RPM/Speedo & 60-120mph vs. 60-125mph

[Master Apex]

While I was responding to the 100-200kph thread, I did a few runs and had interesting observations that I want to share the group.

1) Speedometer needle is 5% faster than actual
- Confirmed by comparing the indicated versus stationary radar
- Indicated Speedo * 95% = Actual Speed (Radar Reading)
40mph (speedo) = 38mph (Radar Reading)
60mph (speedo) = 57mph (Radar Reading)
Which means 120mph (speedo) ~ 114mph (actual)
Therefore, for 120mph actual speed, speedo needle needs to be at 126mph

2) RPM needle is ~2% slower than actual digital reading
Digital reading based on 7.2 self test menu:
http://www.bimmerfest.com/forums/sho...90&postcount=9
Needle at 5KRPM, Digital reading is 51xx RPM, so the needle is about 2% lower.
@60mph speedo needle, digital RPM reading was 4592-4612RPM
Rear tire was 275/35/18, 2.7% shorter than stock 255/40/18



3) 60-120mph speedo (~ 56-112 mph actual), took about ~10.5 seconds
60-125mph speedo (~ 56 - 118mph actual) took about ~12.5 seconds

[BlackMoupe]

Great to see you're utilizing safe driving practices!

But yes, it's a common trend in cars these days to be over a couple percent.