Premium vs. Regular Gas while Towing

[rip-cord]

We purchased a travel trailer in 2010 and have towed it for two seasons. We have a 2009 V6 Rav 4 Sport that we tow the trailer with. The GVW of the trailer is 3,500 lbs so at the top of the Rav's towing abilities. I have installed an external tranny cooler and for the past two seasons have had no issues and the Rav pulls great on the highway up and down hills.

My question is about gas mileage I notice if I use premium when pulling the trailer my mileage improves by about 5 to 10% over regular. Of course it goes down significantly when hauling the trailer overall but I wanted to know if it is better to put in premium when pulling. I am thinking it burns cleaner thus saves the mileage (might be a boost in power but didn't notice). I don't think the savings offset the price but is it better for the engine or should I just stay with regular and burn more of it?

Thanks
Darryl

[RTexasF]

I doubt that it burns cleaner but if you see improvement while towing with premium fuel then there is your answer....use it. I'm all for 87 octane but not when towing a full load like that.

[SteVTEC]

It makes more torque on premium, which lets you hold higher gears for longer. And I'm guessing that under load, the brake specific fuel consumption of the engine is a bit better on premium than on regular also.

[Blogson]

Hmmm - conventional wisdom is that premium gasoline has a slightly lower energy content per unit than regular and so mileage should be slightly lower when using premium, but of course computerised engine management systems possibly could turn that around. Is Canadian premium sans ethanol and does regular contain it?

I've pulled heavy loads when using regular and have had no problems. Ordinarily if there is a fuel related problem it would be engine pre-ignition, which should be audible. If not, according to my engine tech brother-in-law, pre-ignition should show up in code readings. Modern engine management systems are designed to prevent pre-ignition.

[rav4two]

Many Rav owners have noticed that the engine seems to be able to hold onto higher gears for longer with higher octane fuel. These are the same engine operating conditions, of high load and lower RPM, that would cause pre-ignition. So when I know we'll be travelling in the Rav, I always use the highest octane I can find no matter how loaded we are.

[SteVTEC]

Originally Posted by Blogson

Hmmm - conventional wisdom is that premium gasoline has a slightly lower energy content per unit than regular and so mileage should be slightly lower when using premium, but of course computerised engine management systems possibly could turn that around. Is Canadian premium sans ethanol and does regular contain it?

Premium does have a slightly lower energy content per unit than regular, which is why for long distance driving at light loads (not towing) you'll typically get better mileage on regular gas. But premium makes up for it by being able to produce more peak power per unit of fuel than regular due to higher knock resistance, which allows more advanced ignition timing to be run. If you're towing, with the way the electronic throttle control is setup, you'll actually be hitting at or near FULL throttle in top gear before it downshifts to a lower gear, despite the fact that your foot is far away from the floor. So even if you think you're light-footing it, the engine is actually heavily loaded.

Originally Posted by Blogson

I've pulled heavy loads when using regular and have had no problems. Ordinarily if there is a fuel related problem it would be engine pre-ignition, which should be audible. If not, according to my engine tech brother-in-law, pre-ignition should show up in code readings. Modern engine management systems are designed to prevent pre-ignition.

Knocking (or detonation is the more technical term), not pre-ignition. Pre-ignition is something totally different. And yes the engine computer will pull timing if it detects knocking, so it should never be audible on most modern engines. Any spark-ignited engine that has pre-ignition has very serious problems.

[Blogson]

[QUOTE=SteVTEC;901793
Knocking (or detonation is the more technical term), not pre-ignition. Pre-ignition is something totally different. And yes the engine computer will pull timing if it detects knocking, so it should never be audible on most modern engines. Any spark-ignited engine that has pre-ignition has very serious problems.[/QUOTE]

Yeah, I should have been more accurate ref. detonation and pre-ignition. Thanks for reminding me. Unfortunately the results for an engine when either condition continues on a sustained basis are the same . . .

[RTexasF]

Yes it can reduce timing with inferior fuel but here is the big question........When the 3.5l in the RAV is under load is it smart enough to advance the timing when using 91+ octane? From what the original poster said it appears so. Hard to prove but his MPG gain sounds like it has the smarts to do so. Not all car ECU's do. Does ours?

[Dr. Dyno]

I'd run what works best when towing. The timing algorithms are probably set for light non-towing loads. With the engine under heavier load the detonation detection will retard timing but not as much on premium as on regular, thereby running more efficiently and giving better mpg. But if premium doesn't justify the higher price or give easier towing you could go back to regular. It's your choice. Neither one will run "cleaner" than the other. That's purely an advertising word.

[kenm09]

I'm not a fuel expert...but @OP, what kind of travel trailer do you have and does it tow?