How accurate is your speedo ?

On my way to the North west meeting on Sunday, I tried out my new toy, a TomTom sat nav, and was quite taken aback as to how innaccurate my speedo was.
Over the entire speed range, it was 10% out, so when I thought i was doing 70mph I was was in fact doing 77mph. I tried various speeds, and it was exactly 10%.
My car runs on 17" Azev wheels at the rear fitted with 265/40/17. If you have the larger wheels on your car, makes you wonder how accurate is your speedo .....?

Andrew

Yes it's not much kop .I noticed when I had the 17" on the rear comparing it with the Drivesmart
Mine is not wired up since I put the stds back on so I use the Drivefart ....but that couldn't find any satellites yesterday

http://www.carbibles.com/tyre_bible.html

If you go to page 2 of the link above about half way down.
There is a tyre calculator which will tell you how far out your speedo will be against the standard tyre. If you put larger tyres on.

I was just having a look at this today using my Blackberry's GPS. Actually, with the 255/40/17's my speedo seems to be almost spot on. At lower speeds, when my speedo is just getting on for 30, I was doing 28mph on the GPS. At motorway speeds it seems almost bang on.

I have a feeling that the Toyo Proxes I'm using now were ever so slightly smaller than the ContiSport Contact rears (of the same dimensions) that I had previously. Certainly, there was more "width" on the Contis for the same nominal section. So, what tyre you've got on will make a slight difference - but I was pleasantly surprised at how accurate my speedo seems to be.

Turbodog, if you're running 265/40, those are going to be a bit larger diameter than the 255s, although only about 8mm!

Jon

I had the sat nav with me yesterday too and also saw roughly a 10% difference in speeds. I have to say though, you tend to find most vehicles have this difference, It isn't just the Alpine. Just looking again though my difference is the other way, so that when my speedo shows 77mph, I seem to be doing 70mph.

So does that mean we can sue the manufacture if we get a ticket

I had the sat nav with me yesterday too and also saw roughly a 10% difference in speeds. I have to say though, you tend to find most vehicles have this difference, It isn't just the Alpine. Just looking again though my difference is the other way, so that when my speedo shows 77mph, I seem to be doing 70mph.

top wack with revs delimited by 500rpm increases top wack by 4 MPH on standard rubber up from 139mph standard. I have never checked sat nav against speedo on standard rims and rubber tho'

on 17's I have seen 151mph and sat nav varified at 149 thats not bad and having thought about that it seems realistically possible

Si21

Turbodog might indeed have a problem! The speedo speed is higher than the actual speed in the A610 which means if you stick to the cars speedo for limits you will be fine. Of course we own Alpine's so we do occasionally venture above the speed limit. I found out my Sat Nav only goes up to an actual of 126mph and won't go any further though!
Simon (A6 TEN) mentioned he'd seen an indicated 180mph on his 610!!

So does that mean we can sue the manufacture if we get a ticket

I think you'll find there is a degree of variance in every speedo irrelevant of the make n model there fitted to i've found this with the majority of cars i have owned from ford's to porsches and usually found they over read except for the ford but only by the slightest of margins between 3-6% out can't quote on the alpine as the speedo is like an amnesia victim it knows it's there but dammed if it knows what to do!!!! so have to rely on the sat nav!

My GTA with 17" Azev's is the same as turbodog's, it reads about 10% under against satnav, which is predictable because of the wheel size, all my other cars have been about 5% under.

Much better than the world of performance bikes though... there's nothing more fun than seeing the face of a pub-hero Hayabusa owner turning up at a timed top speed event seeing 210mph on his clocks and getting a 175mph timing ticket

Just as a matter of interest, if you have an over reading speedo, just check to see if the speedo needle moves slightly when you switch on the ign.

Just for reference, car manufacturers have to comply to a requirement that the speedo must not under read, but may over read by 10%. To meet the requirement most ere on the cautious side, so an indication of say 75mph really only equates to a true speed of 70mph.