928wolf

Took my first long trip in my 928, mostly highway to the NC Mountains.

Background: My fuel gauge doesn't work properly, it displays the right fuel level at start up but then drifts up to show full. I have been keeping track of fuel level by trip odometer and number of gallons to fill the tank. But all my experience so far has been around town, getting into the throttle pretty good. Pretty consistently I have been getting about 13 mpg normally.

So after about 150 miles of mostly highway driving, I get onto the Blue Ridge Parkway a good bit of the way into the mileage where I normally get fuel thinking I'd get some soon. But as I came to remember, there are no gas stations and very few exits on the Parkway. I started getting really nervous, but a check (turning off and turning back on) said I still had more than half left. I knew I'd get better mileage on the highway but didn't really have an idea how much better.

After a nervous ride up and down the mountain I found a gas station. I had 233 miles on the OD and all I could get into the tank was 7.5 gallons. That's 31 miles per gallon, which astounded me. I assumed it couldn't be right. My manual says that at 74 mph I should be getting about 18 mpg. However, assuming the fuel consumption gauge on my dash is at all accurate, 31 mpg is roughly 8 l/100km on the gauge. On the highway going 80+ mph that gauge was hovering between 5 and 10 for much of the trip, so that's not out of the realm of possibility.

Does this sound right?

Imo000

Nope something is wrong in the calculation. You need to drive it more and keep record to know how much it uses. Some tanks have problem with venting and can not be fully filled. You need more than 8L/100km to go 80mph in a 928.

joejoe

Maybe vent hose has collapsed and cannot fill tank full now.

My '81 did do 29.5 mile per gallon on a trip to sharktoberfest. Filled full at start, recorded and filled full when left, and filled when back. I know it does sound high, but it did do it. Also on trip speed was a near constant 75-80 mph with almost zero slowdowns.

911user

I used to get a steady 27-29 mpg in my 94 GTS on the interstate with good gas and the cruise control on and having over 200k miles on the car. It is possible.
Steve

SeanR

I had an S4 that got almost 30mpg to the east coast and back, the FPR was shot and the car was running very lean.

alabbasi

Only if you drop it out of a plane at very high altitude

928wolf

FWIW, I put 17 + gallons in the tank that morning, so unless I developed a filler problem during the trip or higher altitude (about 2500 feet) made a difference in filling ability or variation in pump shutoff level..... Anything is possible.

More data collection and analysis to come....

mickster

When I was looking for my first 928, I test drove a red 87 or 88 in New Jersey. The owner definitely wanted me to take a long drive (he was great) and I offered to fill up the tank which I thought was fair. When I got to the pump, it only took 14 Gallons even though it was near empty. He replied that this was a "Racing Tank" and had a smaller capacity than normal 928's. He also explained Magic Blower Syndrome as a racing prep issue...needless to say, he had a partially imploded fuel tank due to lack of vent...

prescott

I've obtained 28 MPG going to and from Phoenix at sustained cruising speeds between 85 and 100+ MPH (just keeping up with traffic, and no passenger). I didn't believe it the first time, but I've repeated it twice! Mines a 5 spd, and was loafing both up and down the hill (4,500+ feet elevation change) on I-17. Most of the time around Prescott I get around 17 to 19, if I control my right foot.

Wisconsin Joe

31 sounds a bit high, but it's not impossible.
Especially at steady speeds (speeding up and slowing down kills fuel economy). The car is fairly low drag, so at lower highway speeds it doesn't use all that much fuel.

Part of it could be the "filled it in the morning" aspect. If it was a chiily morning & you filled it with the car cold, and used cold gas, then it would have expanded some as the car and the day warmed up. I've seen cars "drool" out gas under similar circumstances.
Filled to the top when it was cold, then sitting out in the hot sun, the gas expanded to overflowing.

rnixon

31mpg sounds high, but if you ended at a lower altitude than you started, it would definitely help.

Some petrol pumps take more coaxing to really fill the tank. Your next fill-up will probably tell you if it really was full.

Richards 928

I have an '80 5 speed and get anywhere from low to high 20s on the hwy. Sofar never above 30. Or between 8 and 9 liters per hundred kms.

I expect as much as the engine is usually turning under 2000 rpms in 5th gear.

Different story around town though...