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I tried the search function thinking I couldn't be the only one... Also looked in the manual, no joy.
Car is a 2009 C4S. I swapped over summer 19" tires for 18" Pirelli Sottozero winter tires this weekend. It gets cold early in Wisconsin :-) With the 19" wheels and tires, the speedometer was accurate to within a mile an hour or so at highway speeds. With the 18" wheels and tires, the speedo reads 62 mph for a GPS indicated 55 mph. There is a setting in the computer for winter vs summer tires. I changed this and the TPMS works fine. There appears to be no setting to change from 19" to 18" wheels and tires. Is there a fix or should I just remember that I'm 7 mph or so off with the smaller wheels? Pic included with the summer wheels.
I had 18s on my car for 6 years and the speedo read more or less correct - when I switched to 19s it is still fine (both match the Escort RADAR GPS calculated speed very closely). So, as mentioned above, it may have something to do with tire diameter. Tire pressure could also have an effect - wouldn't think it would be that much though.
The recommended 18" tire/wheel size for the 4S in winter is:
Snow tires:
front 235/40 R18 91V M+S 8J x 18H2
rear 265/40 R18 97V M+S** 10.5J x 18H2
Recommended pressure 18 inch wheels Carrera 4S (summer and snow):
partially loaded
(up to 2 persons without luggage)
front 34 psi (2.3 bar)
rear 37 psi (2.5 bar)
fully loaded
(as of 2 persons with luggage)
front 37 psi (2.5 bar)
rear 44 psi (3.0 bar)
How are you coming to your speedometer error conclusions (measured mile, radar, etc.)?
The standard tire sizes for 18" and 19" tires on the car are (rear) 265/40-18 and 295-30/19. The diameter difference on those two sizes is only 0.3", which does not translate into the speedometer difference you're describing.
At 766 revolutions per mile versus 777 revolutions per mile, by my math, the difference on the speedo should be less than 1 mph between the two sets of tires. Did you get an 18" tire size that is taller (higher aspect ratio) than the standard 40?
www.willtheyfit.com
You can play with the numbers to see exactly what size 18's will match your stock wheels for the speedo or the difference speedo error in % with whatever size you are using
With the 19" wheels and tires, the speedometer was accurate to within a mile an hour or so at highway speeds. With the 18" wheels and tires, the speedo reads 62 mph for a GPS indicated 55 mph.
Well, as long as you're going slower than you think you are, does it matter?
(Someone had to give the cheeky and totally unhelpful response -- since this community is so great at giving actual, useful info, as above, I like to balance things out from time to time )
TSP - "19" wheels and tires, the speedometer was accurate to within a mile an hour or so at highway speeds"...
Did you also measure your 19's against your GPS?
With a 19" to 18" wheel, your aspect ratio is higher on your snow and winter tire. The outside circumference should be the same, or at least... close.
With that, if your measuring tire speed. There would be a difference between the circumference of a 8/32 new tire and a worn 1/32 worn out tire, on the same wheel. That would throw off your speedo. How old are the snow tires, how close to new spec? . But we are splitting hairs here.
10-10-2016, 08:20 PM
!8" Wheels and Speedo Error
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