slomarsh

Well, it didn't start out to be a fuel consumption drive. But, as traffic limited my speed I decided to play with the on Board CPU.
Took a trip yesterday of 208 miles. Some hill climbing, some slow freeway traffic and a little in town combined. Most freeway speed was on cruise control at 70-72 mph.
According to the on board Avg. fuel consumption varied from a high of 30.2 to a low of 26.7 mpg
How accurate is the on board CPU? Your experiences?
Tires and wheels are stock as is the engine and Tiptronic.
I did not do a manual fuel check yet.

My previous ride was a C6 Corvette 6speed manual.
It would often show freeway cruising consumption about the same range as the TT experienced yesterday. If the On Board TT is accurate I am pleased because my impression was the Corvette was more fuel efficient. (But far less fun to drive)

Thanks in advance

Third-Reef

Those # seem really high. Good for you. I typically register 19.5 over a period of a month or week of mixed driving. If i reset it for just the freeway portion of my 170 mile "commute" it will regester 22-23. X50, manual, stage 2 tune, stock size ps2 rubber, cruise at 75-80 mph.

slomarsh

Your mileage is more what I would expect. Perhaps add a couple MPG if you didn't have the X50 or the tune.
Also your increased average speed of 75-80.
I'll do a manual check and see how that turns out. This is the first time I actually used the on board MPG on a longer drive.

dprantl

IMO those numbers are too high and unlikely to be correct. An easy way to verify them is to fill up the gas tank completely (until gas starts backing up the filler neck) and zero out the distance counter, then drive until almost empty and fill up completely the same way again. Now you have the distance you went and the number of gallons you burned and you can easily figure out miles/gallon. Just for reference, if you could get 30 mpg and you burned 16 gallons of gas, you would have gone 480 miles. Even at 26mpg, 16 gallons would take you 416 miles.

Dan

Macster

Reads like you might have been clearing/resetting the on-board computer at various times and cherry picking MPG readings.

My 2003 Turbo doesn't have it but my 2006 GTO did have instaneous MPG and at various times the MPG could read a low of around 6mpg to a high over over 30mpg depending upon how I was driving the car.

Pedal to the floor during hard acceleration and 6mpg was the reading. In 6th gear at some pretty low RPMs on level pavement at a very steady speed and 30mpg was the reading.

Don't recall the averages now though.

With my 2003 Turbo with a mix of freeway -- albeit with more congestion that not -- and some town driving it is hard to break 20mpg.

But out on the open road measuring fuel consumption from fill up to fill up 26mpg is the obtainable.

Under similar conditions over the same stretches of road my 2002 Boxster delivered 28mpg.

Kevinmacd

I get about 26 to 27 on a 75mph freeway drive! Around town is about 19. Key is not to get any boost, once that happens the economy is much lower!
I found the ecu mpg calc pretty accurate.

mcbit

Measured from actual fill ups over 48,000 kms I have an average of 16.7 mpg (US), an all time high of 35.6 mpg (US) and an all time low of 13.2 mpg (US).

Third-Reef

My all time low was 6.5 for 2 tanks full of 100 at Laguna Seca.

TheDeckMan

I always use the method dprantl mentioned filling up and calculating via how many miles gone to the gallons used.

Computer on mine tends to say 24mpg when in actual mixed driving it tends to be closer to 20-22mpg.

nbressette

Do you have injectors?

I went through a similar run lately and thought I was getting 30+ on the highway. Then a posted pointed out that the ECU doesn't know how to calculate for larger injectors with the same pulse length. Once I ran the gas pump math I was actually getting 22MPG.

mcbit

Momentarily saw 31 l/100km (7.59 mpg US) on the PCM in traffic today. i generally find the OBC to be very optimistic in relation to fuel consumption.

dprantl

In my stock 996TT I find the mpg counter to be extremely accurate. I always verify it with a manual calculation at the pump and it is dead on almost every time. In fact, I use it to figure out how many more miles I can go before I need to get gas, and I usually fill up with 15 - 16 gallons. In order to have it as accurate as possible, it is important to reset it properly. Do not reset the mpg counter when you are standing still because at the beginning when the program has no data, it can start off with too much error. I always reset the counter while I am driving along at a reasonable speed and try to keep driving as long as possible after the reset so the computer can build up some data. I find that usually even 30 seconds or so is enough.

Now the miles until empty counter, that has some crazy code in there and really gets paranoid when the tank is close to empty. It is pretty much useless when trying to determine when to get gas and typically shows --- when you still have 2 or more gallons left in the tank.

Dan

Dock

I agree.