Speedo UnderReading

[Gibson]

ive always thought i was going a little bit slower than everyone else when my speedo was reading 30mph since i bought the car. and now i have realised that if i set the car at 35mph on a straight flat road and then reset the average speed on the OBC it tells me im doing a steady 30mph...
so my speedo under reads by 5mph at 35mph
similar test reveals that when my speedo is reading 80mph im actually doing 70mph according too the OBD
i think the speedo is at fault because 70mph on the speedo feels slower than in other cars and people get pissed off and overtake me

what the hell is going on?

[G524]

I have the same thing. Are you running non standard wheel / tyre sizes? On the plus side, it's really easy to see '120mph' on the speedo, and your mates will be well impressed.

Capri's are legendary. I wanna sell the E30 and whack naughty great V8 under the bonnet of a dirty old Ford Capri. However, I also want an E36 lump in the Touring but neither of these are happening....

[sam325is]

i was only thinking the other day that a fix would be nice for this problem

[Nemo]

I got the same thing on mine. I was following a mate on the motorway recently at an indicated 65mph and when I mentioned it to him about his Captain Slow impression he said he was doing 70mph but I'm rolling on 17's.

How I've never got any speed tickets I don't know because I've had the 17's on for 2 years now and never realised before

[itec]

I have had 3 E30's and they all did the same the speedo is out by 5 mph,I have a Talex GPS Camera Detector ,which shows the correct speed as with the OBC.

This is why all speed cameras are set with a approx 10 % leeway as the majority of analogue speedo are not accurate.

this means you can do 45 mph in a 40 zone and you won't get flash.

[ibriggsy]

Yeah mine is 5mph under all the way around the clock, measured from a sat nav which are more accurate than speedos. None of my previous cars have been correct either (My (dont laugh) Ford KA was a consistent 10% out and I cant remember the others)

I think the legal tolerances for speedos is to read 10% higher but 0% lower than what they actually are so i dont think what you say about the speed cameras is correct itec.

I also think that manufacturers make them read 10% higher to 'enhance' the performance of the vehicle - makes it seem like they are quicker, more economical (for the speed) and also top speed.

[cros]

It can be adjusted - there's a screw on the speedo circuit board.

You have to take out the clocks and it'll be a bit hit and miss unless you have a frequency generator to hand though: http://www.e30zone.net/modules.php?name ... pic&t=2549

[Gibson]

oh yea a frequency generator i have one in my back pocket!
i getcha. generate the frequency the sensor would be emitting at 70mph and calibrate accordingly.... clever
maybe i should just get my mate and a screwdriver too hide under the dash and adjust while im driving down the motorway... but then i mite get arrested for people smuggling.

oh and by the way.... that capri is halfway threw a 3.9injection Rover v8 transplant.... dirty capri hehe

[Brianmoooore]

The E30 doesn't have an "inaccurate" speedo! It is required, by law, to indicate at least slightly more than the actual speed under all circumstances. The purpose of a speedometer is to enable you to drive without braking any speed limit that may be in force, and this it does adequately.
OBC is usually spot on.

[Gibson]

yes but mine is well out.... so it does have an inaccurate speedo....
its over 10mph out at 70mph so its not just "slightly out"

[itec]

Frequency generator oh yeah I have one those sitting on top of my wardrobe
It was something I put together when I was studying electronics, now I just have to build a power supply for it then I can try this recalibration business.

Ibriggsy regarding cameras I this from gov site
“National guidelines set by the Association of Chief Police Officers (ACPO) recommend that there is a tolerance of 10% + 2mph over the posted speed limit.â€a

I'm not saying we should be going beyond the speed limit, but we can relax if occasionally we step over the limit.

If speedo's are meant to exaggerate true speed that would mean when we testing are
0-60 in 7 Sec we are doing 0-50 and actual 0-60 is 10 sec that a bummer


Gibson- I will let you know what my actual speed is when I do 70

[Gibson]

cheers. yea if u reset the OBC it seems too present a pretty accurate speed....i mite just diddle the screw on the back of the clocks until it matches the OBC

[orangecurry]

I have the same thing. Are you running non standard wheel / tyre sizes?

you still haven't answered this question - if you are then that'll be why the speedo reads incorrectly.

[Gibson]

good point sorry. yes i am but i am running sixteens on 45 profile... so surely the speedo should slightly under-read?
also non standard wheels doesnt explain the difference between the speedo and OBC... do they run from the same speed sensor? and am i right in thinking this is in the diff casing?

[ian332isport]

Mine is actually pretty accurate when compared with my Origin B2 GPS speed display.

It over reads by approx 3 mph at 30 mph (10%), and slowly gets more accurate the faster I go. 90 on the speedo is 90 on the GPS. After 90, it starts to go the other way, but it's never more than about 3 mph out.

Ian.

[itec]

i have test mine with my talex and OBC form 30 and upwards, the Speedo was constantly 5 mph less.

also non standard wheels doesnt explain the difference between the speedo and OBC... do they run from the same speed sensor? and am i right in thinking this is in the diff casing?

I would presume the same with the speed sensor it should be the same, maybe some one can clarify this.

Here is a problem which I have that may help to resolve your problem.

When I got my car 2 years ago the speedo had a mind of its own. i would be doing a constant 60 and it will suddenly jump to 90.On some occasions it went to 140 and over.
My brother 316 speedo does work until you start to drive for 10 minutes then tap it and it will work.

I haven't done anything with mine as yet as I tend to use the Rev counter to gauge speed.

What you probally have to do is strip the speedo and clean it up and check the electrics.

I wouldn't fiddle with the calibration screw until you are sure it working correctly as it is the norm for it to be 5 mph lower.

[slinky2000]

mines the same, at 70mph I'm doing 62 according to my gps, not much to be worried about if you ask me

[George_Of_The_Jungle]

My tech to is also exactly 5mph out throughout the whole revband. Doesnt bother me any more, just read 35 as being 30.
And surely if you are running 16"x45 then it would be reading over the actual speed, as the overall diameter is smaller than standard, so turns more times for the same distance.......?

[Gibson]

what size wheels and tyres are standard on my 320i se 2 dr?

[orangecurry]

diameter is *approx* the same for 195/65/14 205/55/15 or 205/50/16

[Gibson]

im on 205/45/16 i think... shit i had better check.... i wonder if 195/50/16s would solve my arch rubbing problem...