Optimistic speedo?

[gavinm]

is this a common thing on 944s?

I have larger Cup alloys on it, which I'm assuming helps make the reading inaccurate?

jsut seemed to feel that I wasn't going anywhere near as fast as the speedo said I was...

will be out with a satnav to make a comparison tomorrow but wondered if it was a fact.....

cheers

 

[944 man]

With larger 235/45/17 tyres, my car is now virtually spot-on to 100mph; verified carefully using a high sample rate receiver.

Simon

 

[Mark_Bennett]

It's the rolling circumference of the tyre that will affect the speedo reading. Changing rims to larger ones mean you should have lower profile tyres to maintain the tyre circumference.
But...
If you have gone bigger, then the speedo will be reading low, not high.... (not good!)
Smaller circunference will mean the wheel is going round more times for a set distance, therefore the speed will be showing higher than it really is.

 

[gavinm]

hhmmm...

I have 255 40 17 rears and 215 45 17 fronts.....

 

[944s2]

Try this

http://www.alloywheels.com/tyrecalc.asp

Or you could always double check with a GPS system if you can lay your hands on one

 

[gavinm]

ORIGINAL: 944s2

Try this

http://www.alloywheels.com/tyrecalc.asp

Or you could always double check with a GPS system if you can lay your hands on one

interesting....according to that my rear wheels make it over read but the fronts make it under read......

 

[satancom]

interesting....according to that my rear wheels make it over read but the fronts make it under read......

I think your fronts should be 225/45/17 to match the rolling radius of the rears!

 

[Copperman05]

FWIW my stock wheels on my S2 are about 5pmh out when travelling at 70mph, showing 75mph compared to my tomtom's 70mph, which I'm much more inclinded to trust.

Also adds about 10 miles on the odometer to a 300 miles trip that dont exist.

Edd

 

[944s2]

Front wheels dont drive speedo sender its on rear axle!

 

[944 man]

Series one cars speedometer drive is in the nearside front hub. As I have already said, slightly increasing the rolling radius will make the speedo more accurate, as they usually under-read (even after allowing for tread wear).

 

[944s2]

ORIGINAL: 944 man

Series one cars speedometer drive is in the nearside front hub.

Blast never knew that[]

 

[Mike_Pollock]

Hi

I'm runnig 205/60 x 15 (front and back) on my front hub driven 83 Lux. I also have 2 speedos, a satnav which I use a lot in 4 vehicles.
a - Reno Traffic 57 plate -reading 70 - satnav 67
b - Ford Galaxy 99 - readind 70 - satnav 68-69
c - Hyundai Coupe 05 - reading 70 - satnav 65-66
d - 83 Lux speedo 1 - reading 70 - satnav 70
e - 83 Lux speedo 2 - reading 70 - satnav 76 - Which is scary!

The only thing I have changed is the speedo (odometer problems) so I changed it back and back again!

Still the same - any ideas?

So, in answer to original thread, I would say differences in individual speedo heads is likley to be greater than differences in rolling radius unless extreme.

Mike

 

[dirtydirtyharry]

ORIGINAL: A9XXC

e - 83 Lux speedo 2 - reading 70 - satnav 76 - Which is scary!

Dont the police normally give 10% grace, therefore you will be ok*



*This is not law and merely a matter of opinion. For further advice please consult relevant legislation

 

[944 man]

ORIGINAL: 944s2

ORIGINAL: 944 man

Series one cars speedometer drive is in the nearside front hub.

Blast never knew that[]

Every days a schoolday with me Dougie. []

 

[944 man]

ORIGINAL: dirtydirtyharry

ORIGINAL: A9XXC

e - 83 Lux speedo 2 - reading 70 - satnav 76 - Which is scary!

Dont the police normally give 10% grace, therefore you will be ok*



*This is not law and merely a matter of opinion. For further advice please consult relevant legislation

The ACPO guidlines state 10% + 2mph, but an increasing number of Scamera Partnerships are disregarding this: as they make FAR more money that way...

 

[Mike_Pollock]

However they work it, its to close for my licence!

Mike

 

[ChasR]

Mine is on 17" wheels with 255/40R17s on the rear. According to the TomTom it is spot on speed wise.