E30 Zone

Hey, thought i'd ask here first before trawling the rest of the web. I've an e34 540i engine in my e30 and the speedo is quite out, I assume it's something to do with it not being calibrated for that engine although that might not be the right terminology.
Does anyone know what would need to be done to get it to reflect my speed more accurately if this is at all possible.

An example of how 'out' it is was that when travelling up the M4 the other weekend I went through a number of average speed check zones and when i slowed to 50 by my clock I began to cause a tail back quite quickly. I moved over and matched everyone else and they were doing (again by my clock) between 55 and 60.

Cheers

the engine you have fitted should make no difference as the speed reading is taken from a sensor on the diff.

Are you running differnt sized wheels/ tyres?

205 45 16's and I think the diff is a 2.93 as per the 540i.

Your tyres are a bit smaller than standard but it wouldn't throw the speedo out by that much. You need to check your speedo against the GPS on a satnav and see if it over reads by a constant figure (e.g. 10mph) or whether it is a percentage (e.g. 10%) If it is by a constant figure you can take the instrument cluster apart and then remove the speedo needle and then refit it in a position which is more accurate.

I have a Sat Nav so i'll check out my speedo against that and take it from there. Other than the effort required to get to the needle I like the sound of plan b.

Thanks

I was under the impression that the coding plug on the instrument cluster is what needs to be changed here.

What was your engine / diff ratio before the 540i?

Incidentally. Is it really easy to remove the needle from it's home and relocate it to point else where on the clock?

mick_318is wrote:I was under the impression that the coding plug on the instrument cluster is what needs to be changed here.

What was your engine / diff ratio before the 540i?

As far as I know the coding plug has no influence on the speedo.

Taking apart the cluster is very easy, you just need a couple of screwdrivers. I don't think you even need to remove the needle, you can just rotate it with your hand.

Thanks Theo.
I've just read that the difference in speed varies throuout the rev range. That is to say I might be 5mph out at say 45mph but this inaccuracy becomes more pronnounced the faster the car travels.
Wouldn't this make relocating the needle an incomplete solution?
If so, and going by the tyre route should the tires be smaller or larger.

I've just read that the difference in speed varies throuout the rev range.

You read that about your own car?

My speedo was a constant 5mph out, so I tweaked the needle and all is well now.

Your tyres should really be 225/45/16 - which is about 3% larger, this would improve the speedo accuracy a little bit - but not to the extent you mention in your initial post.

Not too sure mate. I'll park that thought until I get the Sat Nav out with me then I can confirm if it is constant. I just read it on another thread so it's not mine.

Regarding the tyres Theo, you'll have to explain to me how come making the tyre wider (205/45/16 vs 225/45/16) affects the speedo. I always thought the change needed to be made at either the tyre wall (45 - say 55 for eg) or the wheel (16 - 17 or what ever) in order to affect a change.

Ok, tyre sizes are very simple.

225 = tyre width in millimeters
45 = tyre wall height expressed as a percentage of the tyre width
16 = rim diameter in inches

Kapeesh?

^ So a Theo says the wider your tyre the digger the side wall so 225,45 is bigger than 205,45