Good Sam Club Open Roads Forum: Effect of tow vehicle weight change on towing capacity
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 > Effect of tow vehicle weight change on towing capacity

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BobJohnson

Texas

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Posted: 08/11/19 10:29pm Link  |  Quote  |  Print  |  Notify Moderator

I am wondering whether reducing the weight of the tow vehicle can increase its towing capacity? Here's why I want to know and why I think it should.

Ignore mfr approval for flat towing and transmission lubrication (I have investigated that). I want to know whether I have the capacity to flat tow my 2019 Miata (manual) behind my 2017 Lexus RX 350. My RX is 2WD and does not have the towing package. Mfr's rated towing capacity is 2,000 lbs (3,500 w/ towing pkg).

The RX weighs 4,222 empty and GVWR is 5,665. Max payload is 1,444, which checks. The Miata weighs 2340 and would be empty except for a braking system, base plate, wiring and half a tow bar, say 2440 lbs. 2,440 lbs would appear to exceed the RX' 2,000 lb towing limit, however...

I am under the impression that a vehicle is approved to tow its rated towing capacity (2,000 lbs here) when it is at its GVWR assuming no other limit is exceeded. Is that right?

If so, my RX is approved to tow 2,000 lbs when it weighs 5,665 lbs (its GVWR). Yet if towing capacity remains constant, it is not allowed to tow one more pound even it loses 1,000 lbs of people and cargo and weighs 4,665 lbs. Why couldn't I tow another 1,000 lbs if the car weighs 1,000 lbs less?

Handling and braking can't account for the 2,000 lb restriction remaining in place, because the towing capacity can be raised by 1,500 lbs by adding the towing package which does not modify any handling or braking components. They just increase heat dissipation and electrical supply (trans cooler, engine oil cooler and bigger radiator, fan and alternator).

So if the RX has the trans and engine oil cooling capacity to move 7,665 lbs when 5,665 lbs is in the tow vehicle and 2,000 lbs is being towed, why could it not move 7,665 lbs without modification if the tow vehicle weighs 4,665 and the towed vehicle weighs 3,000 lbs. The cooling and electrical loads on the RX are not increased a bit by moving 1,000 lbs back 20 feet. (The Miata would be equipped with brakes and would weigh well under 3,000 lbs).

FYI, I normally tow below the speed limit, day time only, watch my gauges, change fluids with synthetics when recommended or sooner, and drive gently. I find no indication on the Internet that the RX has transmission problems.

Your thoughts, please, and thank you for your help!

Bob

rk911

Wheaton IL

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Posted: 08/11/19 10:58pm Link  |  Quote  |  Print  |  Notify Moderator

if we were talking about a motorhome towing a vehicle you'd be on the right track. in addition to the GVWR a motorhome has a GCWR...gross combination weight rating. that's the total weight the MH can propel including itself. discounting tow bar and hitch weight ratings in general for every pound the MH weighs less than the GVWR a pound is added to the GCWR. in the real world its a bit more complex but that's the general idea.

i don’t believe passenger car towing works the same way. the tranny, gear ratios and so on are engineered to tow a set weight and i’ve never seen a GCWR on a pass car, just a max tow weight. i could be wrong about this and if so someone will set me straight.

that being said...how far are you towing? flat or hilly? a buddy once towed a 45’ trailer with his 6-cyl PU from chicago to his seasonal site in northern WI. i thought he was crazy but he made it. i wouldn’t have done that but like Indiana Jones said...”Never tell me the odds”.


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BarryG20

Castle Rock, Colorado

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Posted: 08/12/19 09:32am Link  |  Quote  |  Print  |  Notify Moderator

While I think you are correct in general the pp brought up a couple good points as well. The bigger issue may be the mechanical components of the tow system ie actual hitch on the tow vehicle, hitch attachment strength, frame strength back there stuff like that. However if as you stated the tow package changes nothing but coolers and alt that should be ok too. It is not like you are towing 5000lbs you are really only over 400ish lbs of the exisitng tow capacity but still well under the tow package rating. And if you keep the tv well under its max gvwr I think it would be ok. I would certainly keep my eye on tranny and oil temps. the Alternator should not prove to be a real issue since towed vehicle will only be using some tail lights, blinkers etc.

Is this going to be regular thing or every once in while, short distance, long distance, flat or hilly, mountainous?


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JRscooby

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Posted: 08/12/19 10:15am Link  |  Quote  |  Print  |  Notify Moderator

Never have paid much attention to cars. But trucks, GVWR + Advertized tow rating *when properly equipped, was more than the GCVWR. And to be "properly equipped" you had to have all the power and stopping stuff, but you had to move the seat, run down the windows your self.
Maybe if you strip out all the extra seats, carpets, and the FREDs you might get right.

BobJohnson

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Posted: 08/12/19 04:16pm Link  |  Quote  |  Print  |  Notify Moderator

Thank you all for your replies. They were helpful.

I searched for hours yesterday before posting my question, (Lexus dealers were closed Sunday) but I never found any mention of towing capacity being reduced by anything.

I just called the local Lexus dealer and talked to the shop foreman. They connected me to him after I said I needed to know the answer for sure; no guessing. He confirmed that the rated towing capacity is good right up to the max allowable weight for the car, which is GVWR; 5,665 in this case.

Having learned that, I now can be sure nothing will be over stressed or overheated if I tow a vehicle heavier than the 2,000 lb towing capacity as long as the weight of the RX is under its max allowable weight (GVWR) by at least the amount the Miata is over 2,000 lbs (a 1444 lb gap between curb weight and GVWR will allow me to stay well under limits). The weight changes offset each other. The only difference would be the location of the weight (more in the back, less in the front), which must not total more than 7,665 lbs. Ordinarily, I could not assume that moving more of the dog's weight to its tail would be safe. But the fact that the towing package which allows an additional 1,500 lbs behind the RX contains no chassis mods indicates that it is okay.

You asked about hitch specs, how often I will tow and where. A Curt Trailer Hitch Receiver fits the RX. It is a Custom Fit - Class III - 2" Item #C13272 from etrailer.com. It is rated for 600 lbs of dead tongue weight (I won't have any), 1,000 lb with wt distr and 4,000 lbs of towed vehicle weight. I plan to tow the Miata 1,000 miles between TX and NC twice a year. At the eastern end I climb up into the Blue Ridge, but as discussed before, the car is rated to tow 2,000 lbs anywhere at its GVWR. That doesn't mean it's smart, though. I like to baby my cars, so I will certainly install an auxiliary transmission cooler, am considering putting a fan on it and might install an engine oil cooler. I will not put either in front of the radiator. And I might unhook before the steep parts, park one of them, then go back and get the other. Interesting fact: The RX, although it lacks a transmission temp gauge, monitors it and alerts you to pull over if it gets too high.

If you have installed either type of cooler on a V-6 Highlander or RX I would like to pick your brain.

Thanks.

BobJohnson

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Posted: 08/26/19 10:29am Link  |  Quote  |  Print  |  Notify Moderator

"I read it on the Internet, so it must be true."

Everywhere I searched, web sites said the towing capacity of a 2017 Lexus RX 350 with the towing package was 3,500 lbs and without was 2,000 lbs. Then I pulled out my (massive) Owner's Manual and found that the limit is actually zero.

"Trailer Towing (vehicles without towing package)
Lexus does not recommend towing a trailer with your vehicle. Lexus also does not recommend the installation of a tow hitch or the use of a tow hitch carrier for a wheelchair, scooter, bicycle, etc. Your vehicle is not designed for trailer towing or for the use of tow hitch mounted carriers."

Lesson learned: Get up and go get the Owner's Manual. That's the end of that project.

Thanks for your help, guys.

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