Towing 4th Gear

Not that anyone asked for it, but I got really bored and ran some rough calculations on drag, assuming an estimated drag coefficient of 0.36 for the stock dimensions of truck and trailer (which would undoubtedly be significantly higher in reality) and standard sea-level air density. And this is for a stock truck without any light bars, bed racks, etc. This excludes added drag from geartrains, heavy diff oil sloshing around, rolling resistance of your tires. Which would also increase by significant, if not similar margins.

At 60mph, the drivetrain is pulling against about 230lbs of drag force from air resistance alone, ignoring the rolling resistance etc. Assuming a 4300lb truck and 2200lb trailer(6500lb GCW), this drag vector is equivalent to driving up a 3.5% grade when you are on flat road.

Bumping the speed up to 70mph, it's now pulling against 315lbs of aerodynamic drag. So a 16% increase in speed translates to a 37% increase in drag from air alone.

At 75mph, this increases to 360 lbs of aero drag force, a 57% increase from the 60mph figure with 25% increase in speed from 60mph. This increases that drag vector to the equivalent of driving up a 5.5% grade when on flat road.

For reference this decreases dramatically at 50mph, producing only a 160lb drag vector, or equivalent to 2.5% grade. At 35mph it's 78lb of drag for a 1.2% grade equivalent

Overcoming drag is not the same as powering up a hill. If I'm offroading or hauling a heavy load up a steep grade, yeah I'm gonna drop a gear or two and use higher RPMs to my advantage, to safely deliver the torque needed to get the job done.

But on the highway it's not the RPM or cooling alone I worry about, these engines are designed to produce max hp/torque pretty high up on the tach. I'm concerned with total power output over time in steady-state driving. The faster you go, the more power your engine has to pump through your drivetrain just to maintain that speed for long periods of time. Fighting against all that extra parasitic drag translates to the engine burning significantly more fuel, producing significantly more heat, and cranking that much more torque through your torque converter (or clutch if applicable), transmission, u-joints, differential, wheel bearings, etc, to cover the same distance in slightly less time. And the wear-over-time increases exponentially the closer you get to max rated power. Running 70-75% of your max power output in steady-state is waaaaay easier on the machine than running at 85%+ for hours, before you encounter any hills or headwinds.

Yes, this doesn't take into account other variables like headwinds, hills, accel/decelerating with traffic, gas stops, etc. So it very well may make no difference at all if 3-5 minutes saved gets you stuck at the same red light anyway.

So yeah, if you're driving hours across the country, it might make a lotta sense choosing 75mph over 65 or 70. Up to you to decide if the extra wear and fuel is worthwhile. But for the typical taco owner towing a boat, camper, or work trailer regularly for an hour at a time, I prefer to let up on the gas a little and be kind to the workhorse. In the same manner that I don't romp on the throttle at every green light, just to get to the next red one .75sec before the car beside me.

Side note, all this is why airplanes generally can't "just fly faster", because every 50% increase in airspeed produces a 100% increase in aerodynamic drag to overcome and it's not wise to run any engine at or near the bleeding edge of its capabilities for extended periods. They run up to full power for short bursts like takeoff, but quickly dial back to about 65-75% max for any extended periods for engine longevity.

 

I’ve been towing boats and various trailers all my life. Everyone is very aware that when towing your mpg turns to shit. Usually cuts it by almost half or more. Not a single time was I ever concerned that I could maybe save 1 or 2 mpg if I drove a couple mph slower. I just want to get to my destination as quickly as possible. If I burn a couple xtra gallons, who cares. I don’t know anyone, no one, nobody who intentionally drives 5 mph slower while towing to save a few gallons of fuel. I know when I hook my boat up my mpg go down to about 12 mpg. And some days its 10 mpg. It doesn’t matter, I’m either on my way to have fun or on my way home and either way I just want to get there as fast (within legal speed limit)or as soon as possible. If it really matters then you probably can’t really afford a boat or trailer. And if you are towing slow for “green” reasons then you shouldn’t be towing at all. Get a Prius.
Im not saying you should drive over the speed limit but you should feel just fine driving the speed limit when towing. I don’t even calculate my mpg while towing anymore, it just is what it is.

 

No way to know for sure for any given specific truck, but it's fair to say that the previous two posts, compared with one another, illustrate well why, on average, some Tacomas make it to 240-275K miles, and some go to 400K & beyond.

 

On average 96+% of tacomas do not make it to 250k.

Do the math anyway you want, save .02 a mile driving 57 instead of 60 and save 5$ a tank. Get to your destination 3 mins faster. Whatever. Doesn’t mean shit when some dipshit on their phone blows a red light and T-bones you totaling your truck.

While Toyota does dominate the top 10 longevity category, the wait for it….. Honda ridgeline is the only mid size truck to make the top 10. The Toyota tundra has better longevity stats than the Tacoma.

Do tacomas make it 400k? Yes. Do the majority? No. Not even 250.

This has spiraled quite a bit from ops original question. Can I tow my boat in 4th at 70?

 

Lol. I'm not here to tell anyone to drive like a granny, or to harp on squeezing every last MPG out of our hellaciously thirsty 4liters. I just like making connections between stuff I do and how it affects my equipment, and feel it's important to share that information. Maybe it'll save at least one future "help, my transmission is slipping and runs at 300 degrees" post. I love my truck and would really like to join at least the half-million-mile club someday.

I don't know or care what the mileage statistics are. But I'd be willing to wager that the ones that make crazy mileage are the ones that do the little things that keep the truck happy.

Yes, you can tow at 70 in 4th. It'll do it just fine. I do it too occasionally. Was merely suggesting that easing up a tiny bit (esp when towing on a regular basis) will likely save a lot more wear in the long run than most people think, with very little downside.

 

+1

lugging is hard on engines, in addition to being less fuel efficient. Adding more weight just makes it tougher on the engine.

 

you keep saying that more rpms to get more speed generates more heat. However ,you are not mentioning that the extra speed also creates additional cooling air flow, so some of this washes out.

 

No, I'm not saying the higher rpm is generating more heat. The significantly higher overall power load of that small increase in speed while towing is. I'm saying the higher rpms and extra cooling air become less effective as the total energy output disproportionately increases to maintain speed through the higher drag. Esp in hot environments

At lower speeds where you're not dealing with so much drag, downshifting for higher RPM to keep the engine cool absolutely does help because the engine is outputting less torque to do the same amount of work. Around 60-65 is where the drag curve starts going more vertical. You really start having to command a lot more power to maintain smaller increases in speed, and the small amounts of added airflow can only remove so much total heat. For example, brakes are air-cooled too, and it works well under normal conditions they were designed for. But if you start beating on them at high speed they'll still overheat, warp, and eventually the components will fail. Same goes for the engine and drivetrain components over time.

Yeah, the cooling system should keep up for the most part. But each part that the system is cooling is still running hotter because of the added strain. Cylinder pressures are higher. Pistons are hotter. Bearings/seals are wearing faster even with proper maintenance. Engine oil/trans fluid/diff oil is shearing faster and losing more lubricity over time. I'm not saying "ohmygod you're gonna blow the entire truck up if you hit 70". I'm just saying it amplifies the normal wear patterns to an extent that a lot of people don't consider because it's "only a 10mph difference." But drag is a function of speed squared. Every doubling of speed means 4x the power is required.

Throwing some round numbers out there, if your engine is producing 100hp to maintain speed at 50mph with a draggy trailer, it'll need to produce 200hp to maintain 75. That's a hell of a lot to demand on a regular basis, for hours at a time, from a machine rated for 236hp(these numbers are guesstimated but still demonstrate the concept). Tow package or not, the machinery will not last as long with the extra load on it regularly. It usually won't be immediately catastrophic, but the wear on a vehicle that has been driven at or near-max power regularly, will be comparable to that of a MUCH higher-mileage vehicle

That's why many heavy-hauling engines/drivetrains are super overbuilt, then de-rated to produce significantly less power than what the hardware can handle. And they join the million-mile club on the regular.

 

I will skip over the other mis-placed generalities here, but would like to address the drag misconception. Yes, I have built two airplanes and have undertaken several efforts in drag reduction and feel that I understand the engineering behind drag. You state that drag is a function of speed squared and this is true for airplanes. However, airplanes have two key categories of drag. The first is parasitic drag and the same principles apply to our trucks. THe other is induced drag and this is the drag created by the process of a wing producing vertical lift. Without lift, airplanes fall out of the sky regardless of speed and therefore a LOT of lift must be produced and this creates a lot of drag. Cars have wheels rolling on the ground to resist the forces of gravity and therefore have no wings producing lift and therefore no induced drag. Because over half the drag seen in airplanes is induced drag and trucks do not experience this, it is inappropriate to apply airframe drag principles (i.e. drag = velocity squared) to trucks. Yes, increasing speed will increase parasitic drag, but you have exaggerated the effect.

 

Respectfully, I don't think that's correct. Yes, there is a distinction between parasitic and lift-induced drag, but I wasn't factoring in any lift.

Unless I'm mistaken, Fd=1/2ρV^2(Cd)(A) still applies to ground vehicles. A body with "A" frontal surface area and "Cd" drag coefficient, moving through fluid(air) with ρ density, at V velocity will incur "Fd" drag force.

Induced drag calculation is a whole different beast that takes into account wingspan, wing surface area, aspect ratio, lift coefficient, etc, the result of which is then combined with the parasitic calculation to get your total drag figure. Which I'm sure was tied into whatever you were doing, but you were working with airfoils that I'm sure required much more calculus than simple drag figures for ground vehicles

So while my general numbers for frontal area and drag coefficient are estimated/pulled from google and I haven't run detailed fluid dynamics simulations, the end results are still reasonably in the ballpark, I think. Air calculations are pretty scalable and easy to work with

Though I welcome being proven wrong

 

To simplify, the OM states to tow in #4. It does not mention rpm’s. If it were problematic or harmful beyond a certain rpm (in 4) it would be stated. If higher rpm’s were problematic while towing the Toyota service garages would be full of Tacos with tranny or engine problems and they would soon modify the OM. So tow at what ever rpm makes you happy. Your truck can’t achieve very high rpm in 4 while towing anyway. If saving gas while towing is your goal then drive slow, or don’t tow or drive a Prius.

 

I'll say again for the last time, My main concern isn't about saving gas, and I never said not to tow in 4th at whatever RPM your heart desires. I'll drop down to 3rd or 2nd if I need the power momentarily. RPM =/= sustained power output.

All I said was "hey, if you back off to 60 instead of 70+ when you can, it'll be a lot easier on the truck without costing more than a few minutes of overall trip time...and as a BONUS, it's significantly better on gas." The only other reason I was mentioning gas is because that's where most of the heat comes from, as engines are only ~40 efficient at best. The rest of the gasoline's energy turns to heat. Which becomes a problem when its pushed so hard for long periods in hot weather, dumping fuel into an engine that's having a hard time speeding up.

You saying "Your truck can’t achieve very high rpm in 4 while towing anyway" only illustrates my point; Regularly driving faster than 60-65 while towing is pushing the limits of the machine for extended periods of time. Will it explode instantly? Probably not. But I don't think it's crazy to assume doing so on a regular basis will significantly reduce the overall lifespan of that powertrain. If you want to tow faster without beating on your engine/trans, then get a bigger truck with more than the pitifully anemic 236hp our V6s put out.

You do you, man. I'm just trying to keep my truck in one piece for as long as I can. New Priuses are expensive.

 

Lmao!!
No man, you gotta drive fast to outrun the rust.

 

As vehicles have gotten more expensive and our working dollars are worth less compared to the cost of goods, there is an incentive for us younger folks to take better care of our equipment. Takes a lot longer to save up for a down payment these days and it's not like newer vehicles are being built to last like older ones. (after all, wall street is obsessed with constantly increasing quarterly returns and you can't get those from a company that sells a good reliable product that rarely has to be replaced)

You retired guys have different financial incentives and are probably trying to get the most out of the time you have left on this planet.

 

Thankfully where I live in northern CA rust isn't an issue. No snow, nothing but pure neutral-pH rainwater on the roads in the winter.

 

I understand what you are saying.
But as someone that tows regularly.
The Tacoma with w/4.0 needs to be at around 2,700+ RPM to tow anything at or above 2,500 lbs (as in OP's boat)
And MPG is never going to be good at those RPM.

Sure drop it into a higher gear on down hills and super flat ground.
But don't lug the engine. That's worse that turning "more" rpm.

Personally, I'd rather spend the extra in fuel cost, vs hard parts cost.

 

a locked TC doesnt make more heat with RPM increase

My 2009 taco with a 3500 trailer never had an issue and I drove it in OD unless the TC unlocked too much which happened at lower speeds more like the 40-50 range

 

Well i will give up here. However, in the post above, you state thate if 100 hp is required to do 50 mph, then 200 hp will be required to do 75 mph. Sorry, but that just doesn’t pass the sniff test, for automobiles anyway, regardless of how many formulae you quote. Double fuel consumption to move from 50 to 75 mph???? If that were true, i am certain that at least a portion of the 250 M Americans would have figured that out by now and wouldn’t all be doing 75 on the freeway. Especially a few years ago when gas got to almost $5. If I could cut my gas consumption in half, even I would slow from 75 to 50!

 

Might not be exactly double but it's a massive difference in MPG on any non-aerodynamic vehicle, i.e. Tacomas.

I get about 13 mpg at 75 mph and 19-20 or so at 50 mph.

 

I get 420+ miles per tank doing back roads at 35-45
less in summer with AC
Less with highway use