As the speedo seems to be about 5% (compared to GPS) over reading I presume the in car's tacho could be similarly out?

What experience is out there on digital tachos and accelerometers to provide data on engine and car performance?

Would not want to spend more than necessary on such kit. Use to have a little digital rev display that had a pickup rapped round a HT lead I remember which was pretty good but that was 20 years ago and things have moved on probably.

Is there an accelerometer (with GPS) that has programs for 0-62, SS quarter mile etc software on it?? Looked on the web but was not finding much.

Edited 9 April, 201016 yr by lol

To my knowledge the speedometer on all cars is set to under-read as part of a gentleman's agreement between the car companies. The Tachometer should be spot on in accuracy terms and the odometer figure will vary with tyre wear, but will be more accurate than the speedometer too.

I know the GPS System is very accurate but are civilian GPS devices that accurate.

f not, it is likely your speedo and a TOM TOM, for example, are just giving ballpark figures.

To my knowledge the speedometer on all cars is set to under-read as part of a gentleman's agreement between the car companies. The Tachometer should be spot on in accuracy terms and the odometer figure will vary with tyre wear, but will be more accurate than the speedometer too.

Yes this seems to be true as the average speedo over-reads by 5% approx. In the Octavia Mk2 I think it can have 15, 16, 17 or 18 inches wheels with 60, 55, 45 or 40 profile tyres but all have a rolling radius within a percent or so. Here in the UK, and I expect in all other EU countries the police allow for a 2 % inaccuracy for the various wheel specs that can be fitted to a model of car and ten percent for speedo inaccuracy ie just over 12% in all against the nominal speed limit hence best to use the GPS for speed reading which I think Tom Tom etc reckon on being about a 1 percent point accuracy.

Shame on board computer does not have a revs display option, it would have been so simple to provide as the values are used by the EMU. Analogue will always be rough I would have thought, ie circa 5% depending on manufacturing tolerances on such devices that use magnetic induction against a resitant spring, bound to be fairly variable due to production tolerances against cost would have thought. Anybody have a digital solution?

I know the GPS System is very accurate but are civilian GPS devices that accurate.

f not, it is likely your speedo and a TOM TOM, for example, are just giving ballpark figures.

Civilian GPS are now very accurate ie within a metre or 2. Use to have a biult in error amount so non US military approved entities using the GPS would not have enough accuracy to hit hardened missile silos but that error algorithim was removed a few years ago as it was thought to be unneccesary any more.

Not so much that it wasn't thought to be necessary, more that other countries started to put up their own GPS satellites fed up with the US system's poor accuracy, so they had no choice...

Not sure why you'd want an exact RPM figure for the car either - the main benefit of the analogue display is that you don't need to read it, just look at the angle of the needle or better still, listen to the engine to get a feel for the rpm.