How do you figure out Speedometer calibration value? - Mustang Evolution

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Old 02-22-2010, 11:27 AM   #1
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How do you figure out Speedometer calibration value?

I've got a 2000 V6 with a T5 transmission. I've replaced the rear axle with a 8.8" with 3.73:1 ratio gears. The dyno shop that flashed my computer can't get the speedo right. They said that they needed to figure out what value to enter based on tire size (Mine are stock 17's). This sounds like BS to me. Does anyone out there that flashes PCM's know what value they need to make the speedometer right. They've tried twice now, and it's still off. 28mphs reads as 35mph on the speedo (we have a speedlimit sign with a radar display on it in my town.)

Thanks
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Old 02-22-2010, 01:10 PM   #2
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Re: How do you figure out Speedometer calibration value?

maybe try a different place the dyno shop should know what to do.
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Old 02-22-2010, 01:51 PM   #3
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Re: How do you figure out Speedometer calibration value?

on an SCT you enter revolutions per mile, this takes tire size into account. This is only of course changing the tires, not the rear gears... but you can also calculate extra revolutions per mile that gear ratio changes give you... same basic idea.

I would assume you need to figure out revolutions per miles of your car stock vs 3.73 then figure out what tire size vs stock would have the same effect and plug that in

by same effect I mean not considering rear gears... assume they didnt change but the tires did and figure out what "new" size would offset the revolutions per miles so it all equals out aka fool the computer into thinking the extra revolutions are your wheels not your gears~

3.73 are 14 percent more than 3.27

so you need a tire that mimics this difference (a tire thats 14% taller than stock)

if you have 245/45/17s the height is 17in + tire sidewall

sidewall is 245 * .45 = 110.25mm * 2 (top/bottom of tire) = 220.5mm

17in = 17 * 25.4mm = 431.8mm

So your tire is 431.8mm + 220.5mm tall or 652.3mm tall which is 25.68 inches tall

whats 14% more than 25.68? 29.28in tall is what your new tire should be to slow down the speedo enough to make it accurate~ 14% slower is what you are after.

29.28 = 743.71mm - rim which cant change (743.71mm - 431.8mm) = 311.91 / 2 (top and bottom sidewall)

311.91 / 2 = 155.96mm sidewall tire is what you want, width does not matter.

350/45/17 tire~ try that and see how it works.


I hate messing with percentages so I may be off, but its worth a shot~ just trying to slow down the speedo 14% is what we are after here.
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Old 02-22-2010, 03:37 PM   #4
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Re: How do you figure out Speedometer calibration value?

Yeah, that's what they are trying to do, but it didn't work. The Revs per Mile thing. I don't think they got their math right. I'll figure out what the revolutions per mile of 355-45-17's are and go from there. (I don't think they make anything in an even width.)
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Old 02-22-2010, 04:17 PM   #5
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Re: How do you figure out Speedometer calibration value?

OK, wait. If I have to mimic a tires revs per mile to get the speedo right then the number of revs per mile has to be higher. A smaller tire would go around more times per mile. According to BFG a 245/45/17 goes around 810/mile.

If there is a 14% difference between 3.73's and the stock 3.27's that means that the tail shaft of the the trans is going to spin 14% faster. The same way as it would if you had smaller tires.

So then instead of the 810/mile of the stock set up, the number should be 14% HIGHER right? So 924. 810 + (810*.14)=923.4

Does that sound right? The number they are using according to their SCT software is 813. (what they say is the stock rpm of the tires) The extra 3 is probably just a rounding difference. I don't think they're taking into account the gear difference when figuring the value.

Thanks again. Let me know if you think my math is whacked though. I suck at percentage stuff too.
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Old 02-22-2010, 09:51 PM   #6
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Re: How do you figure out Speedometer calibration value?

the taller the tire the less revolutions per mile. The tail shaft is turning the same, its the rear gears that are increasing the rear tires turning more. At 2000 rpms the tailshaft is moving the same now as it was before gears, it now just takes more rpm to make you go XX mph now.

adding a taller tire just acts to slow the speedo back down~ thats all so you are trying to remove the 14% faster turning rear end/tires.

what revolution per mile did they enter, what should be there, what are they using to modify this info? my friends sct you can just go right to it and bam your enter it in and your done.
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Old 02-22-2010, 10:45 PM   #7
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Re: How do you figure out Speedometer calibration value?

They are using 813. Which is obviously wrong.

They are using a Sniper tuner to flash the PCM.

So ok, this is making sense. I need the speedo to read slower so I need a value 14% LESS than the 813... So 699. gotcha.

I was thinking backwards. Taller tires and shorter gears both make the tires go around slower at a set RPM... I was trying to figure it using the idea that shorter gears increase the RPM for a set speed. Your way is much easier to get my head around.

Thanks for the help.

Again.
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Old 02-23-2010, 08:15 AM   #8
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Re: How do you figure out Speedometer calibration value?

a smaller tire will speed up the speedometer to slow it back down you would need a taller tire as far as I know.
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Old 02-23-2010, 01:02 PM   #9
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Re: How do you figure out Speedometer calibration value?

Wait, but if you tell the computer that the tire is taller, but it isn't, that would increase the reading on the speedometer at a set RPM. I need to decrease the reading on the speedo. I need to tell the car that it has smaller tires, since I can't tell it that it has "smaller" gears. So I need to input a higher revolutions per mile number. On that will then reduce the speedo by the 14%. 810*1.14 = aprox 924.

A taller tire will cause the speedo to read lower than the car is actually traveling since it is covering a larger distance per revolution.

My speedo is reading Higher than I am actually going. If I tell it I have taller tires but don't actually change them, then the speedo will read even higher.
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Old 02-23-2010, 04:38 PM   #10
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Re: How do you figure out Speedometer calibration value?

If you have a taller tire the tire takes longer to turn per revolution, this slows down the speedo. Thats why a taller tire will slow down the speedo, and can slow it down 14% to get you back accurate. At least thats how I am thinking about it.
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Old 02-23-2010, 05:03 PM   #11
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Re: How do you figure out Speedometer calibration value?

Another Solution: find out where the local police department gets their cars speedometers calibrated, or checked. I know in Georgia they have to get them checked and re-calibrated every so often. Most of the time they use independent shops.
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Old 02-24-2010, 09:41 AM   #12
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Re: How do you figure out Speedometer calibration value?

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Originally Posted by SpectorV View Post
If you have a taller tire the tire takes longer to turn per revolution, this slows down the speedo. Thats why a taller tire will slow down the speedo, and can slow it down 14% to get you back accurate. At least thats how I am thinking about it.

A taller tire turns at the same speed at X RPM, but covers more distance per revolution. That would increase the speed of the car... So if I tell the car that I have a larger diameter tire, the car will believe that I am going faster at X RPM, won't it?

To my way of thinking I need to tell the car that the tires are smaller than they are, so the car thinks I'm covering less distance, thus slowing the speedo down.
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Old 02-24-2010, 09:42 AM   #13
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Re: How do you figure out Speedometer calibration value?

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Originally Posted by 99ford View Post
Another Solution: find out where the local police department gets their cars speedometers calibrated, or checked. I know in Georgia they have to get them checked and re-calibrated every so often. Most of the time they use independent shops.
Mostly this is a principle thing. I paid for a tune that included the speedo calibration, and damnit they're going to get it right, even if I have to do all the math on it.

Call me stubborn
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Old 02-24-2010, 09:45 AM   #14
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Re: How do you figure out Speedometer calibration value?

yea dont let them screw you... its possible to fix they just need to figure it out
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Old 02-25-2010, 04:09 PM   #15
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Re: How do you figure out Speedometer calibration value?

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Originally Posted by Horse View Post
Mostly this is a principle thing. I paid for a tune that included the speedo calibration, and damnit they're going to get it right, even if I have to do all the math on it.

Call me stubborn
Good for you! They may eventually get sick of messing with it and you. They may just take it to a place, or have someone who knows the issue fix it. Stick to it!! They will get it right/figure it out the 11th time you go in.
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Old 02-25-2010, 04:31 PM   #16
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Re: How do you figure out Speedometer calibration value?

Ok, I sat down with my wife and her dad (former laser physicist for the Air Force) and a ton of scratch paper and wrote out the equations for figuring out MPH based on 2000 RPM of the drive shaft. After much scribbling I've figured out that if I multiply by the ratio of 3.73:1 and use 696 as the revs per mile of the tire (a taller tire, Spec, I salute you, you were right) I get the same MPH as I do if I use the 3.27:1 gears and 810 revs per mile on the tires. SO final answer: Use a taller tire revs-per-mile to fix it.

That is unless I enter that number and I'm WAY off, then I'll post here in case this helps someone else someday.
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Old 02-26-2010, 10:10 AM   #17
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Re: How do you figure out Speedometer calibration value?

yea you have to slow down and a taller tire slows it down, you are on the right path just got to find the right number to plug in. Taking the 3.73 turns per 1 rev vs 3.27 then the tire into account is a great way to do it as well.
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