Auto vs manual fuel consumption

I have a tiptronic 3.4 carerra 2 and a manual carerra 2 and thought it was interesting/annoying! How the tiptronic uses so much more fuel than the manual. So, I kept a little track of how much each used with the same type of driving and here are the results: Manual Town,125 miles, 30 quid = 22mpg Motor way, 165 miles, 30 quid = 29mpg Combined = 25.5mpg Auto Town, 90 miles, 30 quid = 16mpg Motor way, 120 miles, 30 quid = 21mpg Combined = 18.5mpg Fuel consumption is such a dull topic and not what these cars are about, but, does this kind of consumption align with everyone else's auto and manuals?

Oh and it might be worth noting that the manual has been remapped with a sports exhaust, supposedly worth 40hp. It is noticeably quicker than the auto. Don't know if this would / could improve the mpg.

Big difference!

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Well, it's obvious!! Clearly, you're not wringing out the manual nearly enough!!!

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Contrary to most people's belief, it is possible to get better MPG in a manual compared to older (including the 996) slush boxes. It all depends when you shift and you can adapt to the environment better than the auto can (e.g. up shifting sooner as you crest a hill, etc..). Most people don't have the knack though which is why autos are generally considered better. Indeed. I checked ours the other day and found 14.3mpg I like 3rd gear. A lot

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Oh the manual gets ragged! the tiptronic encourages you to cruise more than the manual, particularly because its a convertible.

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Driving both the exact same, the manual will always get at least 5mpg better than the auto. My manual never gets below 20mpg. The Autos are renowned for drinking more fuel because they slip the clutch, aren't they

Done some research, and supposedly, the remap gives 4-7mpg improvement.

Im somewhat surprised by this - by your use of 'quid' I assume you are UK based? Might be wrong. I have a C4S Tip (UK spec, RHD) in SW France. I regularly drive between there and SE Spain over the Pyrenees, a journey of about 650 miles, and even with some fun in the mountains over the whole journey I normally get 30mpg plus. OK, an hour of real fun and ten hours on the autoroute at 80mph, but very different from your findings. Sadly I calculate brim to brim, and spend a few minutes converting to MPG as I am British and its what I understand........

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Yes i am from England. Comparing that to my tiptronic, i find that astonishing. If i was to drive my manual at 80mph on the motorway i have no doubt it would do 30mpg but the tiptronic would not get near there. Maybe there is something amiss with my tiptronic car, making it more thirsty..

I don't get anywhere even close to 30mpg - about the best I can do is a little of 18 With a MT. R

Can't speak to the tip, but my manual regularly gets 21-22 around town, and reliably 27-28 on the hwy (once got 29 on a 600 mile trip, but the weather allowed the A/C to be off some of the time). I always expect autos to get slightly worse mileage, but the numbers I'm seeing posted here are significant.

We saw 16-18 in town ('01 C4 Cab w/tip), and 27-28 on the open road. This is a little worse for in-town mileage than my manual Cayman (lighter weight, but same size engine 3.4L), and a couple MPG better on the open road. Mostly this is because the Cayman is turning 1000 RPM higher at freeway speeds than the Carrera. Open road where I live means 50+ mile stretches where you often don't even have to hit the brakes. I know that where many folks live, even interstate freeway driving is often somewhat stop (or slow) and go.

Hi Luke. Its always difficult to make comparisons with those 'over the pond' due to the significant difference between the smaller US and the larger UK gallon (how did that ever happen!!) but I have plenty of pals who get 30 mpg from Tips on runs. That said, if I am playing more than touring it will drop to well below 20mpg.....

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I never realised the difference in our measurements until now. The english gallon has 4.55 litres, the American gallon has 3.8 litres. Obviously this means the English MPG is higher. I have adjusted my results accordinly: American MPG: Manual: Town,125 miles, 30 quid = 18mpg Motor way, 165 miles, 30 quid = 24mpg Combined = 21mpg Auto: Town, 90 miles, 30 quid = 13mpg Motor way, 120 miles, 30 quid = 18mpg Combined = 15.5mpg English: Manual: Town,125 miles, 30 quid = 22mpg Motor way, 165 miles, 30 quid = 29mpg Combined = 25.5mpg Auto: Town, 90 miles, 30 quid = 16mpg Motor way, 120 miles, 30 quid = 21mpg Combined = 18.5mpg

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10% was the "accepted" gas mileage penalty for an auto. You are seeing more than that. Are you really comparing apples to apples, though? The 3.4l engine is just plain old VarioCam while the Carrera 2 if a newer model and if fitted with a 3.6l engine could have VarioCam Plus. VarioCam Plus can help with gas mileage. I hope you are not running the A/C on one car and not the other. Also, check those tire pressures.

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I can get 26 American mpg on freeway if not pushing too hard and 18mpg in town on my 3.6 mt...

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They're both 1999 2wd 3.4litre. But the auto is half a year newer and has an electronic throttle body, which the manual doesn't. Yes both have air con on the same amount. What's vario cam?

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Do both cars have the same type/style of AC? The auto climate system -- both of my cars have this -- is quite efficient and imposes a smaller parasitic load on the engine than the other style. If the two systems are different I doubt this would account for but just a small fraction of the differences in fuel consumption though. What are the miles on both cars? Do they have about the same miles? Are both engines in good "tune"? That is are plugs/coils reasonably "fresh"? How is tire life? Is tire wear even? If tire life lousy with one car and tire wear uneven this is a sign alignment is not optimum. Tire wear is friction and this requires more work from the engine to move the car down the road. Again even if this is the case the additional cost in fuel is not all due to alignment. Are the engine air filters and fuel filters ok? Are you using the same oil in both engines? An approved oil? A direct linkage throttle can be more efficient than an electronic throttle under some conditions. With a mechanical throttle one "trick" to maximize fuel economy is to get up to speed and in as a high a gear as reasonable. One doesn't want to lug the engine but at relatively low engine RPMs and "high" load with the throttle open a bit more than it might be otherwise pumping losses are reduced and engine efficiency goes up. E-Gas might not be quite as efficient in this regard because one doesn't have as direct control over the throttle. However, I have driven my 2006 GTO like this -- and it has (had -- I sold the car some few years ago) and e-Gas system very much like the Porsches I own -- and in 6th gear at say 1500 RPMs -- on a flat stretch of ground -- fuel economy as reported by the instantaneous fuel consumption readout approached hybrid territory. But any change in the gas pedal to momentarily reduce speed then regain speed had the fuel consumption falling like a rock in soft water. In spite of the above, I do not drive either of my cars this way. I keep RPMs up a bit higher. I have found with my 996 Turbo that driving too slow is not as fuel efficient as one might think. A few times I have had to drive slow due to weather -- snow -- or restricted speed due to extended stretches of construction or because the speed limit is just set low (60mph in Oregon IIRC or other long stretches of highway where I know speed limit enforcement is very aggressive) -- for an extended time and fuel consumption goes up. Not lots but some. However, and this is nice, at higher speeds up to and above 75mph to say 80mph fuel consumption goes down and the Turbo delivers its best mileage. I like to believe this is explained by these cars are used in some regions of the world where higher speeds are legal and yet where gas mileage is very important due to the very high price of fuel and Porsche did its homework. VarioCam refers to the system of the engine that controls intake valve timing. In the early water cooled 911's and Boxster engines the valve timing was an either or thing. Either it was advanced (where it "normally" would be) or it was retarded. For my Boxster the timing is retarded at engine start and idle and up to around 1400 then it is advanced. It is retarded again at a skosh over 5K to help fuel economy. I do not have the numbers handy for the 996 engine but would expect they are about the same. You didn't ask, but VarioCam Plus not only controls intake valve timing -- and in some models allows for variable timing adjustment -- it also controls intake valve lift. Low lift is selected at cold engine start as this lowers pumping losses and reduces fuel consumption and improves emissions. As one starts to drive away as RPMs rise to some point the high lift is selected. I do not recall off the top of my head but it is at a rather low RPM. I try to avoid operating the Turbo engine at too low an RPM to avoid possibly triggering low lift to save wear and tear on this hardware. While Porsche claims to have tested the hardware for like a bazillion cycles I see no reason to try to push the engine's luck.

Hi Luke, To be honest, the figures you quote are not very scientific at all (I realise they are not suppose to be) but in order to really understand what the differences are you can try the 'brim to brim' method, i.e. full both cars up, drive for a tank or so, refill and note mileage and amount of litres(in the UK) you add to fill both. Measuring it by cost (30 quid, or pounds) introduces a number of other variables, not least the price at the pump. Once you have the brim to brim figures then you can make a direct comparison over perhaps a tank. You might be surprised by the results, although of course it will take some time and miles to produce. If I remember correctly there are 4.55 litres to a UK Gallon. In general your figures seem very low, but of course this also depends on style/roads/traffic and in particular length of journeys - of course short journeys are a real killer of gas mileage. Hope this helps mate.

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Both have the same auto feature air conditioning. The cars are less than 10,000 miles apart in miles. They are both regularly serviced, but as previously mentioned, the manual is remapped and has a sports exhaust. Plugs changed with services, coils have been changed in recent years according to service history. All wheels also aligned in the last service to both cars. This higher fuel consumption doesn't bother me too much, i just found it intriguing how the automatic drinks so much more.

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Hello Twills, Thanks for your post, but i disagree. Measuring in the smaller quantity allows me to ensure the type of driving done by both cars is similar and therefore achieve more accurate results. If i was to fill the cars up i would not be able to keep a track of where i'd been in each car, how much traffic i'd sat in etc etc. Basically, there would be more variables if i was to do it your way. Furthermore, when i have done this I have filled up at the same petrol station, at the same price, so again, no variables there. I stated in my findings what kind of driving each figure was produced from. One was purely on the motorway and one was just driving around town, so you can assume, naturally, there was traffic and stop start driving. Anyway, the pinpoint accuracy of this test is beside the point. Even if there were variable inaccuracies going against the auto, they would not be anywhere near significant enough to account for the difference to the manual. I wondered if anyone else had a manual and an auto and had encountered the same difference in the cars.

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That's right where mine is, or in the close ballpark. I may get a little less hiway due to going faster, and a little less town due to a ton of stop and go crawling driving to work and back, and stop and go in town. I could be wrong, but it looks like this automatic uses a torque converter. This means that the engine is spinning something if it's running - even if you're stopped. So in the city that could easily explain the mpg difference (milage isn't great when these cars are stopped and idling) since it's doing a few percent more work than a manual. Getting going would be the same thing, there is always going to be more loss in the system than with a clutch, and not to mention shifting points and/or awareness to downshift or upshift depending on conditions or emotions. Another example would be my old Moto Guzzi motorcycle, it was one of the "automatic" ones and has a torque converter. Works great as a 1000cc moped really, but the idle needs to be a bit higher due to the extra work the engine is doing, and although driving it in traffic is fun and easy, it does eat up more gas. Once it gets going there is no overdrive locking it up, so it still gets a little less than it would with a standard transmission. IMO

On highway driving my Tip gets pretty good mileage but the city stop and go sucks gas. Having the top down makes the car less aerodynamic I am sure. In city driving it gets better mileage when I have it in manual and shift with the buttons opposed to drive mode. I drive it like I used to drive my old mt Boxster and run the gears out a little. The Tip does drive me crazy with the starting in 2nd gear and how fast it rolls through the gears, I mean 5th gear at 35mph. The up swing, my wife has grown a fondness for Porsche and really enjoys driving the car. Yeah, I am working the his and hers Porsche angle.

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I was getting 22-24mpg going 65-85 for about 150 miles a few weekends ago. Even peaking at a little over 100 didn't really seem like much with the top down.

I get about 21 mpg around town in my 99 (3.4 L) with 6spd manual. The one long trip I took back from Illinois (what I would consider real "highway" driving) I got around 27 mpg.

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Interesting then the Tip fuel consumption is so much higher. Explains maybe why the PDK was developed (resurrected and refined).

I am averaging 19-21 on mixed commute with about 1/2/ to 2/3 spent on the highway. That is a '03 C4S Tip in Auto most of the time. My understanding is that the earlier tips also had limited mapping programs available to the TCU. Is this a possible issue?

I have never reset my average MPG reading and it was at 19.3 MPG the last time I checked. This is city and highway average, but mostly city. I figure if I take trip to Vegas, I'll have a better idea of highway MPG.