Better Gas mileage while towing

mitchntx · June 14, 2004

Wow!

I towed my race car about 300 miles in hilly terrain and a 10mph head wind, keeping the speedo around 70. I used about 26 gallons of gas.

I filled with 89 octane instead of 87 and I used about 19 gallons ... same conditions, but with a 10mph tail wind.

I would have thought the tail wind would have helped a little, but that is significant.

Anyone else significantly helped their mileage with higher octane?

snoman · June 14, 2004

THis is a sore subject with some people but low octane is not alwaya a wise choice in a modern engine especailly when towing. ECM will try to controll knocking under load by retarding spark which lowers MPG and power. THe tail wind helped but so did the gas The only truck I have that I use 87 in it a old 79 J20 jeep truck with 8 to 1 compression, my newer ones use plus or better all the time.

Charlie in TX · June 14, 2004

I have had a similar experience. However, I haven't had enough data to make a final conclution.

I used to get 13-14mpg around town in my normal driving and 14-16 one the highway (mostly 350 mile trip one way trip to the deer lease pulling a light trailer). I distroyed the factory tire on one trip when I got stuck. I put on BFG 265 75s. That day my around town driving went to 11.5-12.5 and 10.5-11 pulling the same trailer on the same roads and speeds. This past weekend I went to the lease again without the trailer. From Houston to Uvalde on a tank of gas, got 12.5mpg. From Uvaldy to the lease back to San Antonio on another tank, got 12.5mpg. Where I stopped gas was relitively cheap, $1.73 for 87 octane, $1.79 for 89. I filled with 89. From there back home I got over 14 while going the same speed as before.

I see a couple of posible reasons. 1. The octane improved engine effecency. 2. In Houston I bought RFG, I don't know about Uvalde, In SA I may have gotten real gas. I will perform more test to find out.

robsawatsky · June 14, 2004

Unless you are experiencing knocking or pinging, the octane rating of the fuel will have no effect on power, economy, operating temperature, engine deposits or almost any other measurable parameter. As stated, engines with knock sensors will retard the timing automatically, sometimes so sensitively that you may never hear any ping. So in some cases the only way to tell if the ignition is being retarded is with an OBD II scanner hooked up while you are running. If it isn't being retarded and there is no ping or knock then using a higher than necessary octane fuel is a waste of money. This is not to say that fuels don't vary in many ways but the octane rating itself alone is not a good indicator of fuel quality or performance.

snoman · June 14, 2004

Unless you are experiencing knocking or pinging, the octane rating of the fuel will have no effect on power, economy, operating temperature, engine deposits or almost any other measurable parameter. As stated, engines with knock sensors will retard the timing automatically, sometimes so sensitively that you may never hear any ping. So in some cases the only way to tell if the ignition is being retarded is with an OBD II scanner hooked up while you are running. If it isn't being retarded and there is no ping or knock then using a higher than necessary octane fuel is a waste of money. This is not to say that fuels don't vary in many ways but the octane rating itself alone is not a good indicator of fuel quality or performance.

This is not at all true as it is possible that it may not knock because ECM can retard spark enough for conditions to prevent it but at cost of performance and MPG. Even my wifes 2000 Jeep Cherokee with a 4 banger and a stick runs better on plus or premium. It will run on 87 without knocking but it runs better and smoother on plus of better and she gains 1 or 2 MPG too when using it. My 89 burb has alway run much better on premium and knock badly on 87 and if you time to run 87 it is a slug and uses more gas. Even my 2000 K3500 runs better too. Engine temp, compression ratio, altitude, load and even humidty determines octane requirements but as a rule of "thumb" if you have 9 to 1 or higher compression you "should" use plus and if it is above 10 to one you "should" use premium but there will always be those that will spend 30 grand or more and truck, a few grand on custom wheels. more yet on an exhaust system and "bitch" about spending a nickle to a dime more a gallon for gas.

mitchntx · June 14, 2004

OK ... we have point and counter-point.

Anyone else?

snoman · June 14, 2004

OK ... we have point and counter-point.

Anyone else?

And your own observations as well!!!

mitchntx · June 14, 2004

I'm trying to learn. And consequently, stay neutral.

But, un-scientifically speaking, it does appear that I got better gas mileage by going to a slightly higher octane.

My race car is LS1 based, so I do realize and understand the affects low octane has on power output of an OBD laden vehicles. However, on my "older" version (98) it does take the ECM several cycles to properly "learn" the amount of knock to expect & the completeness of combustion derived.

I never expected near immediate results.

And, like I said, there was a slight tail wind coming back and would account for some increase in mileage. I just can't quantify how much of a difference it SHOULD have been.

Wingnut · June 15, 2004

Did you go the same speed both ways?

Just wondering, because when I did my little "experiment", I gained about 8 mpg, just by dropping my speed by 10-12 mph.

mitchntx · June 15, 2004

Yes ... very much the same ... between 65 and 70

robsawatsky · June 15, 2004

This is not at all true as it is possible that it may not knock because ECM can retard spark enough for conditions to prevent it but at cost of performance and MPG.

I think if you read carefully at my post again you will see that I'm agreeing with you on the effect of knock-sensors and ECM on performance. The trick is finding out if it really has an effect on ignition timing if you never hear any knock or ping.

If there is no knock or ping, at the optimal ignition timing, then an increase in octane rating will do nothing. I'm trying to get away from the myth that higher octane (in and of itself) rating means a fuel with more energy. The only way more energy can be extracted from a higher octane rated gasoline is because it can be run at higher cylinder pressures (higher compression ratios being one factor) without knocking and thus extract a bit more efficiency out of each power stroke.

Also, the advances in cylinder head design tend to make it rather easy to get an engine with 9:1 compression that will run just fine on 87. My 5.7 Vortec with 9.4:1 runs great on regular, even when pulling a load.

So, the only way to really know is try it - but I don't trust seat-of-the-pants evaluations with respect to performance, unless there are really substantial differences, which is why I stated an OBD-II scanner may be the only way to really know if changing to higher octane fuel makes a difference in the absence of ping/knock.

SxViper · June 19, 2004

Ok I just got back about a month ago from hauling from Maine to Ohio and back. I was pulling a Pop UP trailer weighing in around 3,500 pounds. I found a nice little trick for my truck atleast. When I went out I got 11.5 to the gallon with the cruise set at 70... Now I did push it up to 75 a few times in some nasty Lighting that I was tring to get through but the whole way out I got 11.5... On the way back I tried running at a little slower of a speed.. 68MPH witht the tow haul mode set I was able to keep the truck in over drive the whole time. I got 13.5MPG on the way back.... I will now be towing at 68 instead of the 70MPH... Might take me a few mins longer but I will save a boat load in gas... I was real happy when I saw that.. I was figuring maybe pushing the MPG to 12 if I was lucky... But 13.5 is really good for a 6.0L Crew Cab Long Box 4x4...

How can I tell on a ODBII scanner if the timeing is being retarted? I have one and have not really looked into timing on it.. I am wondering if I jumped to 89 if I would do even better....

Chevyfreek · June 28, 2004

I run 93 octane in my truck and have been sense I bought it. I did try 89 octane once and my truck just seemed to not like it all that much. I think the additives in the premium gasoline help to keep the fuel system cleaner IMO. I have a Westers 93 octane hot tune on my truck I average around 13.5 mpg between the city and highway driving that I do. One thing I can say for certain is that the 6.0L I have gets better gas mileage when my cruising rpms are under 2000. Which for me means just under 65 mph with the 265 tires and 4.10 gears I am running. The closer to 60mph I cruise the better the gas mileage I get. I have seen as good as 16+ mpg on the long road trips out of my truck. My experience has been that higher octane fuel yield better fuel economy in these newer LS1 based engines. No real scientific proof, just my personal experience. It definitly gives you more power when you have an custom computer tune...

snoman · June 28, 2004

I think if you read carefully at my post again you will see that I'm agreeing with you on the effect of knock-sensors and ECM on performance. The trick is finding out if it really has an effect on ignition timing if you never hear any knock or ping.

If the senor is working properly, you will never hear the knock or ping until it gets so bad that the computer can no longer retard spark enough to avoid it. The senor "listens" and "feels" for a harmonic frequency in block caused by knock and it can hear it before you can most of the time so this is not a good test.