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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Well... any mods? What's the tire size, rear differential ratio, and teeth on the speedometer gear? There are a number of things that will skew the reading - non-stock tire sizes for example.
 

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Discussion Starter #3
Well... any mods? What's the tire size, rear differential ratio, and teeth on the speedometer gear? There are a number of things that will skew the reading - non-stock tire sizes for example.
No history, new to me. Tire size is slightly smaller, 65 series as opposed to 70, I believe. That would account for the ECU being slightly off, but the speedo showing 90, ECU registering ~ 82 and GPS showing 80. Thats a speedometer issue. Wondering if anyone has experience with this and a fix other than swapping the speedo?
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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I doubt it's a speedometer problem. We haven't ruled out the simple or small stuff yet. What's the exact tire size, e.g. 215/70R15? What RPM do you indicate at a GPS indicated speed, and what does the speedo report at those speeds? We can infer your rear gearing and speedo gear/tire size needs to get your speedo back where it needs to go.

Often the speedo is not "stuck" on the needle perfectly - factory tolerance was up to 5 MPH fast or 2 MPH slow as I recall - but since you note a different variation at different speeds, that points to a gearing/sizing mismatch.
 

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Discussion Starter #5
I'll get that exact info for you. Tire size: P225/60R15. Door plate shows a R15 should be 70 series; R16 should be 60 series. Doesn't the ECU control the speedo? And wouldn't tire size and gearing etc affect the ECU speed? Or are they independent of each other with different sensors?
Thanks for the help.
 

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No, the ECU does not control the speedo, the signal is sent directly from the speed sensor to both the speedometer and the computer. Your problem is probably a combination of the different size tires and the factory tolerances. Those tires should make the speedo read about 5% faster than you are actually going.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Tire diameter - which is what needs to stay close to factory size for the speedo to be accurate - is a function of rim diameter plus sidewall. Sidewall is calculated as a percentage of the treadwidth. So you need to know how wide, the sidewall ratio, and the diameter of the rim.

The stock sizes are 225/60R16 (26.6" diameter) or 215/70R15 (26.85") and 205/70R15 (26.3"). You have a 225/60R15 (25.6) - so you've lost an inch of diameter due to the rim size, meaning your speedo is only running at 96% of what it should be running at for a given RPM.

If you can read data from the PCM as implied in an earlier post, check the speed the PCM is reporting and compare it to what the speedometer displays. If they aren't the same, then they "should" remain consistently different from one another across the range (e.g. always 2 MPH fast). If the difference increases or decreases as speed changes, THEN I would say there may be an issue with the speedo.
 

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You guys want this tool.

This is from our Bros at SCCOA.

If your transmission is original, its a 7-tooth; later are 8-tooth.

Odds are your stock mn12 is 3.27 gear ratio.

You can get your tire info from your tires' page on tire rack. :)

I have a purple gear in the Tbird, and an orange gear in Lazarus.
 

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1997 Thunderbird 4.6, 1998 Mark VIII LSC
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Discussion Starter #10
Payed attention on the highway this morning.
RPM ~2.5K
Speedo ~ 87
GPS ~ 81
I'll have to pay closer attention to the difference between ECU & GPS speeds to see if it's consistent.
But if the speedo tolerance is 5% high & the tire size being 1" smaller would contribute 5%; that would account for the ~6 mph difference seen above.
-Here's the next question: What is the advantage of a different tire size? These were put on by the PO, they are practically brand new, otherwise I'd change them out.
 

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Scott,

The stock diameter size fills the wheelwell more appropriately for the size of the car, not to mention, gives it a smoother ride. I would just run those till their worn.

I bought my 95 with 215/65-15's on there. They looked too small for the car. They will make the speedo read a little higher, plus give you a falsely higher gas mileage reading.

Like you said, I would chalk up the speedo error on the tire size. Plus at those speeds, it should read ~2 MPH high for margin of error. Most speedos read on the optimistic side to keep you out of trouble.

Al
 
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