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Discussion Starter #1 (Edited)
TL; DR: Got my speedometer to be accurate and now I don't have to mentally adjust every time I look at the speedo. More relaxing to drive. Solution was "manual" tire circumference setting in Tech II to circumference of 1860.

My speedometer has been off by 3mph (high, all speeds) forever. Verified by GPS and those police "your speed is" displays. Stock tire was 215-45-17 but neither that nor the 205-55-16 size corrected it. The only time it was correct was when I was running some 215-55-16's off a 9-5 for a while.

I decided to dig into it a bit and developed a chart of actual tire sizes to visually compare (below). WIS says that there are small, medium,and large choices that correspond to 195-65-15, 205-50-16, and 215-45-17. That already seemed wrong since those aren't in the correct order for S, M, L and by 2002 the 16" was a 205-55-16 as the stock size .

My speedo was high, which means that the car thinks the tires are larger than they are. The fact that the 215-55-16 fixed it verifies that (see size difference in the chart). Clearly I needed a smaller size specified but the only size smaller was the 205-50-16 and that didn't fix it.

Did a little digging and I saw some folks referring to a "manual" setting in the Tech II. I'd never heard of this, folks always said "Pick one of three tire sizes". Maybe that is true with the NG900. But I pulled out the Tech II to check and it listed the three settings but sure enough there was a "manual" setting. The manual setting is set by circumference in mm (on my chart now).

Whatever circumference Saab is using seemed to be "correct" in that it results in 3mph too high. It's their calculation algorithm that's off. Seems to be linear across all speeds so perhaps they are just adding in 3mph for liability reasons.

Did some rough estimates, which turned out to be worthless. Moved to just guessing to hone it in. Final results: A setting of 1860 gives me a speedo that seems to be accurate (visually accurate vs. GPS and police signs although off by 1mph high or low at times depending on road variations).

 

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I seem to remember years ago reading somewhere that most European cars read high since in Europe by law the speedo is not allowed to read under the actual so they build it to be +X/-0.
US laws allow a +/- range so they shoot for on the money.
 

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I wonder if you're thinking of the same Car & Driver article I read way back when? They did a test of speedometer accuracy years ago, and that's exactly what they found. Speedometers from US car manufacturers were the most accurate, but they could read low just as easily as they could read high. The Europeans were the worst. They always read high, and sometimes by quite a bit.
 

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Saab is not the only manufacturer that makes the speedometer read high, usually around 4 or 5 km/h (or 3 MPH).

Increasing the tire diameter can get the speedometer correct at one point, but you start getting errors on both sides of that point. Below the correct point, the speedometer will still read high. Above the correct point, the speedometer will increasingly read low.

The basic problem, mathematically speaking, is that it's like a line graph, showing road speed versus RPM (of the tires). Changing the diameter of the tires changes the slope of the line. However, what we're dealing with is mostly an offset. We don't want to change the slope, we want to make the line straight up from 0,0.
 

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Discussion Starter #5
Yep. I was pleased to find that it was very liner - clearly a static amount added - vs. something that varied with accuracty by speed. But, a little tweak and you're all set - pending approximation for your particular tire size.
 

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My 2004 9-5 indicates 32 when the GPS thinks 30, and points to 74 when the GPS says 70. Not only did the 9-3 do the same, but I agree that every car in the UK in which I've been able to compare, shows the same artificial high speed.
 

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Interesting analysis Bob and others. I recall a tyre & wheel website that did the analysis based of your factory rims ..and calculated that you would be 'x' over the limit due to circumference 'x' etc based on your tyre spec. I'll try to dig it out - I'm sure I linked it when I got some new tyres. I think the value they used was arround 3km/h.
I'm running 205/50/16. Its the spec listed on my tyre card.
Good Work All.
 

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Links as promised:

Based on the difference between 205/50/16 and 205/55/16 > this reports that you would be over by 2km/h.
Given the other potential variables - like tyre pressure, temperature etc - It would be very interesting to test this further.
 
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