Temp gauge re-code

[wpoll]

I took note of the speedo vs BC menu #8 V: value today...



I should have verified these values via a GPS at the same time, as these values are with a set of tyres at pretty much minimum tread depth. I want to perform the same check once I replace the tyres so I can gauge the variation over the life of the tyres. This will affect any correction factor I might code, as no one correction factor will read TRUE speed for the entire life of a set of tyres. I'd like to set a correction factor that doesn't ever result in a speedo reading UNDER the true speed...

[Fifty150hs]

Originally Posted by wpoll

I took note of the speedo vs BC menu #8 V: value today...



I should have verified these values via a GPS at the same time, as these values are with a set of tyres at pretty much minimum tread depth. I want to perform the same check once I replace the tyres so I can gauge the variation over the life of the tyres. This will affect any correction factor I might code, as no one correction factor will read TRUE speed for the entire life of a set of tyres. I'd like to set a correction factor that doesn't ever result in a speedo reading UNDER the true speed...

What I noticed between used (very) tires and new and the gps speed vs the displayed speed was no difference. At 80 mph (gps), the speedo consistently reads 82/83 mph.

[wpoll]

Originally Posted by Fifty150hs

What I noticed between used (very) tires and new and the gps speed vs the displayed speed was no difference. At 80 mph (gps), the speedo consistently reads 82/83 mph.

Interesting - I would expect to see a 3-4% variation over the life of the tyre. That's what I see on my bike and the X5 calculates speed in the same way - via the ABS sensors...

I guess it's possible there is a correction factor built in to allow for the tyre wear but that would require the original tyre size to be known... too complex, even for BMW.

[Fifty150hs]

Originally Posted by wpoll

Interesting - I would expect to see a 3-4% variation over the life of the tyre. That's what I see on my bike and the X5 calculates speed in the same way - via the ABS sensors...

I guess it's possible there is a correction factor built in to allow for the tyre wear but that would require the original tyre size to be known... too complex, even for BMW.

Tire diameter is set. It's virtually the same no matter what diameter wheel as long as the correct tire size is used with the expected diameter wheel. 17" wheel, taller tire. 20" wheel much shorter tire. But the over all diameter remains the same. That's how the tire pressure system works and it wouldn't work if there was any difference in overall tire diameter no matter what size wheel. It's the same for the speedo. If you throw a radically different overall tire/wheel diameter at it, it will not read remotely correctly and you will likely get a tire warning light. And your speedo will be off as well.

[wpoll]

Originally Posted by Fifty150hs

Tire diameter is set. It's virtually the same no matter what diameter wheel as long as the correct tire size is used with the expected diameter wheel. 17" wheel, taller tire. 20" wheel much shorter tire. But the over all diameter remains the same. That's how the tire pressure system works and it wouldn't work if there was any difference in overall tire diameter no matter what size wheel. It's the same for the speedo. If you throw a radically different overall tire/wheel diameter at it, it will not read remotely correctly and you will likely get a tire warning light. And your speedo will be off as well.

Exactly. But the tyre diameter changes as it wears... not by much but it's enough...

[Fifty150hs]

Originally Posted by wpoll

Exactly. But the tyre diameter changes as it wears... not by much but it's enough...

In my experience it doesn't matter. Worn tire vs new it is virtually the same reading on the speedo.

[wpoll]

Originally Posted by Fifty150hs

In my experience it doesn't matter. Worn tire vs new it is virtually the same reading on the speedo.

Count yourself lucky then - my experience has been the total opposite.

Esp. on my motorcycle, which has a larger difference between new and worn diameters and wears a lot quicker - 5000-6000 miles from new to worn out!

The math on this is pretty murky, as the actual effective rolling diameter is dependant on tyre brand, sidewall construction, inflation pressure, temperature, vehicle load.... the list goes on. ;-)

Back to the regular scheduled thread subject.... ;-)

[deepblonde]

Originally Posted by wpoll

I took note of the speedo vs BC menu #8 V: value today...



I should have verified these values via a GPS at the same time, as these values are with a set of tyres at pretty much minimum tread depth. I want to perform the same check once I replace the tyres so I can gauge the variation over the life of the tyres. This will affect any correction factor I might code, as no one correction factor will read TRUE speed for the entire life of a set of tyres. I'd like to set a correction factor that doesn't ever result in a speedo reading UNDER the true speed...

If the true speed is calculated using new factory size tyres on the factory wheels, then , as the tread wears, it will only mean your speedo indicates higher as the diameter gets smaller with wear,
never the other way round,
unless of course you put larger diameter tyres on than standard.
Be interesting to how your indicated speed compares to GPS speed,
assuming that’s accurate.