speedo calibration

[Andy70]

Hi there, if I wanted to change the profile of my tyre, how easy is it to reset the speedo calibration on an e92? As the rolling rotation of the wheel would be different. Thanks.

Standard my car is fitted with a 30 profile on the rear and wanted to go up a bit to 35, so that would mean 8+ mm difference, if I go up a width size too which according to a site I looked at would read the speedo as a fair bit slower, not good news when approaching speed cameras lol

[RUSSELLM]

I’d have a look at your current speed against a GPS device first.

It’ll almost have certainly been inaccurate, long before you considered changes tyre sizes

[Andy70]

What makes you say that? The tyres are on factory spec so therefore accurate? Ish....

[Glosphil]

I will be astonished if the speedo doesn't read fast with standard wheels & tyres fitted - at least 5% at 70mph.

[helix402]

Glosphil said:

I will be astonished if the speedo doesn't read fast with standard wheels & tyres fitted - at least 5% at 70mph.

It will either be spot on or over read by up to 10% in accordance with EU law. (In reality over read). I’m not aware of any software to recalibrate it apart from Carly which will reprogram the car to show the actual speed (rather than the factory over reading).

[Andy70]

helix402 said:

Glosphil said:

I will be astonished if the speedo doesn't read fast with standard wheels & tyres fitted - at least 5% at 70mph.

It will either be spot on or over read by up to 10% in accordance with EU law. (In reality over read). I’m not aware of any software to recalibrate it apart from Carly which will reprogram the car to show the actual speed (rather than the factory over reading).

I believe Speedos to be quite accurate these days but the changes I wanted to make would make it read 8mph slower than I'm really going according a tyre calculator I found so in other words if I'm doing 30, I'll really be doing 38 and that's too much to have to think about... So you reckon there is nothing can be done?

[E-bmw]

Andy70 said:

helix402 said:

Glosphil said:

I will be astonished if the speedo doesn't read fast with standard wheels & tyres fitted - at least 5% at 70mph.

It will either be spot on or over read by up to 10% in accordance with EU law. (In reality over read). I’m not aware of any software to recalibrate it apart from Carly which will reprogram the car to show the actual speed (rather than the factory over reading).

I believe Speedos to be quite accurate these days but the changes I wanted to make would make it read 8mph slower than I'm really going according a tyre calculator I found so in other words if I'm doing 30, I'll really be doing 38 and that's too much to have to think about... So you reckon there is nothing can be done?

I would have thought so too, but a cross check with either a phone gps or stand-alone will tell you.

Last year I wanted to change tyre size on Mrs E's Mazda as a different size was MUCH cheaper.

I checked the speedo wrt gps & it was far enough out that the larger diameter size tyre made it more accurate.

Do a cross check as above & then google "tyre size calculator" any one of them will show you the difference in speed for the tyre size difference, but in answer to your question it is pretty much un-feasible to recalibrate a speedo.

[Andy70]

E-bmw said:

Andy70 said:

helix402 said:

Glosphil said:

I will be astonished if the speedo doesn't read fast with standard wheels & tyres fitted - at least 5% at 70mph.

It will either be spot on or over read by up to 10% in accordance with EU law. (In reality over read). I’m not aware of any software to recalibrate it apart from Carly which will reprogram the car to show the actual speed (rather than the factory over reading).

I believe Speedos to be quite accurate these days but the changes I wanted to make would make it read 8mph slower than I'm really going according a tyre calculator I found so in other words if I'm doing 30, I'll really be doing 38 and that's too much to have to think about... So you reckon there is nothing can be done?

I would have thought so too, but a cross check with either a phone gps or stand-alone will tell you.

Last year I wanted to change tyre size on Mrs E's Mazda as a different size was MUCH cheaper.

I checked the speedo wrt gps & it was far enough out that the larger diameter size tyre made it more accurate.

Do a cross check as above & then google "tyre size calculator" any one of them will show you the difference in speed for the tyre size difference, but in answer to your question it is pretty much un-feasible to recalibrate a speedo.

. Yeah I calculated the difference on a tyre checker site and it would put it 8mph difference so I would download a GPS speed check app thingy? As I.wouldnt trustbtge Google one and thanks for your response in relation to the feasibility of recalibration

[RUSSELLM]

I’d just stick an old TomTom on the windscreen. Go for a drive, and see how far out you are.

With a lot of cars having satnavs these days, there must be a few standalone units hanging about in people’s houses.

I think I’ve Three lurking about somewhere They get the cobwebs blown off them for foreign travel / car hire.

[Andy70]

RUSSELLM said:

I’d just stick an old TomTom on the windscreen. Go for a drive, and see how far out you are.

With a lot of cars having satnavs these days, there must be a few standalone units hanging about in people’s houses.

I think I’ve Three lurking about somewhere They get the cobwebs blown off them for foreign travel / car hire.

no idea who would have one of those to be honest, is there not an app to download?

[helix402]

Most nav apps have speed displayed too.

[Glosphil]

Andy70 said:

Yeah I calculated the difference on a tyre checker site and it would put it 8mph difference so I would download a GPS speed check app thingy? As I.wouldnt trustbtge Google one and thanks for your response in relation to the feasibility of recalibration

8mph error at what speed?

[Andy70]

Glosphil said:

8mph error at what speed?

Ummm not sure I can't remember now. I'll see if I can find the site again

[Andy70]

helix402 said:

Most nav apps have speed displayed too.

Are they accurate? I use Google maps and that can't seem to get the directions right most of the time, so would the speed be accurate? If so then that's good enough for me!

[helix402]

Andy70 said:

Are they accurate? I use Google maps and that can't seem to get the directions right most of the time, so would the speed be accurate? If so then that's good enough for me!

Yes, the apps use GPS, so as far as I’m aware are accurate.

[Andy70]

helix402 said:

Andy70 said:

Are they accurate? I use Google maps and that can't seem to get the directions right most of the time, so would the speed be accurate? If so then that's good enough for me!

Yes, the apps use GPS, so as far as I’m aware are accurate.

Brilliant I'll check the car out first

[E-bmw]

Any old stand-alone unit or G.maps or ANY thing that uses GPS will be 100% accurate at displaying speed over a straight road at a steady speed.

The straighter/flatter the road the more accurate, but in reality we are only talking even a full decimal point if not.

[Andy70]

E-bmw said:

Any old stand-alone unit or G.maps or ANY thing that uses GPS will be 100% accurate at displaying speed over a straight road at a steady speed.

The straighter/flatter the road the more accurate, but in reality we are only talking even a full decimal point if not.

Great thank you

[Andy70]

ok, I found a different site and that said with the sizes I entered that at 70mph the new reading would be 6.8mph, so does that mean relatively that drops as the speed drops, so at 30 it would only be a couple of mph higher?

[BFleming]

You're talking about using a higher profile tyre in the rear only? You run the risk of upsetting the traction control unit by having different rates of rotation front and rear. There's a 4% difference between 255/30-19 and 255/35-19. It might be ok, it might not.