Speedo correction - Ford Mustang Forum
 
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post #1 of 9 (permalink) Old 01-23-2013 Thread Starter
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Speedo correction

3.73s installed w/ 7 tooth drive and 21 tooth speedo gear.

I'm still reading 5+ MPH more than I should. I'm guessing because of my rear tires. Nitto 275/40r17

Wondering if another has used Ford Racings recalibration tool/module?
I've seen good and bad feedback on it and just wanted to see if it's worth the install.

Maybe question is, will the module get me 100% accuate if done correctly?
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post #2 of 9 (permalink) Old 01-24-2013
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Something's not right. 275/40-17 is almost exactly the same diameter as the OEM size. Are you absolutely sure you have a 7-tooth drive gear?

Fyi, if it's something to do with the calibration the speedometer will be off my a percentage, not a set value. Where id you get the 5 MPH figure from?

'94 Laser red 'vert. Still a 5-oh... for now
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post #3 of 9 (permalink) Old 01-24-2013 Thread Starter
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Quote:
Originally Posted by YLDPONY View Post
Something's not right. 275/40-17 is almost exactly the same diameter as the OEM size. Are you absolutely sure you have a 7-tooth drive gear?

Fyi, if it's something to do with the calibration the speedometer will be off my a percentage, not a set value. Where id you get the 5 MPH figure from?

I picked up an inspection scope (camera w/ built in LEDs) from Fry's so I could look up tailhousing without removing the transmission. Yellow drive gear, counted 7 twice.

Did calculations off these two sites:
http://www.fordracingparts.com/download/charts/129.pdf
&
Gear Ratio Calc


On the second site, I looked up what 2000 RPMs in 3rd and 4th would be and it said I was 5 off. [when I'm going 40mph it shows 45]

Then I had my father drive side by side and came up with the same results, 5mi off.



My drive gear teeth was reusable and speedo is new. Only thing I can think of is the sensor is going out or the tire diameter is messing me up.
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post #4 of 9 (permalink) Old 01-24-2013
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Like I said, it definitely is not the tire diameter. You can compare your size to stock size (245/45-17 or 225/55-16) here: Tire Calculator With either size compared to your size, the error (revs per mile) is less than 1%.

With your tire size and gear ratio the 7/21 tooth speedometer gear combo should get you within 3% of dead accurate (and actually, it any error should be reading slow, not fast). The chart you saw is correct for what you needed, but still you might find this useful at some point: Speedometer Gears

Okay, so consider this: Say you are actually going 40 MPH when the speedometer says 45 MPH, and I have no reason to doubt this is accurate because your method is a solid one, imho. That error equates a 12.5% error (simple calculation, but you can see it here). Now, the error produced if the drive gear was an 8-tooth and not a 7-tooth would be exactly 12.5%...

All 94-95 T5s came with an 8-tooth drive gear, no exceptions. I really think you do in fact have an 8-tooth drive gear. The stock 8-tooth drive gear is a pale yellow too (from the T5s I've had apart). Why would this have been ever changed in your trans?

P.S., I actually have the exact same setup you have, 275/40-17 tires, 3.73:1 rear gears, and a T5. I am using the 7/21 gear combo and it's almost 100% accurate.

'94 Laser red 'vert. Still a 5-oh... for now
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post #5 of 9 (permalink) Old 01-24-2013
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search my threads, i have 3.73s and a 255/35/18 rear tire and the speed re-cal and i'm 4 mph off and can't get it any tighter, the reason?

i called brother's customer service and they say that any gear ratio over 3.55 you won't be able to get the speedo dead on! 3.55 gears and under you can get dead on with your speedo.

1995 Mustang Cobra, 5.0 V8, T5 transmission

*currently under restoration*

could you repeat all of that, I wasn't listening???
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post #6 of 9 (permalink) Old 01-24-2013
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Quote:
Originally Posted by PANTYEATR View Post
search my threads, i have 3.73s and a 255/35/18 rear tire and the speed re-cal and i'm 4 mph off and can't get it any tighter, the reason?

i called brother's customer service and they say that any gear ratio over 3.55 you won't be able to get the speedo dead on! 3.55 gears and under you can get dead on with your speedo.
The reason is you took the easy way and tried to correct it electronically, not by actually installing the right speedometer gears.

It seems to me that you should be able to get it closer than that by learning the actual function of the unit, but I have no direct experience with that exact unit. I do know that by installing the correct gear combo you can get a lot closer, if not dead on.

And, once again, if your speedometer is off by 4 MPH the needle has been moved. Calibration error will show up as a percent.

'94 Laser red 'vert. Still a 5-oh... for now
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post #7 of 9 (permalink) Old 01-24-2013
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As YLDPONY said, it came with an 8-tooth internal gear. The only way it would be a 7-tooth would be if someone serviced the transmission and swapped it.

You can't count the teeth because they are spiral. You can count the teeth if you can see the edge of the gear where each spiral tooth starts, but that edge can't be seen with the transmission closed.

The 7-t yellow gears are BRIGHT yellow. I call it "Crayola Yellow."
The 8-t gears are supposed to be light green, but some are a cream/ivory color and others seem to be pale yellow.

I would think a Dallas Speed-Cal would be able to bring it into range. You might call their support number and ask.

Stan -
1965 5.0 Coupe /MAF/EFI/AOD/B303 Roller Cam

Mustang Manual Transmission Specialist
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post #8 of 9 (permalink) Old 01-24-2013
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Keep in mind speedometers have an error tolerance of around 2%. Add in the 1% difference from the tires. A couple of pounds of tire pressure can throw it off even more (these are all additive). I think you're only noticing the difference because you're recalibrating and want it perfect. Even my newest car without any gear work and stock tires is off a few miles from the gps. But yeah, if it's a constant, perhaps someone pulled a needle for some whiteface gauges or something and missed the mark reinstalling. I'd just quit worrying about it, you're close enough. If you happen to see a cop adjust for the known difference.
post #9 of 9 (permalink) Old 01-24-2013
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Quote:
Originally Posted by YLDPONY View Post
The reason is you took the easy way and tried to correct it electronically, not by actually installing the right speedometer gears.

It seems to me that you should be able to get it closer than that by learning the actual function of the unit, but I have no direct experience with that exact unit. I do know that by installing the correct gear combo you can get a lot closer, if not dead on.

And, once again, if your speedometer is off by 4 MPH the needle has been moved. Calibration error will show up as a percent.
i called bothers tech line and they said to me that the brospeed calibrator cannot get dead on accurate with any rear end gear sets higher than 3.55 are you saying that they are wrong?

either way, i'm gonna eventually go to 3.55 or 3.31 anyhow.

1995 Mustang Cobra, 5.0 V8, T5 transmission

*currently under restoration*

could you repeat all of that, I wasn't listening???
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