Z3 I don't trust my onboard computer

The onboard computer on my recently acquired '98 Z3 tells me I'm getting around 28 mpg at highway speeds. I don't believe it. I've got the 2.8L with the 5-speed manual, burning 91 octane, and I expected something more like 22 mpg. I need to drive 100 highway miles or so and refill the tank to get a good guess at it, but I wondered what other owners think. If it's that inaccurate, can I trust the displayed cruising range? Running mile markers and using a stopwatch tells me the average mph is about right, if that matters.

if your computor [hal] is calling you dave you can fill up ,write down the mileage ,repeat at next fill up then do the math. gallons used /mileage traveled

Additionally, your speedometer is probably reading faster than the actual speed: most BMWs do. I'm not sure where that fits into the computer logic or odometer reading.

No, the computer isn't calling me Dave, but I'm pretty sure I heard it whisper "spank me." I haven't had time to do the drive/fill thing, I've just watched the mpg on the display. Stop-watch runs tell me the speedo is faster than real life by about 3 mph at highway speeds, which sounds like it's better than a lot of people encounter. I'm still having fun getting the feel of the car, trying to make sure I don't trust the computer telling me I've got 80 miles left when it's really only 10!

1) The odometers are correct with stock tire sizes, the speedos read high due to TUV requirements. Some very high. Spec is 2.4 MPH + 10% maximum. So actual 60 MPH could read as high as 68.4 MPH.

2) The computer bases the fuel mileage on the fuel injector pulse width, so they are not that accurate. Even less so if you have high flow injectors.

And 28 MPG on the highway is not out of line. I can see 28 or so on my E46 M3 with pure highway running at a reasonable (that is reasonble for a BMW) speed.

Originally Posted by Pinecone

And 28 MPG on the highway is not out of line. I can see 28 or so on my E46 M3 with pure highway running at a reasonable (that is reasonble for a BMW) speed.

I would never have guessed. With a rating of 24 highway, I expected a typical 22. If I'm really getting 28 or close, I'll be ecstatic. Now on to other questions and issues. Ah, the joys and trials of a used car...

Originally Posted by tkdkerry

The onboard computer on my recently acquired '98 Z3 tells me I'm getting around 28 mpg at highway speeds. I don't believe it. I've got the 2.8L with the 5-speed manual, burning 91 octane, and I expected something more like 22 mpg. I need to drive 100 highway miles or so and refill the tank to get a good guess at it, but I wondered what other owners think. If it's that inaccurate, can I trust the displayed cruising range? Running mile markers and using a stopwatch tells me the average mph is about right, if that matters.

See MPG.

I downloaded an app to my Iphone called "Speed Box free" which uses GPS to provide a real time speed output. I can set my Iphone on the cluster while driving and compare actual speed with the speedo. My speedo reads fast as most BMW's do.

Thanks, everybody! You know how to make a guy feel welcome!

For the erroneous speedo, I installed the SpeedoDRD. Now my speedometer is dead-on. You might still need the Iphone app to calibrate it.

http://www.bimmerfest.com/forums/sho...d.php?t=551995

As pointed out, it doesn't affect the odometer.

My speedometer on my 98 Z3 reads 4-5 miles faster than three different GPSes claim I'm doing - shifty Germans!

glad that I saw this post.. I just picked up a 1998 Z3 and noticed that when I am driving my Z3, I am getting passed by many cars.. I don't notice that while driving my Jeep. I just downloaded the iPhone Speed Box Free app (thanks Flossy21).. I will compare my speedo and the iPhone readout tonight on my drive home.

You drive by using your speedo????

Only time I worry about the speedo reading, is when the radar detector goes off, or I see a law enforcement vehicle.

I have to drive using the speedo in MA.. There are speed traps everywhere in MA.. I have been stopped for speeding a few more times than I care to think about..

Electronic Countermeaures.