Is your odo/speedometer off now that you have purchased the SCT X-Calibrator 2?

[pitpup]

Is your odometer/speedometer off now that you have purchased the SCT X-Calibrator 2 for your Stang? If so, an easy fix. This is long, but worth the read.

 

1. When I got my Stang, I was too busy having fun with it to take it out to the interstate and see if the odometer consistently clicked off exactly one mile (or something very close) with every mile marker I passed. If anyone has a stock Stang, I'd be interested in hearing how they clock from the factory.

 

2. Enter the SCT tuner. When I got it, I uploaded the tunes and again, was having way too much fun to think about odometer/speedometer accuracy. The car just rocked. So I continued to enjoy it.

 

3. After reading the manual on the SCT-2, I learned that in addition to being able to program in the rear gear, you could also adjust for tire diameter. Our 235/50/18's are 27.25 inches tall when inflated to 32psi.

 

4. Because I'm a math freak, I decided to throw this adjustment in too, BEFORE CHECKING THE ODOMETER ACCURACY FOR THE STOCK SETTING. So! I found out from the Falken Tire website that our tires roll 740 revolutions per mile. TO CHECK THAT (I'm anal, remember), I did it the old fashioned way. 27.25 inches x 3.14 = 85.6592 inches. This is how far the tire travels in 1 revolution. 5280 feet x 12 = 63,360 inches per mile. Ya with me so far? 63,360 divided by 85.6592 = 739.675 revolutions in a mile.

 

5. So, math boy plugs in '740' on the SCT and does not give it another thought. Meaning, I never checked it!

 

6. Anyhoo, I was on my way to Maryland on Tuesday, 12/26. Mile markers out the wazoo, but did I even think about checking the odo? No. But here's how I found the snafu. I put my cruise control on 63 miles per hour and at the next mile marker, I click 'start' on my watch's chrono. I'm anticipating covering a mile in 57 seconds or there about. 60 mph is 60 seconds, a mile a minute remember. But what do I find? I cover the mile in exactly 60 seconds. NOT 57 seconds. So I realize I'm off by 3 mph!

 

7. So I reset the odometer at the next mile marker and it's game on! Well don't you know that as each mile clicked off on the car, the mile marker was appearing just a little bit further and further out front with each passing mile. After going 4 miles, the Stang was clocking 4.1 miles, give or take. I tested this over 3 separate 4 mile runs and was consistently overclocking.

 

8. I realize at this point that '740' in #4 above is not correct. My tires have only 7000 miles and are fully inflated, so it's not like I've worn them down. I get back from Maryland and immediately grab the SCT-2 and head out to Interstate 295. The tuner lets you bump the value for revolutions per mile by increments of 5. So I had to dial in either 745 or 750 or 755. Some value more than 740 to tell the 'puter that I had to go further down the road before it registered 1 mile. That value turned out to be '750'. I made two checks on I-295. Both times, as the mile marker came up, WldTang would click off a mile on the odo. It was dead on.

 

9. I then put the cruise control on 60 mph and hit the chrono at the next mile marker. When the following mile marker came up, my watch showed 60 seconds. Not 1:03 or anything else. I did this for 3 separate checks. All 60 seconds. But the story continues!

 

10. So now I'm curious as to what the 'stock' value is on the SCT-2. This is the value that is downloaded to your car if you don't play with this setting. It does not give you a value. It just says 'stock'. So I hit I-295 again and this time, instead of overclocking (4.1 miles clocked for a true 4.0 miles) IT UNDERCLOCKED! It registered 3.8 miles for a true 4.0 miles. Two tenths the other way. While this would understate your mileage, IT WILL BE OFF FOR THE SPEEDOMETER. This could get you in big trouble. Say you're on a highway with a 65 mph speed limit. You set your cruise for 74 mph, or 9 over. You would actually be moving somewhere around 79-81 mph (approximately). So now you're 15 over the limit when you think you're 9.

 

In closing, I don't feel this is a 'Brenspeed vs. Bamachips vs. YOUR TUNER' thing. It's just something that you should be aware of. I'm not aware that the custom tuners even touch the 'stock' value from SCT, so SCT is off here. And for those of you with 17 inch wheels, the 235/55/17's have a height of 27.17 and travel 742.670 revolutions in one mile. This is very close to the stock 18's with 739.675 revs.

 

Forgive me for being this anal, but when my car is telling me it's going 70 mph, I want it to be humming along at 70 mph. Not 67. Not 73. But 70! And I want the mileage to be dead on. Not that I'm worried about resale, but when I take a trip to Maryland, I want to run off 155 miles, not 159.

 

To wrap it up, the stock value (unknown) is garbage and the calculated value of 740 is garbage. The value that works for me is 750. And even this may vary as the tires wear, so I'm going to make a point to check it once a month.

[RUFDRAFT]

Whew!

 

Great post - And I SURE DO undertand all the math..... :happy feet:

 

Good to have you back, bro. How's the new site coming along?

[kraegar]

I've been meaning to check mine. I plan on taking two GPS units with me (a tom-tom, and a handheld garmin, so I'm double checking my results), and measuring speed & the odometer on the highway. Haven't gotten to it yet, but that alone might be a good reason to purchase the SCT tuner.

 

- Tony

[yamahar1]

Very precise dude. I am thinking of enlarging the wheels, but I guess with the smaller diameter tires, it will be close to the same overall diameter anyway. I am going to get the predator from diablo, I wonder if they have the same capability of changing tire/wheel diameters also? I used to be good at math like 15 years ago, I might need your help when I get the new wheels and tires. I agree, it sucks if you set the cruise for like 77, and it is acutually running over 80, surefire way to get a ticket in Michigan or Ohio for sure. Thanks for the in depth coverage, very informative.

[RUFDRAFT]

Ask a trooper to clock you.

 

I'm not good at running software while driving!

[pitpup]

Whew!

 

Great post - And I SURE DO undertand all the math..... :happy feet:

 

Good to have you back, bro. How's the new site coming along?

 

 

Hey Ruf, new site is coming great. Welt is on the way... enuff for all!

 

Back to the tire thing, I found out about 1010tires.com from modularfords.com and when I plug in 235/50x18 there, it calculates 763 revs per mile. I'm using 750. Originally I had 740 so we can see the 750 is a step in the right direction. I wonder how precise their website is. I'll have to make another 4-5 mile run on the Interstate and double check the odo. Bottom line, if I'm off, it's not by much. But I still want to know.

[yamahar1]

Hey Ruf, new site is coming great. Welt is on the way... enuff for all!

 

Back to the tire thing, I found out about 1010tires.com from modularfords.com and when I plug in 235/50x18 there, it calculates 763 revs per mile. I'm using 750. Originally I had 740 so we can see the 750 is a step in the right direction. I wonder how precise their website is. I'll have to make another 4-5 mile run on the Interstate and double check the odo. Bottom line, if I'm off, it's not by much. But I still want to know.

 

Hey, I think I would be interested in that welt pitup. I e-mailed that Mike guy, and he never even bothered to e-mail me back, so scew him I guess he is too good for us Mustang crowds. I like the look that matches the interior upgrade. I think it was silver tinsel or something. Anyway, I went to that 1010tires.com site, how did you find the calculation for revs per mile, I could not find it on the site after searching around for a while. I don't have a lot of patience, if I cannot find something in a few minutes, I lose interest. Anyway, good to know the # is like 750-760 revs per mile. Looks like the Diablo Predator can do the changes for tire sizes also, that is probably my future tuner, but if I get the 18X10's with thinner tires, and match the same overall height I will not have to change it anyway, right? I am not too anal about the speedometer thing though, all the motorcycles have an error also of anywhere from 5-10%. No biggie, I think I am like Ruff on the bike, if they start rolling after me, they will have to call the air patrol. Later.

[pitpup]

Hey, I think I would be interested in that welt pitup.

 

 

I'll have over 2000 feet of silver tinsel by next week. I'll hook you up. As for the 1010times.com thing, there is a link at the VERY bottom. Here's a direct link:

 

http://1010tires.com/TireSizeCalculator.asp

 

NOW! That said, BF Goodrich has a table I found last night. We run the 235/50x18 KWDS tires and here is a link to that. It will show you the revs per mile are 759. Wish I'd found this before I punched in 740 from my manual calculations. Just about every time manufacturer has this kind of data. Here's the BFG site:

 

http://www.bfgoodrichtires.com/specs/g-for...-a-kdws/40.html

 

Interesting it shows the revs at 45mph. Wonder what they are at 65mph. (Don't you think the tire would expand as centrifugal force would pull the tire outward... stretch it) Perhaps it has one rev per mile at 45 and another at 65. Man, my head hurts just thinking about it.

[dtharmon633]

Very precise dude. I am thinking of enlarging the wheels, but I guess with the smaller diameter tires, it will be close to the same overall diameter anyway. I am going to get the predator from diablo, I wonder if they have the same capability of changing tire/wheel diameters also? I used to be good at math like 15 years ago, I might need your help when I get the new wheels and tires. I agree, it sucks if you set the cruise for like 77, and it is acutually running over 80, surefire way to get a ticket in Michigan or Ohio for sure. Thanks for the in depth coverage, very informative.

 

 

This site might come in handy for you.

 

http://www.miata.net/garage/tirecalc.html

[pitpup]

This site might come in handy for you.

 

http://www.miata.net/garage/tirecalc.html

 

 

Wow, these calculators should be taken with a grain of salt. It came up with the same number I got when I manually calculated: 740. I'm using 750 after observing a 4 mile run. TWICE. The BFG site says 759, which should be gospel.

 

But thank you for the link. At least I know my initial 740 was not pulled out of my fanny!!!! LOL.

[kraegar]

I'm going through this fun with pirelli 235/55zt17's now. Pirelli's website says 765, I figure 741 by hand. Some online sites says 741, 1010 says 765.

 

I called pirelli's tech support to ask about it. They wouldn't even discuss the information. Fun.

 

Time to get out the GPS & stopwatch.

 

- Tony

[pitpup]

I'm going through this fun with pirelli 235/55zt17's now. Pirelli's website says 765, I figure 741 by hand.

 

I would go with the website figure of 765 as a baseline. I've had the best luck with those numbers (website). On a similar note, I've adjusted my revs from 750 to 755. It's damn near perfect. Note the BFG website sez '759' for the stock KDWS tires. My SCT goes in 5 rev increments. Tried 760, but for me, 755 with tires inflated to 32 psi at the time of the test was the way to go.

 

I figured by hand too, but it's off for whatever reason. Go with website figures...

[kraegar]

Grr, don't know why I kept saying #'s in the 200's instead of the 700's. fixed my posts.

 

- Tony

[Five Oh B]

I have the 235/50/18's on my 07 GT, and set the Diablosport Predator tuner to 755 revs per mile and it is dead on per the mile markers on I-5. I, too, started at 740 after doing the math (20168/27.25"), but our tires are not really 27.25" tall, but rather about 26.5" with 32psi and mounted on ours cars. I think the wheel width has a little something to do with the real height versus a calculated height. The wider the wheel, the shorter the tire would become.

[kstrong]

You got it all wrong, Your car is GRABBER ORANGE. They go faster remember.

They go farther faster.

Just ask Grabber, he'll tell you.

[pitpup]

B)-->

QUOTE(Five Oh B @ Mar 6 2007, 12:48 PM) 110619[/snapback]

I have the 235/50/18's on my 07 GT, and set the Diablosport Predator tuner to 755 revs per mile and it is dead on per the mile markers on I-5. I, too, started at 740 after doing the math (20168/27.25"), but our tires are not really 27.25" tall, but rather about 26.5" with 32psi and mounted on ours cars. I think the wheel width has a little something to do with the real height versus a calculated height. The wider the wheel, the shorter the tire would become.

 

Very cool, 'B'. Nice to know that I'm not making this crap up! Yeppir, 755 with our stock tires does appear to be THE setting.

[kraegar]

Confirmed via a double-gps test (measuring at +/- 15 ft. average accuracy on both) that both the dealer stock setting, and skipping the tune options altogether (leaving it at "stock setting" for the revs / mile) is dead on accurate for mine (stock tires & rims). Tires are inflated to 35psi.

 

Tested at 35, 40, 50, and 60 mph, for at least a mile each.

 

- Tony

[Grabber]

Confirmed via a double-gps test (measuring at +/- 15 ft. average accuracy on both) that both the dealer stock setting, and skipping the tune options altogether (leaving it at "stock setting" for the revs / mile) is dead on accurate for mine (stock tires & rims). Tires are inflated to 35psi.

 

Tested at 35, 40, 50, and 60 mph, for at least a mile each.

 

- Tony

Tony,

I thought you were a sensible guy ?

[kraegar]

Tony,

I thought you were a sensible guy ?

 

What did I do?

 

- Tony