Speedometer Confusion

Rick Simons

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In an attempt to increase my speedometer's accuracy I've been experimenting with the tire revs/mile setting on my SCT (Bama) tuner. However, I'm getting different results than expected:
My tires are 26" tall, which should mean a circumference of 81.64 in. Divide this into the 63,360 inches in a mile and I get 776 tire revs/mile. But if I set the revs/mile to 776 in the tuner, the speedo reads WAY too high, as in 90 +/- at a calculated 70 mph.
Before I started, the tire revs/mile was set at 755, and the speedo was close, maybe 2-3 mph fast.
I have 3:73's and this has been changed in the tune by Lito already. Can it somehow think it still has 3:55's?
Or am I missing something obvious? (always a possibility with me)
 

Juice

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Lets just say there is more to calibrating the speedo than gear ratios and tire size. But yes, you have to set gear ratio first, then tire revs per mile.
I'm not a fan of how the X4 deals with this actually. You have "stock size" and the option to change it. But what is the stock value?? the x4 does not display that!
From what I recall, 2-3 mph difference only needs 10 or 15 revs per mile change. You can ignore the actual number of the revs per mile. So for 755 to be 3mph fast, try 765.
 

Sammy70

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I just went through the same thing with Lito about a month ago. Thought my speedo might be off after flashing in his tune (it was GPS verified spot on with the previous Bama tune). Lito had me take readings every 10mph of indicated speed on the speedo and GPS speed readings. My car is stock 17" wheels/tires with 4.10 gears (which of course he knew about). It took him 2-3 revisions to get the speedo to agree with the GPS readings and it's all good now.
 

GlassTop09

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In an attempt to increase my speedometer's accuracy I've been experimenting with the tire revs/mile setting on my SCT (Bama) tuner. However, I'm getting different results than expected:
My tires are 26" tall, which should mean a circumference of 81.64 in. Divide this into the 63,360 inches in a mile and I get 776 tire revs/mile. But if I set the revs/mile to 776 in the tuner, the speedo reads WAY too high, as in 90 +/- at a calculated 70 mph.
Before I started, the tire revs/mile was set at 755, and the speedo was close, maybe 2-3 mph fast.
I have 3:73's and this has been changed in the tune by Lito already. Can it somehow think it still has 3:55's?
Or am I missing something obvious? (always a possibility with me)
Using the 26" you gave, using (pi=3.142 x d = circ) comes out to 81.12" circ instead of the 81.64" circ you quoted, which when divided into 63,360"\mi comes out to 781 revs\mi by my calc's instead of the 776 revs\mi you quoted.

Hope this helps.

PS---Stock revs\mi is 747 (27" x 3.142 = 84.83"; 63,360" ^ 84.83").
FYI.........
 
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Laga

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Either measure the circumference, instead of figuring from diameter. Or, lookup the revs per mile from the spec sheet of the tire.
 

Juice

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The math never worked exactly for me. Got me close, but not exact. Had to tweak the revs/mile, even with the bone stock v6 setup. It was still off enough to bug me.<2mph.
 

Rick Simons

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Lets just say there is more to calibrating the speedo than gear ratios and tire size. But yes, you have to set gear ratio first, then tire revs per mile.
I'm not a fan of how the X4 deals with this actually. You have "stock size" and the option to change it. But what is the stock value?? the x4 does not display that!
From what I recall, 2-3 mph difference only needs 10 or 15 revs per mile change. You can ignore the actual number of the revs per mile. So for 755 to be 3mph fast, try 765.
One thing I have learned so far is that the higher the revs/mile number I put in, the larger the error (on the high side)
Using the 26" you gave, using (pi=3.142 x d = circ) comes out to 81.12" circ instead of the 81.64" circ you quoted, which when divided into 63,360"\mi comes out to 781 revs\mi by my calc's instead of the 776 revs\mi you quoted.

Hope this helps.

PS---Stock revs\mi is 747 (27" x 3.142 = 84.83"; 63,360" ^ 84.83").
FYI.........
I still get 81.64 on my calculator......
But it doesn't matter a whole lot now- last night on my way home from work the speedo was reading almost dead-on, without me changing anything from when I adjusted the revs/mile yesterday morning and got it 15-20% off. Yet another mystery. In any case, I'm done screwing around with it for now. Thanks for all the input!
 

Mach2burnout

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I just went through the same thing with Lito about a month ago. Thought my speedo might be off after flashing in his tune (it was GPS verified spot on with the previous Bama tune). Lito had me take readings every 10mph of indicated speed on the speedo and GPS speed readings. My car is stock 17" wheels/tires with 4.10 gears (which of course he knew about). It took him 2-3 revisions to get the speedo to agree with the GPS readings and it's all good now.

I’ve had the same issue ever since getting my tune from Lito. I have lost his contact info. How are you contacting him these days?


Sent from my iPhone using Tapatalk
 

GlassTop09

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The math never worked exactly for me. Got me close, but not exact. Had to tweak the revs/mile, even with the bone stock v6 setup. It was still off enough to bug me.<2mph.
You're right, the math will never work out exactly cause no one never accounts for true radius length (the axle speed rotation will only correspond accurately to the actual true radius length....center of axle to ground in vertical plane)....simply cause they can't, so the only way to get as close as you can using the math is to accurately measure the true radius length of your car's rear wheels from axle center to ground contact point then apply the math....but even doing this, what are you gonna use as a benchmark? Any change in tire inflation pressure will also affect the true radius length as well as any load changes over the rear axles so if the math can get you within 2-3 MPH of whatever benchmark you use (most use GPS thru their nav system....if they have 1 installed or thru their phone) to compare the results I'd call it good.....

But that's just me................

Rule of thumb.....lower revs\mi corresponds to a larger dia\circ; higher revs\mi corresponds to a smaller dia\circ.
One thing I have learned so far is that the higher the revs/mile number I put in, the larger the error (on the high side)

I still get 81.64 on my calculator......
But it doesn't matter a whole lot now- last night on my way home from work the speedo was reading almost dead-on, without me changing anything from when I adjusted the revs/mile yesterday morning and got it 15-20% off. Yet another mystery. In any case, I'm done screwing around with it for now. Thanks for all the input!
Curious....what was the ambient air temp when you drove home?

I ask this cause if we want the math to be as close to accurate we really need to measure the actual loaded true radius length of the rear wheels (from center of axle to tire ground contact patch in vertical plane w\ full normal weight over the rear axles & normal running tire pressure....also to use an air temp correction for a std such as 60*F since air expansion\pressure is affected by temp but this is getting nit picky) to then determine the true running tire dia THEN use the already known formulas to get the true revs\mi setting cause this is what the PCM really cares about (what sets the ABS tone ring rotational counts pattern).

Otherwise I'd just find an area of acceptance & be happy. I've found that my calc's have consistently fallen within 2-3 MPH lower of actual GPS calc'd MPH which is perfect for my liking as w\ this info I have a fighting chance of disputing an errant radar check\speeding ticket as long as I have my CC set for the current speed limit using my car's speedometer.

YMMV.
 

Rick Simons

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I ask this cause if we want the math to be as close to accurate we really need to measure the actual loaded true radius length of the rear wheels (from center of axle to tire ground contact patch in vertical plane w\ full normal weight over the rear axles & normal running tire pressure....also to use an air temp correction for a std such as 60*F since air expansion\pressure is affected by temp but this is getting nit picky) to then determine the true running tire dia THEN use the already known formulas to get the true revs\mi setting cause this is what the PCM really cares about (what sets the ABS tone ring rotational counts pattern).

Otherwise I'd just find an area of acceptance & be happy. I've found that my calc's have consistently fallen within 2-3 MPH lower of actual GPS calc'd MPH which is perfect for my liking as w\ this info I have a fighting chance of disputing an errant radar check\speeding ticket as long as I have my CC set for the current speed limit using my car's speedometer.
More good info; thanks!
This could easily get a lot more science-y (yeah, I just made that word up), but FWIW I did the math again before leaving work yesterday, and came up with 67 mph in 5th @ 2000 rpm w/26" tire diameter and tire revs/mile @ 755. Actual observed (speedo) mph was 68 +/- @ 2000 rpm. Close enough for me.
 

Juice

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One thing noone mentioned, is the speedo/cluster itself accurate? If the pcm is sending 60mph, could the speedo be displaying say, 63 mph? That will affect the math.
 
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Norm Peterson

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In an attempt to increase my speedometer's accuracy I've been experimenting with the tire revs/mile setting on my SCT (Bama) tuner. However, I'm getting different results than expected:
My tires are 26" tall, which should mean a circumference of 81.64 in. Divide this into the 63,360 inches in a mile and I get 776 tire revs/mile. But if I set the revs/mile to 776 in the tuner, the speedo reads WAY too high, as in 90 +/- at a calculated 70 mph.
Before I started, the tire revs/mile was set at 755, and the speedo was close, maybe 2-3 mph fast.
I have 3:73's and this has been changed in the tune by Lito already. Can it somehow think it still has 3:55's?
Or am I missing something obvious? (always a possibility with me)
You're missing something in your math, but it's not an obvious something.

Your math is correct, but only for a completely rigid wheel (like on railroad cars with no tire).

Tires, being of rather flexible composite construction, distort under load. One of these distortions is the tread rubber "compressing" in the circumferential direction where the tread passes through the contact patch. Basically, this makes your theoretical 81.62" circumference tire use up somewhat less road distance to actually turn one full revolution.

For most tires, a 3% downward correction on circumference (or a 3% upward correction on the resulting revs per mile) is appropriate. Though for some tires it can be somewhat less or slightly more. 26" tall tires are up there in the 799 - 800 revs/mile range.


Something else is going on if you're reading 90 at anything close to a calculated 70, and I haven't read through the whole thread to see if anything else has been suggested.


Norm
 

Norm Peterson

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The math never worked exactly for me. Got me close, but not exact. Had to tweak the revs/mile, even with the bone stock v6 setup. It was still off enough to bug me.<2mph.
It's because of the tire distortion effects.

That none of the online tire calculators I've ever seen know about either. It's really an SAE level topic, well beyond the simple math you learned in school.

Try calculating the revs per mile from the diameter of any tire in Tire Rack's specs pages, apply the 3% upward correction factor to your answer, and compare that to the listed revs per mile.


Norm
 

Juice

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Math is work! lol. IIRC, the x4 changes 5 revs/mile/click. One click is just a bit less than 1mph change in speedo. But that only helps if the tune options are unlocked.
 

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