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Discussion Starter · #1 · (Edited)
Speedometor reads 5mph below the actual speed

Have been wondering about this for years. It used to be about 4mph below what I was actually going.
This was verified with multiple different radar signs that police departments put out on the street to
tell you how fast you're going, and a speeding ticket!

Well, stopped by a mechanic who took out his tech II and connected it as a courtesy.
We noticed that the computer was set to 15" wheels, but the car has 16" wheels.
I thought ok, maybe this is why the speedometer reads 4mph below my actual speed.

So with the tech II he set it to 16" wheels. Now, the speedometer reads 5 mph below, not 4!

This, along with an engine coolant temp gauge that reads at 10 o'clock instead of 9 o'clock is annoying.
He was going to recalibrate the temp gauge but didn't want to without taking a pyrometer out to verify
the engine's running temp. A couple of years ago, the temp gauge red in between 9 and 10 o'clock.
I replaced the engine temp sensor, and the thermostat (I forget what temp rating the thermostat was) and
the gauge stayed the same. A couple weeks ago when the car approached overheating because of a bad
radiator fan, the needle went up, and now at normal operating temperature, the needle is reading even higher at 10 o'clock.

Do I have a bad instrument cluster? I don't get it!
 

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That depends on what speed we are talking about. I am not sure what the stituation in the US is with regards to this, but in europe its mandatory that the speedo either is accuarate or overreads, claiming a higher speed than you are travelling to within a percenatage of the true speed (I can't recall the figure now) most do overead, as there would just be too many tickets! undreading would be very bad, can you imagine the lawsuits?
 

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Have been wondering about this for years. It used to be about 4mph below what I was actually going.
This was verified with multiple different radar signs that police departments put out on the street to
tell you how fast you're going, and a speeding ticket!

Well, stopped by a mechanic who took out his tech II and connected it as a courtesy.
We noticed that the computer was set to 15" wheels, but the car has 16" wheels.
I thought ok, maybe this is why the speedometer reads 4mph below my actual speed.

So with the tech II he set it to 16" wheels. Now, the speedometer reads 5 mph below, not 4!

This, along with an engine coolant temp gauge that reads at 10 o'clock instead of 9 o'clock is annoying.
He was going to recalibrate the temp gauge but didn't want to without taking a pyrometer out to verify
the engine's running temp. A couple of years ago, the temp gauge red in between 9 and 10 o'clock.
I replaced the engine temp sensor, and the thermostat (I forget what temp rating the thermostat was) and
the gauge stayed the same. A couple weeks ago when the car approached overheating because of a bad
radiator fan, the needle went up, and now at normal operating temperature, the needle is reading even higher at 10 o'clock.

Do I have a bad instrument cluster? I don't get it!
Think you'll get to get the cluster re-programmed with a Tech 2.

Only the temp gauge can be reset without the "hardware key" that's needed to program the rest of the cluster so many knock off Tech 2 units can't do the job.

SAAB dealer or reputable independent should be able to get all the gauges indicating correctly..
 

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Discussion Starter · #4 ·
It was a GM tech II, but maybe he didn't reprogram the cluster completely. I've never had a card that is a full 5mph off like this. Hapens at 20mph, 40mph, 60mph, etc.
 

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I have the same exact issue with my 2002, car was programmed to 15" wheels and Charles River Saab reprogrammed it to 16", still 4-5mph faster than actual speed. If you find a fix please let me know, its a minor aggravation but annoying nonetheless.
 

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I have never had a car of any brand that was not 3 to 5% higher than actual, tested via GPS and/or radar. It is done for a reason, most of which is liability (they do not want to pay your tickets) some legal and some practical. The same size tire of different models/brands will have a slightly different circumference which impacts speed, actually as any tire wears it reduces circumference, air pressure is also a factor.

I have never seen it be off by exactly the same speed regardless of speed though, that is odd, almost like the needle is set in the wrong place.

A half inch change in diameter (nominal 25" tall tire) for any reason could be 2% in speed right there (may be off I did the math in my head...). Unless the speedo is GPS based it will be off. Even if you set it perfect now it will be off again in the future (worn or new tires).
 

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Discussion Starter · #7 ·
It would make sense if the speedometer said 30 when the car was actually going 25, but this is reverse. When it says 30, you're actually going 35!
 

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Does it read exactly 5 MPH under no matter what speed you are doing, i.e at 30 you are doing 35, at 50 you are doing 55, at 70 you are doing 75. Or is it a percentage, i.e. at 30 you are doing 33, at 50 you are doing 55, at 70 you are doing 77.
 

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I believe it is a cluster problem then, especially if other gauges are off too.
I would try replacing it, with a junk yard unit, a new cluster will need to be married, but you should be able to swap odometer units to maintain your mileage.
 

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the speedo only reads properly with the right size tire to rim ratio. if you have a tire to big for the rim (profile) the speedo will be off just as if it is too small. even if the guy changed it with the techII, how do you know that the tires on those 16's are correct profile for the calculations from sensor pulse counts to mph displayed on the gauge cluster. what you will need to do is find what the factory tire profile is, then do the calculations to figure out the correct tire profile that should be on the 16's
 

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no, all you are doing is turning the sensor faster or slower, the tire is not growing in size as your speed up. (not meaning to sound mean)
 

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no, all you are doing is turning the sensor faster or slower, the tire is not growing in size as your speed up. (not meaning to sound mean)
Nope, jake is right. Say the tire diameter is off by 5%, that means each wheel turn is 5% more (or less) distance than the cluster realizes. That means the numerator in the speed equation (miles) is off 5%, but the denominator (hours) is still right. The faster you go, the further off (in miles) the numerator will be.

Take this hypothetical. Your actual tire circumference is 4 feet, but the car is calculating your speed based on a circumference of 5 feet. That's a 25% error. Now say you traveled 4000 feet in one minute. That's 1000 rotations, but the car thinks that 1000 rotations is 5000 feet. that's a difference of 1000 ft/min. Your actual speed was 45.45 m/hr, but your speedo would have shown (on average) 56.82 m/hr (125% of your actual speed). Now, do the same trip in half the time. Your actual speed doubles to 90.9 m/hr, and your indicated speed also doubles to 113.64 m/hr. The speed diference went up from 21.37, to 42.74 (also double), but is the same 25%.

An incorrect wheel/tire size will definitely cause the indicated speed to be off by a percentage, not a fixed number of mph.
 
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