new wheels vs speedo accuracy

someguy from the other zafira forum told me that when i changed my wheel the accuracy of my speedo will change. he said that since the wheel is bigger than the stock my speedo will be 1.7mph less than what its telling me (30mph = 31.7mph). is this true? the diameter of the stock wheel and tyre (205/15/65) and my new ones (215/17/50) are almost the same so i was wondering how it will affect my speedo. sorry guys my math skills are rubbish can anyone enlighten me on this. thanks

hi mate, it will affect your speedo but only a minimal amount i wouldnt worry about it. if you have a satnav take the car out for a run and compare the natnav speed against the clock speed.i wouldn't worry about it as you are allowed a 10% inaccurancy. vauxhalls of yesteryear were very inaccurate in my experience. clock reading 150 satnav reading 128mph.
most notably a mk2 astra gte with c20let conversion the clock stopped reading @ 130. sat nav read 158

usually the wheel height remains around the same as the wheel gets larger the tyre walls fitted are smaller,,, just measure the whole circumference of both wheels around the outside of the tyres and see how much difference there is...

scottie2065 wrote:

hi mate, it will affect your speedo but only a minimal amount i wouldnt worry about it. if you have a satnav take the car out for a run and compare the natnav speed against the clock speed.i wouldn't worry about it as you are allowed a 10% inaccurancy. vauxhalls of yesteryear were very inaccurate in my experience. clock reading 150 satnav reading 128mph.
most notably a mk2 astra gte with c20let conversion the clock stopped reading @ 130. sat nav read 158

i have a tomtom satnav even before i made changes in my car and ive notice that there is always 5-7mph difference between the satnav and my speedo (car 80mph + satnav 75mph). i went to ashford two weeks ago and its the same story with my new wheels and tyres. to be on the safe side i always follow my speedo and not the satnav because i always think that there is a certain amount of delay between the satnav and the satellite. thanks for the input mate

supersterl wrote:

usually the wheel height remains around the same as the wheel gets larger the tyre walls fitted are smaller,,, just measure the whole circumference of both wheels around the outside of the tyres and see how much difference there is...

yup! they both have the same outside circumference.

scottie2065 wrote:

hi mate, it will affect your speedo but only a minimal amount i wouldnt worry about it. if you have a satnav take the car out for a run and compare the natnav speed against the clock speed.i wouldn't worry about it as you are allowed a 10% inaccurancy. vauxhalls of yesteryear were very inaccurate in my experience. clock reading 150 satnav reading 128mph.
most notably a mk2 astra gte with c20let conversion the clock stopped reading @ 130. sat nav read 158

ive been told they dont give the 10% anymore its down to each officer

denzzz28 wrote:

supersterl wrote:

usually the wheel height remains around the same as the wheel gets larger the tyre walls fitted are smaller,,, just measure the whole circumference of both wheels around the outside of the tyres and see how much difference there is...

yup! they both have the same outside circumference.

then the speedo should read exactly the same as before..

supersterl wrote:

denzzz28 wrote:

supersterl wrote:

usually the wheel height remains around the same as the wheel gets larger the tyre walls fitted are smaller,,, just measure the whole circumference of both wheels around the outside of the tyres and see how much difference there is...

yup! they both have the same outside circumference.

then the speedo should read exactly the same as before..

thats exactly what i had in mind.

to be honest i just go by the speedo as well because in the majority of cases @ 60mph on the clock the car will be going slower.
rule of thumb when dealing with the law is they allow 10% but as Madgezza said its up to the officer. but most officers go by the 10% rule.