karman

I wonder about the tire size too.
I may re-calibrate mine.
What was your calculation for the rev/mi. ratio G.T?

1meanZ

The odometer reading to my buds in my Lightning, Silverado, and Hayabusa is 98 miles.

The Mustang is 103 miles.

hamstang

GPS and those roadside radar trailers confirm my speedometer reads about 2mph fast.

pitpup

OK, John. My thinking was this. When my car was stock, it was fine. The speedo was right. The odo was right. When I got my SCT Xcal2 I noticed it was off like 2/10ths every 4 miles. So thats about 250 feet every mile (a halft tenth). Check with your dealer as you do not have a tuner where you can set this. On my Xcal2, I set the revs per mile to 755. I have the stock 235/50/18's on the polished bullitts.

See the dealer. Mine was dead on from the factory...

G.T

well i'm running 285/40ZR18 rear tyres.
so my calculation is as follows

first, calculate the sidewall height
285mm*40%=114mm

Convert sidewall height from mm to inches
114mm/25.4=4.88189in

Overall tyre diameter = rim diameter + 2 sidewalls
(2*4.488189)+ 18 = 26.976378 inch

Overall circumference = pi * radius
3.141592 * 26.97637796 = 84.748791 inch

1 mile = 63,360 inch

Tyre rev/mile ration = 63360 / 84.748791 = 747.62

depending on your handheld tuner, you might be able to set 748 rev/mile, 745 or 750. and this should be as accurate as it gets.

NJJOE

So, if you are running a XCAL tune and the tuner did not know your tir size, you may be miscalibrated? I think that is what I am reading. If so, how can we check using the xcal-2?
Is the a chart to use? tire size = rev/mile?

As stated above 235/50 = 755 rev/mile
285/40 = 748 rev/mile.

I would assume there is a chart somewhere for this.

Would like to see a write-up on how to check and change the rev/mile using a xcal-2.

G.T

you can't check the current value with the XCalibrator, or even the Predator but i could be wrong.

the miscalibration isn't your tuners fault.. essentially it started with Ford not properly calibrating the ECU for the tire sizes they're shipping the vehicles with.

i haven't found a chart but there are javascript based calculators all over the net.. and if you got a calculator, you can do it yourself as per my example above.

finally, once you've got your tire revs/mile ratio figured, you can set that with your XCalibrator.

stock XCal-II units with preloaded calibration allow you modify optional parameters, tires/rev ratio is one of them

most custom tunes (notably those from Doug @ Bamachips) also allow you to do the same.

pitpup

Louie,

I calculated it by hand too. And I was close. (And prould of myself, too.) But I went to BFGoodrich's site and they, along with Falken and others, give you the roll out.

Here's the trick. They had the 235/50/18 KDW and I looked at that value. THEN I found the KDWS, remember we have the KDWS, not the KDW and it was actually a different value. Maybe 4-6 revs per mile, but it was different. I thought that was interesting.

I believe (it's been a while) BFG had '759' for the KDWS. I tried that. I tried 750. I tried 755. I ended up with 755 and it's just about right on. They also give you those numbers based on 45 mph. I did notice a slight change at 65. Think about it. They're spinning faster so they are ever so much taller. I used 755 because it was a good average and dead on for 50 mph when I tested on 3 separate 4 mile jaunts. Remember, I was off 2 tenths before I took things into my own hands.

By the way, here's the complete thread I wrote up on my website. It includes a link to the BFG tables:

http://www.grabberorange.net/forums/viewtopic.php?t=122



Stan

neil07gt

does anyone sandbag their revs per mile?

G.T

pitpup: any idea where my calculation went wrong?
my formula makes sense, but as you mentioned, the number is off...
i checked a few online calculators and you're right.. 285/40ZR18 is 771 Rev/mile, not 747.
however, i found another formula that says
Revs Per Mile = 20168/tire diameter (inches)

i ran that on my tire diameter, and got 747 rev/mile too

neil07gt

Revs per mile indicates the precise "rolling" size of the tire (physically measured). It is slightly different than the value you get from calculating the specs. That is why the value is sometimes different between two tires of the same specs. The difference is slight enough so that when accumulated over a mile of distance you can be a couple dozen off. Keep in mind the "rolling" size will change as the tire tread wears.

John SCB

Seems like this would be a minimal factor to consider but if this is true, would that explain why the speedo is off nearly 1/10th with the stock tires? They have about 30k on 'em.

neil07gt

It probably is minimal. I just wanted to mention it--to further distinguish the differences between the actual physical size versus one that is calculated strictly on tire sprecs.

You can probably find more info by Googling "tire rolling circumference" or "tire rolling diameter". I found this quote below from Continental Tire. It indicates a change of less than 3% requires no adjustment.

http://www.conti-online.com/generato...lus_sizing.pdf

John SCB

I just checked with AAA...they don't know when the speedo-calibration vehicle will be back in service in SoCal. Just curious if anyone out there has had their speedo/odo checked by a third party...not at the dealer?

vsop

In FL I opened up the yellow pages and found a local place that did speedo calibration on rollars. they were able to certify the speedo

theedge67

I personally am not that worried about being 1mph off on my speedo, especially since it seems that the tendency is to be a little high. Might save me a run in with the law some day, and save a bit of gas. The odometer worries me though. I already passed my 36K mark, and would be very upset if it turned out I only drove 33K miles or so.

Giddyup

I haven't got pulled over yet, so I assume that it's right...

DavidM

Quick question may seem dumb if so sorry.....
How do you change your rear axle ratio with the SCT x-cal2 I have 4.10's on order and am going to need to change them after the install have searched for an hour can't seem to find a good thread on this. Any info appreciated.

dave_bought_another_black_stang

my gps shows that I am right on!

pitpup

Hey Dave,

When you upload, you're asked if you want to make any custom changes (I forget the exact wording). You can set the rear end. You'll actually see a setting for 3.55, 3.73, and yes, you're beloved 4.10s. You'll also see a setting for REVS PER MILE (again, I forget the exact setting. I have the manual and can find the exact wording this weekend. It's late. I'm trying to help you from memory.) Check out the website for your tire manufacturer. It will give you the roll out. (revs per mile) If not, you can work off of the tire dimensions. For instance, the stock 235/50/18 is 27.25 inches in diameter. But I found the best way is to check the tire makers website. If you can't find it, give me the make and size of your tires, I'll see if I can help you out.

Stan