I installed an STI gauge cluster in 04' WRX and speedometer questions

[ajpturbo]

I installed an 04' STI gauge cluster in my 04' WRX. All works well even the rpm warning indicator. Dccd light is on and center diff temp light. Using the same year for your swap seems to be the way to go.

But something that people seem to have missed when I looked for this topic was I noticed that when the speedo said 40mph my GPS said 37mph. I figure this must be because the STI has 235/45/17 tires which have a diameter of 25.4" compared to my WRX's 205/55/16 which have a diameter of 24.9".

How can I correct this? Is is a tranny speedo gear or in the cluster itself?

I'd like it to be accurate becuase it will actually rack up the miles faster on my car.

Thanks for your help.

[ajpturbo]

Anyone? please

[Rambo]

I know a shop near Seattle offers a speed correction service to fix this problem. I can't remember the name, but you can ship the cluster to them, and they'll fix it for fairly cheap. They can also correct the mileage, if the odo is wrong.

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[Heide264]

Almost all cars I've been in have a 3mph reading compared to a GPS. No worries. I think they do it for liability/clearance reasons. Okay to be under, but not over sort of thing.

[ajpturbo]

Almost all cars I've been in have a 3mph reading compared to a GPS. No worries. I think they do it for liability/clearance reasons. Okay to be under, but not over sort of thing.

That's not true. All of my cars are right on. I'm pretty sure it's becuase of the tire size. It' might even be 4 mph off. And the faster I go the more off it is.

Hey Heide where around pittsburgh are you?

[Heide264]

North Hills (North suburbs, getting towards Butler).

We keep all the stock tire sizes on all of our cars but my van, and they are all like that at least. I know our saab 9-5, saab/trailbrazer b****** child, and my van all are about 3mph slower than gps's show. For what it is worth at least. Worth some thought from other memebers though.

[zax]

That's not true. All of my cars are right on. I'm pretty sure it's becuase of the tire size. It' might even be 4 mph off. And the faster I go the more off it is.

Hey Heide where around pittsburgh are you?

Almost all cars I've been in have a 3mph reading compared to a GPS. No worries. I think they do it for liability/clearance reasons. Okay to be under, but not over sort of thing.

Actually, this is true. Most car speedometers will read about 3-4% higher than the actual speed. It is law that all cars must read MORE than the actual speed, but never LESS. My Bugeye reads approx. 2-3 MPH higher than the GPS speed at 50 MPH even when I'm on stock tires, stock wheels. Unless you changed the wheels speed sensor, there is no difference between the two clusters.

[RayfieldsWRX]

Ever car that I've owned since my first GPS (10ish years ago?) has read slightly faster than reality. I did, however, notice that my STI cluster reads a bit higher than even that. I don't really worry about it...it helps to keep me out of trouble.

[zax]

Keep in mind that a standard GPS will also read only lateral motion (motion tangential to the radius of the earth). The implications are as follows: If you are driving up hill at 50 MPH, your tangential motion (as read by the GPS) will be the horizontal component of the true velocity vector. The vertical component of your true velocity -- the component of your velocity that increases elevation -- will not be registered by the GPS due to the typical software calculation (uses Lat-Lon differential rather than x, y, z position deviation). Thanks to Pythagoras, the components of a vector will always be less than the magnitude of that vector. Put simply, the speed as measured by the GPS will be less than the physical speed of the car when traveling in any direction other than parallel to the earth's spherical surface -- or at least that's how I understand the system.


/Nerdism

[Heide264]

Keep in mind that a standard GPS will also read only lateral motion (motion tangential to the radius of the earth). The implications are as follows: If you are driving up hill at 50 MPH, your tangential motion (as read by the GPS) will be the horizontal component of the true velocity vector. The vertical component of your true velocity -- the component of your velocity that increases elevation -- will not be registered by the GPS due to the typical software calculation (uses Lat-Lon differential rather than x, y, z position deviation). Thanks to Pythagoras, the components of a vector will always be less than the magnitude of that vector. Put simply, the speed as measured by the GPS will be less than the physical speed of the car when traveling in any direction other than parallel to the earth's spherical surface -- or at least that's how I understand the system.

True in concept, but I don't feel like working out the math. I doubt it is a difference over 1mph. Also, even on flat roads (we have a couple around here... er, I noticed it in Erie lol) I've noticed it being pretty consistent.

Interesting thought though.

You know they are using barometric pressure sensors to back out altitude now also? I doubt they use it to compensate for speed yet, however. I tried to use one of these sensors in a senior project. I will never use another component without easy to use source code.

[zax]

I tried to use one of these sensors in a senior project. I will never use another component without easy to use source code.

Did you use the SirfStar chipset? Most embedded systems now offer easy integration with mentioned chipset.

[zax]

True in concept, but I don't feel like working out the math. I doubt it is a difference over 1mph.

Your intuition serves you well. A 20% grade will cause a deviation in a car traveling 50 MPH by 1.00375 MPH.

[Rambo]

Keep in mind that a standard GPS will also read only lateral motion (motion tangential to the radius of the earth). The implications are as follows: If you are driving up hill at 50 MPH, your tangential motion (as read by the GPS) will be the horizontal component of the true velocity vector. The vertical component of your true velocity -- the component of your velocity that increases elevation -- will not be registered by the GPS due to the typical software calculation (uses Lat-Lon differential rather than x, y, z position deviation). Thanks to Pythagoras, the components of a vector will always be less than the magnitude of that vector. Put simply, the speed as measured by the GPS will be less than the physical speed of the car when traveling in any direction other than parallel to the earth's spherical surface -- or at least that's how I understand the system.

Hmm... never thought of that, but I guess you're right. I never use my GPS speed anyway. On stock wheel and tire sizes, I'll take whatever the speedo gives me.

If your cluster needs the ODO reset to the proper mileage, I did find that company: Electronic Speedometer Service

[RayfieldsWRX]

Wow, nice math deviation..all true, but not worth worrying about in this area. Lots of rolling hills...figuring out the math of how accurate your GPS actually is would make you nuts.

[monchis]

Not to thread-jack here, but if I installed an 05 STI cluster on my 05, it would be a direct bolt-on?