

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