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Right from the factory, speedometers in most classic Volvos usually read a bit on the "optimistic" side -- the speedometer might read 60 MPH when the car is really only moving at 56 or so. Now, when many of us drive cars with transplanted transmissions, rear ends and different tires, speedometer readings can be wildly off.
You can easily calculate your real speed by knowing a few numbers about your particular car and by having an accurate tachometer, even if you just rig one up temporarily. Electronic tachs may be presumed to be accurate. P1800 and 1800S owners, please see the footnote.
You will need to know the rolling radius of your tires. Check them for proper inflation, then use a tape to measure perpendicularly from the middle of a rear wheel to the ground (front wheel for the 850 - V/S/C70). Record this measurement in inches.
You need to know the rear-end ratio of your car. Stock ratios were usually as follows:
This is enough information to check your speedometer error in 4th gear without overdrive. Apply the following formula:
Let's try this out for a 120 4-door with no OD. The rolling radius of the tire was measured to be 12.5 inches (yours will vary -- this is just an example). We want to know how many RPM equate to 65 MPH.
Obviously, you don't want to be driving down the road punching a calculator or trying to work this out in your head, so there are two things you can do. The first is not elegant, but it is completely effective: work out the formula for four or five different speeds (that can be driven in 4th gear) and write down the results. Take an assistant for a ride and drive at the resulting RPM. For each speed, have your assistant write down what your speedometer actually reads. When you get back home, mark those speeds on paper tape and stick them at the appropriate places on the speedometer's face. Speedometer error is usually linear, so you can extrapolate for lower speeds as well.
The second solution only works if the error is slight and you have time to tinker. The speedometer cable is driven by a gear at the rear of the transmission or OD unit, and this gear is available with varying numbers of teeth. A gear with more teeth will slow the speedometer down; one with fewer teeth will make it read higher. You can access this by removing the cable and undoing a nut (holding a clamp on the OD units). The drive assembly will then pull out (oil will run out; have a pan handy and more oil available). The gear is then easily replaced. The transmission does not have to come out of the car to do this.
Frankly, the less elegant method works well enough for us -- we've won a number of TSD rallies with a strip of tape under the face of the speedometer in our 122S. After a few weeks of driving with it, you'll know where the indicator points at "real" speeds, and you can do away with it and impress your passengers: "Wow! We're going 100 MPH!" (but you'll know it's only 70).