'89 325 Touring speedo reading inaccurate

[Podge72]

For some reason have noticed that speedo is showing wrong mph. I think it's about 9-10% out...any ideas and how can this be sorted as it's really annoying.

Cheers

[daimlerman]

Tyre size?

[Podge72]

Good point..thery're 15" diameter but not standard alloys. Is that enough or would you need the full rundown. If so I'll have to check in the daylight. Sorry know sweet fa about this sort of stuff, can you tell?!

Cheers

[Captain_Birdseye]

I don't either, looking forward to find out as my new wheels+tyres together are a bit smaller than standard.

[Sanchez]

Has it got the correct clocks? with the correct coding plug.

[Podge72]

How would I know if it had or not? Is there any way to tell?

Cheers

[Grrrmachine]

All the info you need on Wheels:

http://www.e30zone.net/e30zonewiki/index.php/Wheels

and Tyres:

http://www.e30zone.net/e30zonewiki/index.php/Tyres

I'd have a look at those numbers first before you start exploring the clocks and coding plug. Remember that the gauge will never be 100% accurate; it overreads slightly.

[capri_rob]

With 15" wheels you need 55 profile tyres. If they are something other than that the speedo reading will be wrong.

[Brianmoooore]

Coding plug does not affect the speedo in any way.

[Rav335uk]

To work out your rolling radius for different size wheels and tyres use this.

www.willtheyfit.com

[Podge72]

Cheers guys. I'll let you know how I get on...

[Grrrmachine]

To work out your rolling radius for different size wheels and tyres use this.

www.willtheyfit.com

There are two separate tyre calculators in the Wiki link I posted, as well as a table of correct sizes

[Rav335uk]

To work out your rolling radius for different size wheels and tyres use this.

www.willtheyfit.com

There are two separate tyre calculators in the Wiki link I posted, as well as a table of correct sizes

I didn't know now did I

[Podge72]

Tyre size is 195/45....

[Brianmoooore]

Tyre size is 195/45....

Well, there's the problem, then!
560mm diameter, when it should be 610mm = approx. 9% error.

[Podge72]

So then, lower profile tyre + wheel = smaller diameter overall size therefore 1 revolution = 9% less than standard hence the increased mph reading....I get it now..it's like being back at school only learning about something you're actually interested in!

So in other words live with it or go back to standard...

Cheers Brian.

[Brianmoooore]

The tyres you have account for most, but not all of the error. The speedo will still read less than actual speed, even with the correct tyres.
This is because the speedo is not like the other instruments - those are all provided by BMW for your convenience and information, whereas the speedo is there by law, and it's purpose is not to inform you how fast you are travelling, but rather to enable you to conform with whatever speed limit is applied to the road you are on.
To ensure that you do not unknowingly break a speed limit, all speedometers intentionally indicate a higher speed than you are really travelling at.

[Jos]

Hence the reason why you can be done for 31 in a 30, if you're doing 31 on radar then your speedo probably reads 35!

[Suki_BiH]

you have a tire size problem in combination with manufacture default higher speed readings.

earlier models have a calibration option inside the cluster, i'm not sure for later models :/