speedo calculations

[cschollum]

Can someone explain how alterations to wheel sizes affect speedo calculations?

my car has different wheels from standard and ive always suspected that the speedo was out - im aware electronic speedos, especially bmw have a small difference in speed stated to actual due to allowances for errors, meaning you are going slower than stated but im guessing wheels play a major part in this

do the markings on the speedo allow calculations to be made as to what size the wheels should be for the speedo to be acurate? it says something like k=

any help would be good
chris

[SRR2]

The speedometer is calibrated to read a few % high with the nominal OEM tires. Larger tires than nominal will cause the speedo to read lower at a given road speed. Smaller tires, higher. The difference is directly proportional to the tire rolling circumference. You don't actually need to calculate the circumference; all that's necessary is an accurate measurement of wheel center to ground of both the OEM tire and your replacement. Naturally tire wear, load, and inflation will all have some influence on this dimension.

Instead of driving yourself trying to calculate this, and to guess at the OEM dimensions, why not get yourself (or borrow) a small handheld GPS unit and measure the actual speed. It will be absolutely accurate and free of approximations and guesswork.

I believe that the 'k=' has to do with the selection of speed sensor and cluster. There's nothing you can do with it or about it.

[BigKriss]

cschollum, tire size calculator. (http://www.miata.net/garage/tirecalc.html)

[Jay 535i]

this looks good, but id need to know what sizes i had when it left the factory - also werent the originals metric and now we use something else?

Factory size is 225/60/15, I believe.

[SRR2]

You need a better GPS. The two I have are perfectly stable and can display speed in either km/hr or miles/hr (and one of them can do m/sec and ft/sec too). Dunno where you're coming up with this knots situation. Speed is speed, too. Makes not a bit of difference whether you're on a flat road or on an incline. A properly functioning GPS will give you an accurate speed to 0.1 mph.

The best way to calibrate speed IMO is to set the cruise to put the speedo exactly on 60 mph. and then read the true speed on the GPS. The cruise should be able to hold within 0.25 mph under ordinary conditions. IIRC the cruise on my E34 would hold to +-0.5 on any hill.

[DaCan23]

Well

1 mile/h = 0.868976 knot


thanks, i understand i cant do anything about it, I have gps and the speed is out, but you cant rely on GPS to give an acurate measurement unless you are on a flat surface and even then it jumps up and down and never stabalises
and remember gps signals are given in knots and not MPH, as the figures are similar many people forget this and so you have to do the conversions.

i was just wondering how much my current non oem tyres are from stock regards diameter - anyone know what the diameter of the standard wheels would have been? im sure the k= on the speedo has some bearing over the type of calculations it does based upon the expected tyre sizes

[Bellicose Right Winger]

The "k" value is the number of pulses the speedometer needs to register 1 mile. The pulse generator in the dif creates 6 or 7 pulses per axle revolution. You can do the math to check this after you measure/calculate/estimate the tire circumference.

To check speedo accuracy, set cruise control and measure time required to travel 5 miles using milemarkers on interstate. MPH = (Miles travelled)/(Time in hours).

What size tires are on your car?

Paul Shovestul




Can someone explain how alterations to wheel sizes affect speedo calculations?

my car has different wheels from standard and ive always suspected that the speedo was out - im aware electronic speedos, especially bmw have a small difference in speed stated to actual due to allowances for errors, meaning you are going slower than stated but im guessing wheels play a major part in this

do the markings on the speedo allow calculations to be made as to what size the wheels should be for the speedo to be acurate? it says something like k=

any help would be good
chris