Tyre radius question

I know it's a dumb question, but it's really annoying me.
The weight of a car flattens the tyre where it meets the road, so the radius is less. Does an underinflated tyre affect the speedo reading?

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It can , yes.
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Not a dumb question.

Speedometers on BMWs are driven off the differential IIRC, so it will have less impact (averaging).
Additionally, this effect can be used by an ABS system (wheel speed sensors) to discover if a single tire is going down. (BMW uses individual Tire Pressure Monitors in each wheel.) This system is used on our Toyota Highlander, for instance.
FloydR
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wrote

My '03 Z4 still uses the wheel speed monitoring for a low tire warning.
Tom K.
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"Newer" BMWs use the wheel speed sensors for speedo, tyre pressure warning and ABS.
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wrote:

Haven't some of the "newest" BMWs switched over to the individual pressure monitors? Some of the latest BMW motorcycles have them.
Tom K.
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Early models had a pulse sensor in the diff, however from memory, the later models use the L/H/R ABS sensor for the speed reading, so if it was that tyre then it is possible. Despite this, I find it unlikely that it would affect the speedo reading enough to get you off a speeding conviction because speedo's deliberately over read for situations such as this.
Whilst you are at it, you could also argue that driving around a corner would also affect your speedo reading.

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That is true. Parts of the car that are further away from the centre of the turning circle will be travelling faster.
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radius
Of course not, the circumference remains the same - and that is the distance travelled by the wheel.
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Except that the *RADIUS* changes, which affects the *EFFECTIVE* circumference. The consequence is that the radial speed changes.
FloydR
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No it doesn't. The *effective* diameter is reduced by lower pressure.
If the pressure drops low enough, the effective diameter is that of the wheel plus the thickness of the compressed tire tread.
-- Larry
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No. An underinflated tire (as we spell it here in the Colonies) will affect performance, handling, and mileage, but not the speedo. Okay, it will affect the speedo, but the tires would have to be severly underinflated -- about half of the required spec. -- in order for any impact would be severe enough to be noticed on the speedo. You would fill the tires long before they were so low as to affect the speedo.
What makes you ask?

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Hi Jeff.
Well, I had read the thread in this newsgroup about speedos being out by around 3%. I used a GPS unit and found that my car is out by 10%. When the speedo says I'm travelling 100kph, I'm actually travelling 90kph. Some local drivers tend to get very close behind you and urge you along if you're travelling below the speed limit. I would like my speedo to be accurate and travel close to the limit, but not over it (or not too often). I would be happy to get new rims and tyres to do it next time my tyres need changing ,so the next question was to find the right sizes.
I checked the popular on-line calculators to find one that would tell me what I should replace my 205/60/14 tyres with. None would tell me a tyre size that would decrease the speed shown to the right value. I suspect a 215/55/17 would do the trick, but need to check my figures.
We discussed it at work and got completely sidetracked on whether it was the circumference or the effective radius that was important. As we were trying to find a value that was going to give a 100% accurate value, the flattening of the tyre as it met the road might have been important.
I offered to "ask the experts", as this newsgroup seems to have some people with good technical knowledge.
Thanks.

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Then you need larger tires, not smaller ones. If you take air out, the speedo will be even faster than it already is. It will read 102 at 90 instead of 100 at 90. Since you know it reads fast, why not simply drive at 105 kph in a 100 kph zone and let the others worry about the cops. Or, you could drive in the other lane so the faster cars could drive by ...
My E36 cars (two of them) have the speedo dialed in to within 1.5 mph at 85. I am running 225/45x17s.

Overall Diameter = Width (225, 215, 205, etc.) X Aspect Ratio (50, 55, 60, etc., expressed as a percent -- 0.55 for example) X 2 / 25.4 (to convert mm to inches) + Rim Diameter (15, 16, 17, etc.). You can then multiply the result by pi to get the circumference, then divide that number into 5,280 to find how many revolutions in a mile. If the current tire gives 800 rev/mile, then you want a new tire size that gives about 792 rev/mile. My car with 225/55x15s gives 815 rev/mile, with 225/45x17s gives about 807 rev/mile. Since you live in the land of metric measurments, the calculations will be a bit different, but the end result will be similar -- you need a larger tire than you have if you want the car to go faster at the same indicated speed.
At your current size, you will be looking at a 205/50x16, or a 215/50x15, or 215/40x17. The last of these is nearly identical to what you have, and I'm not sure it is even an available size.
Personally, I suggest getting different tires if you want a different look and/or feel to your car, and constrain the options to those that have the least impact on the other stuff -- speedo, gear ratios, rubbing on fenders and undercarriage, etc. I would not change the tire package just to make the speedo more accurate, expecially on such an old car. If the limit is 100, and you need to do 105 to not be slow, then do 105 and be done. Your indicated speed and actual speed are only different by about 4 mph, this is not bad -- certainly not bad enough to buy a new tire and rim package.
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