Speedometer Adjusting - Mustang Evolution

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Old 12-20-2013, 11:33 PM   #1
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Speedometer Adjusting

2013 5.0 here. I just wanted to get an insight into adjusting my speedometer.

My tuner gave me the ability to change my revs/mile on my SCT X3 device, and according to this website the factory 235/50 R18 tires come out to 740.4 revs/mile.

I have set it up to that, however, my speedometer seems to be about 4 to 5 mph too fast. I have determined this by using those radar devices the state sets up on the highways to show you how fast you are going. I have also used an android app called Speedview which uses GPS to show you how fast you are traveling.

Knowing this information, should I adjust my revs/mile up or down, and by how much to bring the manual speedometer down around 4 to 5 mph?

I would ask my tuner, but he only answers his email once a week (he provides darn good datalog tuning though.)
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Old 12-21-2013, 10:21 AM   #2
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It's actually very hard to measure revs per mile and the website estimates are not to be trusted, as they vary greatly. Think about it, the diameter of the wheel is changing constantly as the wheel rolls with weight on it, its twisted by traction and braking forces and it responds to the centifugal force at high speeds. Using circumference is a bit better.

Here's a way to do it in your driveway:

Get a dab of grease an put it in a spot in the middle of the tread.

Roll car forward so grease patch touches down twice.

Measure distance between centre of the two patches

There are 63360 inches in a mile. Take that and divide it by the inches you just measured, say, 85.

63360/85 = 745 revs per mile

Input that via your tuner and done.

Oh and, btw, your needle speedo will likely always be a bit off, even with proper data inputs. You could tweak it a bit a few revs at a time and test but, it's just never going to be a precise instrument. The digital speedo in your info centre is likely bang on though, at least mine was.
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Old 12-21-2013, 12:09 PM   #3
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Speedometer Adjusting

I had the same problem adjusting the speed on my last truck. Not only do you need to put the revs/mile but you also need to select the right rear differential. Putting those in should get you within 1-2 mph
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Old 12-21-2013, 05:09 PM   #4
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this a question i was about to post a thread about. When i get tunes for life do i give them my tire size and rear dif to set the speedo right? and how do i fix it when i put in my 3.73's in a few weeks? do i have to get a new tune from them or can i adjust it with tuner. Oh, btw, i'll b using the sct itsx so can i adjust that using the ipad?
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Old 12-21-2013, 06:04 PM   #5
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I don't know about your tuner but this isn't a flashable parameter, it's just a setting adjusted through the diagnostics tab on mine.
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2012 Premium GT, Candy Red, Brembo package, 3.73, MT82, Comfort, Tech, et al. Procal tune, Roush UCA, UMI poly LCA, GT500s, Steeda red bracket, Hoosier 28x10x16 bias drags. Best e/t 11.91 at 115.23 RIP
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Old 12-21-2013, 06:07 PM   #6
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Oh ok, but mine should have that too, right?
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Old 12-22-2013, 02:48 PM   #7
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Thank You for your input 5LHO!

Yesterday, I just made my revs/mile 10 higher than the calculated value and my analog speedometer seems to only be one or two mph faster at 70 mph than my GPS speedometer says. I might tweak it a little bit more.

My axle ratio was correct as well.
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