Boxerworks.com

My tacho or my speedo ?

I was pulling 7500 rpm in 5th today and the speedo read 155 km / 96 mph

but later they gave me 80 km / 50 mph at 3100 rpm.

This is on my ST with a slightly low final drive.

Any idea which of the two would be closer to reality ?

I'm thinking the tach would be more accurate, getting its info direct from the engine. The speedos were known to be optimistic by quite a bit, I think it was 10%, there's a BMW bulletin regarding this. Combined with a possibly mismatched rear drive, and all bets are off...

Sibbo, if you have one of those clever little telephones you can get a free app called HUD (Heads Up Display). It is a GPS speedo that should be accurate enough to solve your problem.

ps: The next knock on your door will probably be the police.

Charlie , my phone is sort after by phone museums .

Sibbo wrote:Charlie , my phone is sort after by phone museums .

Ah, I see. I do have a little telephone that works without wires for travelling but it is not 'smart'. Not sure if I'd like one that could beat me at chess.

Ask around among friends. Somebody will have a GPS device that you can tape to your tank to do some test runs and get some clues as to how accurate your instruments are.

Tach should be normally be more accurate but I think not in your case at 7500 RPM. Assuming you have a 37/11 or 3.36 final drive and a stock 4.00x18 rear tire RPM vs MPH should look like this. If your final drive is different to stock post up what it is and I'll run the spreadsheet again.

Thanks for the offer Barry .... but I have no idea what my final drive ration is having never pulled it but it certainly seems low. The web tells me an ST should be good for 106 mph but that 7500 rpm came up pretty easily and quickly and I think she had a bit left .... so I certainly must be pulling a unusually low ratio.

The drive ratio should be stamped next to the swing arm fill plug.

Thanks.

37/11. = 3.36.

This sees to indicate my tacho is inaccurate because the 7500 rpm I saw would give 115mph and that seems pretty unrealistic .

The speedo in both the R100LT I had and my R80 both read 10kms slower than what I am doing according to a GPS.