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My wife and I are looking at purchasing a new Subaru Outback. In the manufacturer's specifications, it states that for the 2.5L (four-cyl), that the towing package allows us to tow up to 2,700lbs.

In contrast, the 3.6L (6-cyl) allows for 3,000lbs.

This seems really odd to me. For one thing, I don't think that a 2.5L engine produces enough torque to tow 2,700lbs. Additionally, if that were true, why wouldn't the addition of over 1L of displacement generate enough of a torque difference to tow more than an extra 300lbs?

When I ask the salespeople (and others) about this, they give me a hand-waving answer about how it "has something to do with the symmetric all-wheel drive and attachment to the frame at a lower center of gravity."

I understand that Subaru uses a Boxer-style engine, with symmetric all-wheel drive, but could someone give me a better answer (ideally with specifics about the physics of energy transmission in the drivetrain) as to why a Subaru 2.5L engine can tow 2,700lbs, but my current Mazda3 with a 2.4L can't really tow 500lbs?

  • Jetta will tow 2000 plus with t brakes, anywhere.😀 – Supersr Sep 7 '18 at 19:09
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In terms of towing, in reality, all you need is enough torque to get over initial resistance, as rolling resistance is much lower. This is why you get people who can tow a bus, or a jumbo jet, for example, so absolute power isn't the be-all and end-all.

As I wrote in my comment, the flat-4 boxer engine is a good one for pulling, as it does give smooth power, per revolution - it isn't 'lumpy' but the most important difference between your Mazda and a Subaru is the engineering of the entire drivetrain. Subaru, built by Fuji Heavy Industries, really lives up to the 'heavy industry' part - it is heavily over-engineered, with the engine block, differentials, gearbox and transmission well able to cope with more than the car will normally have to deal with.

As Paulster commented, the actual safe weight towable also depends on the handling and braking characteristics, so while the power of the 3.6l may be much higher, the rest of the car is not much changed (ie same tyres, suspension, brakes, similar weight) so even if you had a much bigger engine you wouldn't be able to tow much more.

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While I won't speak directly to the Subaru and its specific towing capacity, there are several reason why some vehicles have better towing capacity than others.

The main reason is most people think about towing as how much a vehicle can actually pull, when there are more factors involved. You also need to look at how much the vehicle can safely slow down. Realistically, you should not tow more weight than what the vehicle weighs (without a trailer brake). If your towed weight exceeds the vehicle weight, the towed weight will have the propensity to push the vehicle around during stopping. (Think of a jack knifed trailer.) I would assume with the Subaru, the larger engine weighs more, which allows for the additional towing.

Another thing to consider is there is more to towing than what the engine can handle. Everything in the powertrain (engine, transmission, transfer, differentials, drive axles) are all in play. If these pieces aren't built to take it, it won't be able to handle it. I know the Subaru's can take some punishment, which is probably why they can handle a lot more than your Mazda3.

Some vehicles just weren't designed to tow other things. Consider, also, when the towed weight capacity is given, you should reduce that much by the weight of any cargo you are hauling in the vehicle itself, such as occupants, fuel, other cargo. All of the weight which would not be there with a vehicle by itself needs to be taken into account.

I know there are other reasons, but hopefully this gives you an idea why it changes from vehicle to vehicle and model to model. The change isn't quite linear and therefor needs to be taken into consideration.

  • Interesting. So, it sounds like the manufacturer's specification is trustworthy, then (i.e. it's not likely that they are advertising that it can tow significantly more than it actually can). Do you know how the symmetric all-wheel-drive factors into giving the car more towing capacity than, say, a 2-wheel drive vehicle? – jwir3 May 15 '15 at 18:04
  • Sorry, what I meant by the previous comment was not that the manufacturer's specs weren't above board, as it were. I simply meant that it seems like towing 2,700lbs with a 4-cyl engine would put undue wear and tear on that particular engine, or is that not likely to be the case? – jwir3 May 15 '15 at 18:05
  • that isn't the case - it doesn't matter that it is a 4 cylinder engine (although a flat 4 boxer engine is very balanced, which helps with pulling large loads) but what is important is that the Subaru engine and power train (excluding older clutches!) is heavily over-engineered. – Rory Alsop May 15 '15 at 19:33
  • In your second paragraph you mention that the weight of the towing vehicle should be higher than that of the trailer, but in the fourth paragraph you suggest that increased weight in the vehicle diminishes towing capacity. These two thoughts can be reconciled, but you've made no effort to do so in your explanation. – Nathan L May 15 '15 at 21:34
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Just returned from an 1100 mile round trip from Rochester NY to Dayton and back. Our 2013 Subaru outback limited (4 cylinder) towed an Aliner. Trailer weight without gear is 1525 lbs. There were 2 passengers (me and wife) and I estimate we carried approx. 500 lbs of camping stuff. Rochester to Dayton does not have any severe hills. Camper is equipped with electric brakes. Subaru performed very well. Running on level road at 70 mph, engine ran at 2100 rpm and the temp gauge never went above normal (non towing) value. Most driving was done in 80-85 deg. F weather. I plan to ask subaru reps. if I am doing some "hidden" damage to drivetrain as things seem to be going too good! Average fuel usage was 23.7 mpg.

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In addition to the other answers:

The Mazda3 is an odd case. Apparently the US specification says 'no towing', but the EU spec has these towing capacities for the same car:

  • Braked trailer kg 900
  • Unbraked trailer kg 550

That is 1984 lbs braked and 1212 lbs unbraked, not too far off from the Outback, and about what I'd expect.

The Outback weighs about 300 kg more than the Mazda3, the extra weight helps stability when towing.

  • Interesting. It's odd that the US version and EU versions are different. Are they actually different vehicle specifications, or is it more of a legal thing? – jwir3 Jun 8 '16 at 20:20
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Same as VW. 2012 jetta in EU has towing spec, but towing not allowed in US. In US, base vehicle weight and wheelbase must be above a threshold set by NHTSA for safe towing. You will find it odd that towing is not approved for many vehicles in the US, yet every hitch company makes a hitch for most vehicles. go figure.......

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Two interesting data points:

  • 2011 Toyota Yaris can tow 900 kg with 1.33 liter engine (scaling to 2.5 liters gives 1692 kg)
    • The engine produces 99 hp
    • The car is 1490 kg max plus trailer
    • Car + trailer max is 2390 kg or 2.39 tons
    • Power per ton is 41.423 hp per ton
  • 2016 Toyota RAV4 hybrid can tow 1650 kg with 2.5 liter engine
    • The hybrid system produces 197 hp momentary or 155 hp continuous
    • The car is 2270 kg max plus trailer
    • Car + trailer max is 3920 kg or 3.92 tons
    • Power per ton is 50.255 hp per ton momentary or 39.541 hp per ton continuous

Your 2700 lb is only 1226 kg. I hardly see it as surprising that 2.5 liter engine can tow 1226 kg.

I suspect the 3.5 liter engine not being able to tow 3.5/2.5 times what the 2.5 liter engine can tow is due to braking, not due to acceleration.

Volvo's 750 hp engine can tow 60 tons. This gives 12.5 hp per ton.

If heavy trucks manage with 12.5 hp per ton, why do you think 40-50 hp per ton would be a problem in passenger cars?

The true limit is the brakes, not the engine.

  • I seem to remember that UK construction & Use regs specified trucks to have a minimum power to weight ratio of 4.4kW per 1000Kg – Solar Mike Sep 7 '18 at 20:07

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