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

First things first. The stock tire size for 240 wagons is 185-14, not 185/70-14 (that's the stock size for the sedan). The former has an aspect ratio or 78, so by calculation the OD is 25.36 in. (My spreadsheet for calculating tire sizes is in inches and diameters.)

185-14 25.36
185/70-14 24.20
185/65-14 23.47
205/65-14 24.49

Thus your winter tire will show a +7.5% speedo error, and your summer tire will have a 3.4% error.

Bear in mind that these are 'theoretical' diameters. Tire mfrs show slightly different diameters in their specs, but not enough to bother with. Tread wear, vehicle loading and inflation pressure also contribute to small variations in 'true' diameter.

I'm curious how you attempted to calibrate your speedo - did you have a measured mile (or km)? Calibrating against the tach is unreliable unless the tach itself is calibrated. At any rate, if you are showing an indicated 80 at a known 65 kph, that's a 23% difference, far more than what tires sizes would account for. Is it possible your rear axle gearing has been changed, or that your car's speedo was replaced by a previous owner with the incorrect unit?

You may want to pull out the manual. The sender may out of adjustment and not picking up correctly. Just a thought.