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Discussion Starter · #1 ·
I recently took my first long trip in the 97 9000 CSE my father gave me, and I borrowed my wife's GPS, mostly just to follow along where I was and find convenient fast food along the way. I discovered that the speed reported by my speedometer is roughly 3mph faster than that reported by my GPS (at about 65mph). It's hard to argue with the GPS, so I'm thinking my speedo is off a little bit. It's not a huge deal, but a mild annoyance, and I'd like to be able to fix it; does anyone know if it's possible to adjust the speedometer? As far as I know, the tires that are on it are the same size as those that came with it (although I don't have the car in front of me at the moment to find out what that size might be), so I guess it's always been this way.

As I understand it, the speedo isn't cable-driven, so there might be a rheostat somewhere I can adjust; if I'd have to do something like replace a gear near the tranny, I'm going to ignore it. Any ideas?
 

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The only way it'd be off is if the tires aren't the right size.
According to the owner's manual, you should have 205/60R15.
 

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Discussion Starter · #3 ·
Yep, 205/60 R15 Z-rated Michelins are what's on the car. I dunno why the speedo would be crazy; should I assume there's no easy way to adjust it?
 

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well its actually a good thing in a way becuase if your speeding, you aren't actually going quite as fast as you thought so in a way it might help you in the long run. Mine on the other hand doesn't work at all, so I'm going 20 all the time which actually got me a speeding ticket a month ago when I thought I was going 125, the hill I was on caused me to 133 (km/h ofcourse).
 

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Discussion Starter · #5 ·
xxbrianxx said:
well its actually a good thing in a way becuase if your speeding, you aren't actually going quite as fast as you thought so in a way it might help you in the long run.
That's probably true, but it still bothers me. :) It's like when budding musicians are told it's better to be sharp than flat; that's probably true, but either way, you're out of tune.
 

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My 97 Aero speedo is off by 3 to 5% (I'm not sure by how much exactly).

I find the difference between the speed displayed on the trip computer (which is accurate) and on the speedo is 3 to 5 % off.

Correct tire sizes should be 195/65x15 and 205/55x16.
205/60x15 will cause the speedo to read about 1.2% fast compared to the stock sizes.

Almost all speedos are optimistic by 2% so 3.5% is well within normal accuracy.

Manufacturers have two choices given the cost of making a truly accurate speedo is prohibitive. They could aim for maximum accuracy which would produce some speedos bang on, some reading low and some reading high. Reading high is not a problem but reading low could be in that you could drive too fast or get tickets and so on. Nobody suffers if the speedos are optimistic. So, given manufacturing tolerances they aim for every speedo to be no slower than bang on. Most speedos will over read using this strategy.
 

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Not sure about other countries, but I know for a fact that speedos to get homologation in Europe NEED to have a built-in error, this means that they mus over read by some extent. A car (or a bike) with a perfectly spot on speedo would be illegal to sell in Europe.
 

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Just get yourself bigger tires, 17 inch wheels will surely fix it:cheesy:

Klim
 
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