7 replies to this topic

[stellis]

ello, ive gopt a 998 with quite a few mods, diff head cam, etc, an ive had it since november and for ages the top speed was 85 an no more, so i got my carb set up because petrol comsumption was naff, but the engine excelerated more but still only 85 topend, but the other day race my mate in his mini to tamworth, ( this is after a good day of thrashin about, even though i used to do this ), my car startd exceleratin well good and now pull straight through to the fuel gauge with the needle, ( centre speedo), i was jus wonderin is there sumot up with the enginem or has it jus decided to stop bein a ball bag and go minttt, ??, not really a prblem i knw i was jw , cheers for any replies ste

[cowboy]

could be a problem with the speedo cable or head.

one way to check is to get a freindly olice man to za you with a speed gun as your hurtling down the road at 85 LOL

or get your mate to travel at differant speeds along a duel carraigeway and find out the differance with your speedo and his. if there the same now. no problem.

if they are differant then you will need to look deeper for the problem

[Jimmyarm]

Dont forget that comparing speedos is a bit of an innacurate way of doing this, other cars speedos arent notoriously reliable either.

Evo 6's will read 180 when they are only doing 159.97

Either a RR or a GPS would be a more accurate measurement.

[edjohnson]

Stellis, please take the time to write your posts a little more understandable.

And as for your car, it could be a number of different reasons. Irregular speedo cable, nice cold air on the dual carriageway, higher octane fuel, burnt off carbon deposits, slight gradient, wind etc

and GPS isn't mega accurate, because it only monitors your progress over the earths surface as a whole, and does not take into account inclines.

So if your on a hill, it is telling you how fast you're travelling across, not up and across.

[Jimmyarm]

Stellis, please take the time to write your posts a little more understandable.

And as for your car, it could be a number of different reasons. Irregular speedo cable, nice cold air on the dual carriageway, higher octane fuel, burnt off carbon deposits, slight gradient, wind etc

and GPS isn't mega accurate, because it only monitors your progress over the earths surface as a whole, and does not take into account inclines.

So if your on a hill, it is telling you how fast you're travelling across, not up and across.

That last bit isnt true, GPS doesnt just do horizontal it does vertical as well. My basic understanding of it is that it maps you in a 3d world rather than a 2d one so it can take into account the altitude change as you drive (this requires it to have a fix on three satelites)

If it didnt do vertical how would aeroplanes use it to tell when they were entering certain airspaces ?

Admittedly its not going to be that advanced for you average tom tom but its still a more accurate way of measuring your speed than an average cheapo speedometer and the manufacturers will have taken into account that most people dont drive around on perfectly level surfaces !

Edited by Jimmyarm, 06 March 2007 - 01:15 PM.

[stellis]

lol, cheers for the replys, i thnk it might be the gearbox were the speedo cable comes out givin up teh ghost, bin in a v long time, now, lol , i only thnk this because bout 2 - 3 weeks ago i replaced the speedo and cable , anyway im sure ill find sumot else in abit to go on about , cheers ste

[Dan]

A GPS speedo is not very accurate really. It is digital and digital means average. It only knows where you are to within a few metres at any one time nomatter how many sattelites it is using and it averages your speed between those points, assuming you are traveling in a straight line between them. Computers don't understand curves remember. Try watching what it says if you drive under a bridge too. The only real way to determine you speed is to physically measure the amount of road travelling under the car, or by doppler radar tracking the road surface. It's all relative remember!

[Jimmyarm]

Some light reading...

http://www.expansys....2333&thread=232

Whilst I agree that GPS is not completely accurate, most sources still say its a much closer reading of your true speed than an in car speedo.