Buddy G's post on getting 28 mpg in a modded car triggered replies about changes in jetting resulting in errent calculations. My car has been flashed, and I was surprised to see my mileage improve as well.
On the last 3 tanks, I compared the computer calculation for mileage to my actual calculation of dividing miles by gallons pumped into the tank

My computer is .5 to .7 mpg optimistic on each tank. As there can be a difference in howwmuch gas your car takes at different pumps or stations, I varied my purchases.

Here is my question. Has anybody checked their unmodded car for accuracy on this computerized calculation?
More paranoid, can the reflash intentionally or unintentionally modify this calculation?

To be clear, my car has stock intake and fuel internals, so the error is either built-in or installed. Any thoughts? AS

 

How low are you running the gas quantity before filling up?

 

Down to the warning light- about 12.5 gallons typically. AS

 

I've been told that because of the way the gas tank is constructed, the fuel quantity is estimated by a computer when the quantity gets below approximately 1/4 tank remaining. With the quantity being estimated by the computer, the mpg is also being estimated by the computer...and maybe sometimes not very accurately.

 

I've put as much as 14.6+ gallons in at one fill up but cannot remember the mileage at that time. I've been getting as high at 24.3+ and about 21-22 average on my latest commutes.....Bone StockT2

 

T2, do you double check the mileage manually, or are you depending on the computer? My point is that the computer is a bit optimistic. AS

 

I had a strange experience yesterday that I htink backs up Dock's assertion that the computer is guessing the quantity below 1/4 tank.

I took my car in for a service yesterday with a little under 1/4 full. When I picked the car up, I found that they must have disconnected the battery as all the defaults were reset (eg. my aftermarket stereo lost its settings), and after I started driving it away from the dealer I noticed the gas tank was completely empty, nothing registering on the gauge and 0 miles left on the computer "miles to go" display (well kilometres actually ).

Now I'm thinking what the hell could the service center be doing with driving my car around to use 1/4 of a tank of gas. But the odometer did not show any extra miles so I guessed they let it idle for a couple of hours but no idea why as it was only a minor service

Then when I went to fill up the car, it took $20 less than it normally does and I always fill it up before the computer "miles to go" is around 50km so this time around it should have cost me more as I was running on empty.

So I'm speculating that if the computer does guess the gas level below 1/4 of a tank, resetting the car by unplugging the battery lost the estimated gas level, so it showed fully empty instead. After fillup, its all good now.

 

AS,

I read it off the computer screen and also do the gas in to mileage run and they are not that far off. I have noticed lately that I run the tank alot lower than I used to (i.e. instead of a 12+ gallon fill up it's a 13.8-14+ gallon fill up) and the mileage to go before gas seems alot more accurate. T2

 

AS,

I don't have my data with me at work, but I'll give you some insights based upon memory.

Dock is correct, the amount of fuel in the tank is measured for the first 2/3 to 3/4 of the tank, then estimated from there. It seems that due to the shape of the tank in a TT or C4S, the float mechanism doesn't reach all the way to the bottom. Once it gets to this point, it makes a calcuation based upon the injectors flow and the size of the tank. The problem is that the sender doesn't accurately know where it is at the time. Is it at 2/3 or 3/4 of a tank?

At about the 2,500 mile mark, my TT failed it's fuel pump, As it's in the tank, the removal of the fuel level sender mechanism needed to be removed. When it was put back in, my fuel level indicator became very inaccurate. I took the car back to the dealer twice to have the sender recalibrated, After the second try, I just gave up. To calibrate the sender, the tank must first be emptied, then filled with specificed amount of fuel and calibrated. This process is repeated 2 or 3 times. From my experience with this "Exact Science", if Porsche was aiming for the moon, they's miss it by several million miles.

As a result of all of this, I began to keep records after every fuel fill-up. I'd record:
Miles Driven
OBC Estimated Miles to Empty
Fuel Level Remaining based upon Avg MPG and Estimated Miles to Empty.

What I discovered is that my Avg MPG was about 2-5% higher than reality. The Miles to Empty was a cruel joke. It would show me 5 EMTE and I could only add 13.5 - 14 gallons to the 16 gallon tank.

So what can you make of this? The fuel consumed by the engine is merely a calculation by the OBC made by measuring how long the constanf flow injectirs are held open. This is probably accurate to 2-5 %. The odometer in my car is accurate to with in 2% as measure over a 10 mile stretch of intersate. Your fuel level and EMTE can be off quite a bit. So, watch your odometer, check your Avg MPG and make your own estiamte of MTE.

Cheers,

 

FredR; what are your actual MPG #s on the TT ?

 

Around town with stop and go, and a little heavy on the accelerator, typically I get 14 to 16, with a one time low of 12.4....on the highway at a constant speed without too many spirited passes, I get 21 to a max of 24.7....but that was only once!.

 

Mixed city highway - 18.5 -19.0
Highway 75 mph cruise - 25-27
Road America - 7.5

 

I'm really sorry guys....but I have a really hard time seeing a TT that has gas mileage in the 27 mile range. Maybe because I'm driving an X50? Is it possible that the non X50's are better on mileage? I've literally tired every trick I know to get above 24 and just can't do it. Some of the guys have posted that their mileage actually IMPROVES after the cars have been flashed.....wouldnt' that remap the fuel consumption for more power=more gas to attain that power? My 99 996 could get 28...but that was a 3.4 L engine....I dunno....just cant' seem to accept this high mileage thing on the TT's.

 

I was taken by surprise by the 27 mpg cruise number, so today while coming home on I94, I set the cruise to 74 and watched the mileage. It started at 30 (!!!) but then settled down at 28.3. It stayed there for 5 miles, when I couldn't stand to drive that way. After a few bursts in traffic and a good off ramp, I had it right back to 22 (whew!)
My car is an X50 that has been flashed. My mileage indicator also improved after the flash, which made me wonder if the computation was intentionally manipulated. I may be a bit paranoid. The only time I had tried that pre-flash, my car did 26 mpg, by the computer. I had never considered the manual cross-check. AS

 

Once I added a sport exhaust city milage went from 15 to 16mpg on computer. I have not checked for accuracy nor do I recall what it did stock.