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Did anyone ever try to calibrate the electric speedometer? At about 65 kph the speed shown is 80 kph. The tire size is correct, even though it's not the factory reccomended size. I calculated the tire size according to Paul Grimshaw's Volvo Performance Handbook. He says that, in order not to have speedo variations, you have to maintain a maximum 3-4 % variation in total length of the wheel. I'm using 185/65R14 for winter tires and 205/65R14 for summer tires. Both sizes match the 3-4 % variation according to my calculations, as you can see:
1. According to the sticker on the fuel cap the stock tires for the 1991 Volvo 245 are: 185/70R14 or 185/65R15
2. This means for the first size:
Width = 185 mm
Height = 70% of the width --> 185*70/100=129.5 mm
Rim size = 14 " --> 14*2.54=355.6 mm
Length = 2*pi*r= 2*3.14159*Height+Rim size/2=2*3.14159*307.3=1930.82 mm
We'll consider that as standard length - 100%
The second size (original) is:
Width = 185 mm
Height = 65% of the width --> 185*65/100=120.25 mm
Rim size = 15 " --> 15*2.54=381 mm
Length = 2*pi*r= 2*3.14159*Height+Rim size/2=2*3.14159*310.75=1952.5 mm
The length ratio gives us 1952.5/1930.82*100=101.12%
My winter tires 185/65R14:
Width = 185 mm
Height = 65% of the width --> 185*65/100=120.25 mm
Rim size = 14 " --> 14*2.54=355.6 mm
Length = 2*pi*r= 2*3.14159*Height+Rim size/2=2*3.14159*298.05=1872.7 mm
The length ratio is somewhat to the limit: 1872.7/1930.82*100=96.99%
My summer tires 205/65R14:
Width = 205 mm
Height = 65% of the width --> 205*65/100=133.25 mm
Rim size = 14 " --> 14*2.54=355.6 mm
Length = 2*pi*r= 2*3.14159*Height+Rim size/2=2*3.14159*311.05=1954.38 mm
The length ratio is almost identical to the second standard size: 1954.38/1930.82*100=101.22%
So, as you can see tire length is not an issue of speedo variation. Any other ideas?
THX
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