Speedomerter

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Permalink Reply by MY MOTIV on February 14, 2010 at 6:26am

The wheels will make a difference. Or rather the total diameter of the wheels and tyres. BMW's are known for showing 3-4 MPH over the actual speed for some reason. I asked a policeman friend of mine what speed you would actually get pulled over for on a motorway and he said you usually get 10% allowance. This combined with BMW's speedometer's made me think 80MPH would be safe. Not quite your question but I have thought of that too.

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Permalink Reply by Gordon Cotter on February 14, 2010 at 6:57am

Thank you. I to have a traffic Policeman friend mostly on Motorway patrol he too said 10% allowance. The major problem with me is that the faster I go the gap gets much wider potentionally. at 90+ the gap has widened to 20MPH approx. I travel down to the midlands quite frequently, and make a 4am start some mornings being so early and few cars about I usually boot it to around 120 on cruise to see a steady reading, satellite speed readings are 90mph to 100mph. I know that sitting at 70/80mph on car speedo I wont be done for speeding. I thought of booking car into a gagrage with a rolling road just to see axactly what is happening. Incidently Ihave joined a scheme whereby for £4.95 a month I have the full services of a company solicitor and full court defence council. It is called "Drive Protect" Also for a little more, if you should lose your licence you get a driver supplied for length of Ban. OR TAXI FARES. nae three bad eh Jimmy?
Gordon

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Permalink Reply by MY MOTIV on February 14, 2010 at 7:02am

Haha. Not a bad deal...
I have some box standard OEM E38 wheels and tyres and just measured the diameter. 26". Any idea of the size of you current set-up?

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Permalink Reply by Gordon Cotter on February 14, 2010 at 7:11am

JUst been out to car with tape measure. From Rim to Rim 18 1/2 inches. I have my 18 inch rims in the shed I measured one of those and it read 19 1/2 inches across the rim.

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Permalink Reply by MY MOTIV on February 14, 2010 at 7:24am

What's the measurement from the edge of the tyre? (Rolling surface).

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Permalink Reply by Gordon Cotter on February 14, 2010 at 8:10am

21 1/2 inches. BUT YOU CANNOT MEASURE THIS DIAMETER, as some tyres have low profile walls. You can get a 225x55x 18 you can also get 225x45x 18 or 225 x40x18 THE LOWER THE PROFILE THE SMALLER THE SIDE WALL, ALSO WITH TYRE WEAR THE SIDE WALL WILL AGAIN BE SMALLER. The measurement has to be from rim to rim surely? AS THIS IS CONSTANT
Gordon

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Permalink Reply by MY MOTIV on February 14, 2010 at 8:26am

It will be the actual rolling surface that will make the difference. A smaller tyre (rolling surface) will have to roll more times than a larger one to cover the same distance. 21.5" will give a circumference of 67.54" whereas the originals will have a circumference of 81.68" (26"diam). A difference of over 14" (if the measurements are correct) would definitely make a big difference on the speedo.
Hope that helps :)

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Permalink Reply by Ken (Scottish Regional Rep) on February 14, 2010 at 8:31am

All car speedometers read over by design, I really wouldn't bother about it. GPS speed will also vary depending how many satellites the receiver has locked on to. My car is 9mph over on stock 17" wheels and correct tyre size fir those wheels.

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Permalink Reply by MY MOTIV on February 14, 2010 at 8:32am

Tire/Wheel Compensation calculator using those measurements:

Indicated speed: 10 mph. Actual Speed: 8.26 mph.
Indicated speed: 15 mph. Actual Speed: 12.4 mph.
Indicated speed: 20 mph. Actual Speed: 16.5 mph.
Indicated speed: 25 mph. Actual Speed: 20.6 mph.
Indicated speed: 30 mph. Actual Speed: 24.8 mph.
Indicated speed: 35 mph. Actual Speed: 28.9 mph.
Indicated speed: 40 mph. Actual Speed: 33.0 mph.
Indicated speed: 45 mph. Actual Speed: 37.2 mph.
Indicated speed: 50 mph. Actual Speed: 41.3 mph.
Indicated speed: 55 mph. Actual Speed: 45.4 mph.
Indicated speed: 60 mph. Actual Speed: 49.6 mph.
Indicated speed: 65 mph. Actual Speed: 53.7 mph.
Indicated speed: 70 mph. Actual Speed: 57.8 mph.
Indicated speed: 75 mph. Actual Speed: 62.0 mph.
Indicated speed: 80 mph. Actual Speed: 66.1 mph.
Indicated speed: 85 mph. Actual Speed: 70.2 mph.
Indicated speed: 90 mph. Actual Speed: 74.4 mph.
Indicated speed: 95 mph. Actual Speed: 78.5 mph.
Indicated speed: 100 mph. Actual Speed: 82.6 mph.

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Permalink Reply by Dennis Cooper on February 14, 2010 at 11:10am

Hi Gordon,

You need to find out the BMW standard fitment tyre size for your car. Now I know some E38s came on 17 inch wheels, some on 18 and others on 20" (usually the Alpina 20" option)

The tyre size will have what's called a rolling radius as already mentioned and can be found when cross referencing with online tyre size calculators.

You need to have the correct size tyres on in order to as closely match the original rolling radius as the OEM equipment tyre size specifies. Only then will you speedo closely indicate as best you real road velocity/speed. For instance, 'IF' your rolling radius on the correct tyre sizing is 650mm, you'll see the list of tyre sizes for 16/17/18/19 inch and other sizes will include their rolling radius depending on their width size and sidewall ratio. If you find a tyre that's say 10mm out, that will have an adverse effect on your indicated speed. The tyre chosen should be within a max of 3mm I believe.

When you find out that information, check with the tyre sizes on your car, they'll be incorrectly sized. You need to puchase tyres of the correct size and have those fitted to 'fix' this issue. Don't be tempted to fit one ratio of sidewall up or down depending on price, you'll still have this issue - it's annoying when the pricing differential can be quite substantial!

As already mentioned, GPS is a great indicator of speed, but can be innacurate depending on satellite signal strengh and no of signals recieved. The 'error' of the 10% on the speedo is a max figure as well, I think most BMW's and modern cars have their speedo's reading much better than +/- 10% nowadays and will be quite accurate at high above legal motorway speeds. I think only after you start getting past 125 mph will you need to have specialist equipment to accurately guage your speed (I read it somewhere so im vague and im not 100% sure it's 125mph and up, but you get the idea)

Hope this helps, Cheers, Dennis! West London & Slough UK!

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Permalink Reply by Gordon Cotter on February 14, 2010 at 11:47am

That is very fullsome in it's context. I do see this, I will get the correct OEM I found the spec of my car under the back seatsquab, well my friend Ken did. I still have it. I do know that they had fitted 81/2 inch x18 BMW alloys with 255x45x18 all round when new. The 45 being "Low Profile" One thing is that because the reading is higher than actual road speed. I should never be caught for speeding. Thanks again.
Gordon

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Permalink Reply by Gordon Cotter on February 14, 2010 at 11:57am

These figures quoted is almost exactly the same as my readings. At 70 on my speedo clock the Satellite readings on all three read between 56 and 60mph... Do you know how to get a closer reading to the actual?
Gordon

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