Speedo error?

[stephenh]

My 97 C2S seems to have a vastly over reading speedo. I have put in my Garmin Sat nav which I am presuming is accurate, and it reads 33mph when the speedo says 40. At 90mph on the speedo the sat nav says 81. The car is pretty standard running 18" wheels with 225/40/18 tyres at front & 285/30/18 rears which I think were standard when the car was new. Any solutions, or are they all similar?

 

[clyde]

I find my speedo is slightly out when compared with the Garmin but it is a consistent 9% or so over across the whole speed range (or at least as far up as I've taken it)

Your higher figures don't seem too far wide of the mark at a bit over 10% error but the lower figures are way off. I'm not sure how you would get differing percentage errors across speed ranges as it's an electronic rather than a mechanical speedo head. Is the percentage difference linear, that is to say does it get more accurate the faster you drive? Maybe by the time you reach top speed it would be reading spot on []

 

[Stewart Rix]

My old C4 speedo was reading 10% over consistently across all speeds when compared to a portable satnav throughout the whole trip to Le Mans.

 

[Mike M]

I believe all speedos are out under tolerance but always over. Thats why if your caught doing 40 measured by the Police in a 30 limit, they wont let you off as your clock speed will inevitably be indicating even more than that! All the speedos on my moders cars have been out.

 

[szklarek]

Yes I've always assumed that the speedo reads about 10% over the speed you're doing but my satnav seems to indicate that the speedo is under-reading but much more than that.

Regards
Anton

 

[Jon Kent]

I'd say it could be the tyre circumference that's the reason.....a bit anorak, but a buddy gave me a link to a tyre/wheel comparrison site
http://www.rimsntires.com/specs.jsp
Rim size is not really relevant here, but I've compared against my standard sized 265 rears.....check the circuference and revs per mile.......

Wheels:
Wheel 1
Wheel 2

Wheel size
18x10 ET 47
18x10 ET 47

BackSpace
186 mm (7.3'')
186 mm (7.3'')

Typical Weight
~ 33 lbs / 15 kg
~ 33 lbs / 15 kg

Tires:
Tire 1
Tire 2

Tire size
265/35-18
285/30-18

Section width
265 mm (10.4'')
285 mm (11.2'')

Sidewall
93 mm (3.7'')
86 mm (3.4'')

Tire diameter
643 mm (25.3'')
629 mm (24.8'')
14 mm (2.2%) shorter.

Rim sizes
18x8 to 18x10
18x9 to 18x10.5

Circumference
2019 mm (79.5'')
1975 mm (77.8'')

Revs per mile
797.1
814.8

Rgds,
Jon

 

[szklarek]

Plus I guess you've also got the added complication of tyre wear and pressure differences that would affect the circumference.

Regards
Anton

 

[clyde]

But is the correct size for 18" wheels not 285 x 30 Jon? That's certainly what it is for the turbo, is the S different?

 

[stephenh]

Thanks guys for your comments, looking at Jons figures my 285/30/18 tyres are doing more revs/mile as they are slightly less diameter - this may have an effect.
The 'S' came with 9Jx17 255/40 or 10Jx18 285/30. Just thought the speedo optimism was a bit excessive.