Lies, damned lies and BMW speedos

Well, the track day atBruntingthorpe airfield proved interesting. Huge complex with jets everywhere. When I asked for directions I was told, "Follow the track, and turn left after the Jumbo Jet."

I didn't need the standing start procedure as it was rolling starts. But I tried it, and after I remembered to turn off DSC, dropping the clutch at 3,000 revs worked fine.

Most of the cars were Porsche Carerras, I don't think there was any Boxsters. I was teamed with an S50-engined M coupe for our runs. The timing traps were towards the end of the 2-mile straight but due to the slight hump in the runway you can't see it until you're 400 yards away.

All my runs were in excess of 160 mph on the speedo, with one at what I thought was 163 and another at 162, but when the timing results came through all my runs were 150 mph. So much for speedo accuracy.

Nevertheless Zmurf was considerably faster than the S50 coupe, even when I had a passenger. Some of that may be down to the very slightly more powerful S54 engine, but I think the 'race' spec Eisenmann exhaust also helped.

One of the faster Porsche's achieved 170 mph.

Tim

Do a search on speedo error on this board. The BMW standard is something like 10% + 3 MPH. So you could have seen as much as 168 at an actual 150.

I think the top speed was 151mphish on the dyno but the speedo speed was aobut 5-8mph faster.

Another interesting thing is that the speedo doesn't match what you see in the diag functions which was off more.

I believe the most accurate part of the speedo is the top center so when you are pegged on the right you are not at the most accurate portion of the gague.

Pat

Originally posted by Mini-///M
I think the top speed was 151mphish on the dyno but the speedo speed was aobut 5-8mph faster.

Another interesting thing is that the speedo doesn't match what you see in the diag functions which was off more.

I believe the most accurate part of the speedo is the top center so when you are pegged on the right you are not at the most accurate portion of the gague.

Pat

Actually the OBC is spot on. The speedo is ALWAYS high. BMW builds them that way for some convoluted logic that I never quite understood, nor agreed with.

European laws.

They state that the speed must never under-report the speed no what wheels you decide to put on it. Even if your car comes with 16", the speedometer must account for the fact that you could possibly put on 18" wheels.

American laws are much more lax. That's why American cars are usually accurate with stock wheels while euro cars overreport speed.

Actually... American laws state that the speedo can't be running lower than the actual speed with stock tires/ rims. All manufactrurers have speedo error built in just so you can't get pulled over and use your speedo as an excuse.

I think in some brands the possible error can be quite large, to the tune of 10+%.

BTW I don't think the diag test function gives actual mph because there was a discrepency between the dyno, speedo, and diag functions when we were looking at 2-3 years ago on bimmer.org

Pat

My Ford truck can be programmed for any revs/min to adjust the speedo to your current wheelsize. Sadly I can't say the same for my M Coupe.

Tim, Is your exhaust a complete system of just a weld job in front of the rear silencers? How did she feel at top whack?

Originally posted by Mini-///M
Actually... American laws state that the speedo can't be running lower than the actual speed with stock tires/ rims. All manufactrurers have speedo error built in just so you can't get pulled over and use your speedo as an excuse.

I think in some brands the possible error can be quite large, to the tune of 10+%.

BTW I don't think the diag test function gives actual mph because there was a discrepency between the dyno, speedo, and diag functions when we were looking at 2-3 years ago on bimmer.org

Pat

BMWs can be within spec if it shows: actual speed + 10% + 2.4 mph. So if you were actually going 60 mph, the speedometer could read 60 + 6 + 2.4 = 68.4 mph and still be "within spec"

Has that changed to be more aggressively off in recent times? V-man's '92 5 is dead-on, speedo-wise, but the M is at least 10% over.

The exhaust is a 'cat back', so includes two pipes and two boxes. You can order it in one of three states of tune, I chose the least restrictive which is a tiny bit noisy.

The standard M silencer box has twin 63.5cm tailpipes. Eisenmann's are a choice of 70cm or 83cm. They reckon the larger pipes need bodywork alterations but I found they fitted just fine.