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YOUR RESEARCH60 k, new plugs, tires, oil...lost some MPG
Leading reported evidence
Replace with the seasons (summer
Performed · outcome unconfirmed
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Read evidence and context
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In some regions gas blend is changed with the seasons (summer to fall/winter and winter to summer) which can affect gas mileage too.
Leading evidence: 'Replace with the seasons (summer' (no explicit attributable resolution; this is a reported action or suggestion). Across 1 matching discussions, The most frequently discussed action is 'Check fuel gage i can leave the station' (1 discussions). For that action, 0 discussions report a resolution and 0 report an unsuccessful attempt. No action has an explicit, attributable resolution report under the current rules. These are forum reports and suggestions, not a confirmed diagnosis.
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Since I reset the on board computer with its gas mileage reading at fill up this can result in a drop in fuel economy that does not reflect reality.
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I read another thread on another forum about an owner who did the plugs in his Cayman and used some other brand of plugs -- that at least one cross reference sheet claimed were a match for the Beru plugs -- and he had issues afterwards that eventually drove him to replace the plugs.
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Assuming the right plugs were used there can be other reasons.
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Suggestion
Otherwise the shut off occurs too soon and unless I check the fuel gage I can leave the station with less than a full tank.
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Suggestion
While the lower octane seems attractive due to its lower cost at the pump fuel mileage can suffer and one's actual fuel cost can go up as one is filling the tank more often because the lower octane gasoline is less energy efficient.
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Suggestion
By using the upside down nozzle fill ups are consistent and gas mileage results from tank to tank are consistent as well, barring a change in vehicle usage which sometimes happens as I end up using one of the cars, the Turbo, as a cab ferrying my parents around town which kills gas mileage.
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Suggestion
I read another thread on another forum about an owner who did the plugs in his Cayman and used some other brand of plugs -- that at least one cross reference sheet claimed were a match for the Beru plugs -- and he had issues afterwards that eventually drove him to replace the plugs.
Use pencil and paper over courses of known mileage
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Suggestion
As the speedo on 996s tends to be inaccurate, and as tire diameter is probably somewhat different with your new tires, you might use pencil and paper over courses of known mileage to find your true mpg, if you have not already done so.
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Rennlist: 1 discussions
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Outcome without a verified action link
Low octane gasoline has the same or close enough to the same to make no difference BTU content it is the low octane grade that requires retarded ignition timing to avoid detonation that results in less mechanical energy being obtained from the chemical energy in the gasoline that accounts for the drop in fuel mileage.
Well I just hit 60k a few months ago. I got new plugs, did an oil change. After the new plugs I started getting less MPG on long trips?? I did get new tires, Conti DWS but I cant seem to figure out why I lost 5 mpg. I did a search and saw some post but just wanted to pick peoples brains and see if anyone had any ideas as to why.
No need to troll about the MPG....we all know that it is a sports car. I have a 2003 C4S...thanks in advance for your help.
I the last few weeks I picked up a fabspeed x pipe with 200 cell cats...great sound btw. Did the PSE hack on the relay...very easy and still have the PSE when I need it.
As the speedo on 996s tends to be inaccurate, and as tire diameter is probably somewhat different with your new tires, you might use pencil and paper over courses of known mileage to find your true mpg, if you have not already done so.
Hotelguy, I'm also paying attention to mileage. A 03 C4S. Daily commute of 140 round trip miles. Some stop lights mostly high speed runs with semi occasional full throttle passing bursts
I average 23.6 currently. What are your driving habbits slow and steady or enthuastic romps through the gears ?
Check your tire pressure and change your engine air filter too, drop in a bottle or two of techron over the next month. I have DWS tires and I am getting between 29 and 31 on the highway at cruising speeds (70 to 80).
I hope you used the right plugs. I read another thread on another forum about an owner who did the plugs in his Cayman and used some other brand of plugs -- that at least one cross reference sheet claimed were a match for the Beru plugs -- and he had issues afterwards that eventually drove him to replace the plugs.
These replacement plugs have the sunken center electrode for what reason I have no idea.
Assuming the right plugs were used there can be other reasons. As another poster noted a change in weather can affect gas mileage. Colder temps mean a decrease in gas mileage. This can also result in under inflated tires -- every 10F temperature drop has the tire pressure drop by one PSI -- which have a big impact in fuel economy.
I have to point out if you fitted non-standard or non n-rated tires this tire choice can play a role in fuel economy. (I've run N-rated Conti tires on my Turbo and they are a fine tire so I'm not disparaging the Conti tires per se.)
In some regions gas blend is changed with the seasons (summer to fall/winter and winter to summer) which can affect gas mileage too.
There are as I covered above various things that can account for the gas mileage change. There is also the human element.
One factor is fueling. I have found to ensure consistent shut off of the gas nozzle when fueling my cars I have to turn the nozzle nearly 180 degs. Otherwise the shut off occurs too soon and unless I check the fuel gage I can leave the station with less than a full tank. Since I reset the on board computer with its gas mileage reading at fill up this can result in a drop in fuel economy that does not reflect reality.
By using the upside down nozzle fill ups are consistent and gas mileage results from tank to tank are consistent as well, barring a change in vehicle usage which sometimes happens as I end up using one of the cars, the Turbo, as a cab ferrying my parents around town which kills gas mileage.
On the subject of gasoline be sure you are running the right octane grade. While the lower octane seems attractive due to its lower cost at the pump fuel mileage can suffer and one's actual fuel cost can go up as one is filling the tank more often because the lower octane gasoline is less energy efficient.
Low octane gasoline has the same or close enough to the same to make no difference BTU content it is the low octane grade that requires retarded ignition timing to avoid detonation that results in less mechanical energy being obtained from the chemical energy in the gasoline that accounts for the drop in fuel mileage.
I have an 04 C4S and just replaced the tires (Sumitomo HRT Z III), oil & filter, plugs & coils at 57K. I have it parked because of snow and haven't used it as a commuter yet this year. I was getting 18-20 / tank (mix of city/hwy). I will keep an eye on it and post my results once I empty a few tanks.
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