Before I start my tires are stock size, 215/45/17. And my speedo has been dead on til recently.

So the other day I. Am driving across the bay bridge the other day for the dan mvkk meet,doing about 40 and I go past those speed check deals, and it says I'm doing 37.

So I don't think much about it I figure they are just off by a few mph.. But then 2 days ago I'm going across the bridge again in my wifes car and it is dead on, so then I start thinking, wtf?

So now yesterday the wife and I are driving home from work, seperate cars because I was running late, and I have her follow me. When I am doing 70, I am actually doing 72-73. But when I am @ slower speeds like 35-40, I am actually doing 33-37.

What would cause this and what can I do?

 

is the speed sensor in the trans

 

My STi says I am going 70 when I am actually going 63-64. I have 225/45R17 tires, they are the stock size.

 

I dunno, I. Wouldn't have the slightest idea

 

That's odd, my STi always says I'm doing 55mph, but apparently I'm going 80mph...at least the officer said I was.

 

I think if I go with a 245/40R17 my speedo will be correct or at least slightly more accurate or just keep going with the slight difference in speed and not really care.

 

Every car I've ever owned or driven past one of those speed boxes always tells me I'm going slower than my speedo.

I believe speedo calibrations are specified as -0% / +x% so you can never tell a cop you were driving fast because your speedo said you were slow.

I always assumed they are built-in 5-10% high on purpose, because they know everyone does at least 5 over.

 

Yeah, I have the same problem with my 02 wrx. I just got it and noticed the speedometer and the tach are both above actual. Tach is about 500 rpm above actual.

 

It's a given that there is a variance of 5% and sometimes 10% of speedometer readings that is normal. Then you if you consider that it's possible that worn tires, different tire pressures, & air temperature can effect speedometer readings even more!
What about the calibration of the radar units and if the radar beam was really directed at another car?

 

hahahahaha!lol