Slipping gas mileage, any suggestions?

Messages
26
Location
Tennessee
My '98 frontier ka24de gas mileage has been slipping just a bit. For a couple months I consistently averaged about 26 mpg. For the last few weeks it's been closer to 24mpg. I drive the same route to work each day, about 30 miles each way. I don't run the a/c a lot. I've filled at the same gas station for the most part, with a few attempts at variety to see if it made any difference. I usually run 87 shell. My last tank was 93 octane, mpg was 24.08. Any ideas or suggestions?
 
Messages
7,108
Location
MIchigan
The answer to your question is staring you in the face. High octane gas (93) burns slower than (87) therefore it gets ejected out the exhaust valve. Check the owners manual to see what they suggest using probably 87. I use mostly shell 87 also because it's one of the few out there that doesn't contain ethanol.
 
Messages
2,513
Location
Richmond, VA
I got less mpg with high octane gas too. I need premium for my lawn equipment and I'll usually just top off the truck when I'm getting gas for my mowers. Every time when I check my mileage after using premium gas I get about 1 mpg less.
 
Messages
5,435
Location
KC
using gas at an octane other than what your user manual specifies can have adverse affects. If your manual calls for 87, you'll get your best performance with 87 unless you have modified your car in a way in which it needed higher pressure.
 
Messages
7,077
Location
Ontario, Canada
I once tried dosing the fuel in my previous car with large amounts of MMT octane boost, and the gas mileage went way down. But MMT is a combustion poison, so it probably wasn't the increased octane number that caused the poor economy. However, if they use ethanol to make the high octane gas, and the regular has no ethanol, like what Shell is doing over here now, the regular will have more energy per gallon and definitely will give better mpg.
 
Messages
1,910
Location
Vista, CA
quote:
55 sez "High octane gas burns slower? Nah, that's incorrect."
Octane is not a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
 

merc80

Thread starter
Messages
26
Location
Tennessee
Using 93 octane is not the problem. I run 87, my truck specs are for 87. I tried one tankful of 93 just to see if there was might be any improvement, which there wasn't. I put a bottle of techron additive in my tank today and I also replaced my distributer cap. My plugs have a few thousand miles on them, but they look ok. Air filter is less than a month old. Any other ideas?
 
Messages
1,983
Location
New Brunswick
quote:
Originally posted by LarryL: Octane is not a measure of power but of the fuels’ resistance to ignition from heat. A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
Come again? I was always told, and I think I've even read it on this site, that Octane is a measure of gasoline's resistance to self-ignition. How would that make it burn slower?
 
Messages
9,103
Location
MN
Merc80, 1) Take your ride out in a run if you can with the Techron in it, 100-200 miles in 1 shot. Rev up the engine in 1st or 2nd gear as high as it will let you. You'll see a little white smoke come out of the exhaust, that'll blow some of the carbon out. 2) If possible, run some Seafoam through the intake/carb, whatever you got and rev it up. 3) Fill your tires up to near max psi, that could be part of it if it's low... 4) Were you driving into the wind or less time behind a larger vehicle...? With the wind or (safely) behind a large vehicle will increase your mileage... 5) Check a few other things out too, PCV O2 sensor, etc. Let me know via BobMail... Good luck!
 
Messages
1,910
Location
Vista, CA
55, maybe you should do a Google search and some research of your own. If gasoline ignites before the spark arives, it's possible that heat caused it, from compression, and maybe a hot exhaust valve or a bit of glowing carbon in the combustion chamber. In such a case the mixture often explodes, when is not good. It can even push back on the pistion that is still moving up. This reduces power and sometimes damages the engine. The type of gasoline that resists this also burns slower. You gain back the performance by increasing the compression ratio and adding some spark advanse to start the burning a little sooner. You want a nice even flame front across the chamber not an explosion, but a good hard push on the piston just as it starts back down. Some high octane gasoline even has less potential energy than the same amount of regualr gasoline. Most are about equal, but there is a potential for just a little less with the high octane stuff.
 
Messages
2,187
Location
Arizona
quote:
Originally posted by LarryL: Octane is not a measure of power but of the fuels’ resistance to ignition from heat.
That I'll agree with, though of course it's more complex than just that.
quote:
A higher-octane fuel, under identical combustion chamber conditions, will burn slower.
That's flatly incorrect, according to Chevron's paper(s) on the subject. I'd post the direct link for you, but since Chevron re-did their site my bookmarks don't go anywhere any more.
 
Messages
43,667
Location
'Stralia
Flame speed in petrols (gasoline) is pretty well constant regardless of octane rating, unless you are at borderline detonation, then the lower octane stuff can spontaneously ignite, with a seemingly infinite flame front. Octane has nothing to do with flame speed in normal (quiescent or turbulent) combustion, nothing to do with the conversion of hydrocarbon/air mixtures into heat, and certainly doesn't lead to either excessive fuel consumption or engine deposits (unless additves like TEL or whatever are doing the depositing)
 
Messages
22,188
Location
Colorado Springs
I wholeheartedly disagree shannow. I get less mileage and less power running premium fuel in my vehicle, especially in winter. High octane fuel, in and of itself, will make less power in an engine if detonation is not present, which leads to more fuel being burned at lower temps, which equals more deposits. I see people on message boards all the time say that they were expecting this miracle jump in power only to find the opposite after they fill with premium (in cars that don't need it). Man, there was a HUGE article in a Ford Mustang performance magazine about this very subject about a year ago. I'll try and find it. They got the most power out of a heads, intake and cam swapped 5.0 on 87 octane fuel.
 
Messages
1,251
Location
Akron, OH
Higher octane fuel *DOES* have less power per ounce. This is tangential to our discussion, but I saw a guy doing dyno testing on sport motorcycles who saw a 6% drop in horsepower when he switched from 87 to 93 for one dyno run.
 
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