I discovered my speedometer is reading 10% high. So when the speedometer displays 88 km/hr the car is actually travelling at 80 km/hr, 66/60, etc. Discovered when using an iPhone GPS navigation app. Then supported by following my wife in other vehicles. The car has about 140,000 km on the odometer but runs great, other than the usual transmission misbehavior.

The rear seal was changed at 64,000 km's and shortly after the transmission began experiencing shift problems. Any ideas as to what may be causing the speedometer to read high? Could it be a transmission problem, an error made by the mechanic when the transmission was re-installed or maybe the transmission was swapped with a different year without my knowledge??? Unlikely that it was swapped but I had suspected the speedometer was reading incorrectly ever since the engine rear seal was done. Any comments are appreciated.

 

By design most speedos indicate a little high but 10% is excessive. Your tires have a smaller circumference than factory spec.

 

Rear speed sensor error triggered by servicing the rear perhaps?

 

Are the rims the factory ones and with equivalent size tyres (tires)?

 

Not long after acquiring my S-type, I noted the same disparity in the speedometer reading vs. GPS. After considerable faffing about, the root cause was determined to be tires that were considerably below the diameter of factory original OEM tires. This was partially because they were just worn out, but primarily due to differences from one manufacturer to the other.

A replacement set (of another brand) reduced the disparity from about 8% to about 2%. This number is now drifting back towards 5% as the tires wear.

I will solve the problem by turning off the speed indication on my GPS unit.

 

If you have navigation you might want to do a check with the built in GPS speed?

I checked it on my 2005 STR and surprisingly my speedometer matched the GPS reading almost exactly! It only differed by 1 mph at the most. Seems too accurate for a factory speedometer.
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Why?

 

All the other ones I have checked are at least 5 mph off.
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Yup. Same thing here.

 

yep mine is 10% out aswell !

 

That's what I love about this forum. Lots of quick and knowledgable responses. I had not considered tire rear and when you raised the possibility, I though "Why didn't I think of that!". Da! I have summer and winter tires on their own rims and intend to check both. The summers are Continentals and the winters are Blizzaks. One set of rims is original, while the others are a Tire Rack model ("Sport" I think). Shouldn't be the rims, but could most certainly be the tires.

Thanks very much for your assistance.
Terry