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Old 07-12-2012, 10:24 PM   #1
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Speedometer Calibration

I have stock 235/50/18 tires on my 06 mustang manual GT. I bought the car used about a year ago, and noticed that I had to tune the revs per mile to 800 to get the proper MPH with my GPS. I figured it was the bald tires that was miscalibrating my speedo. I bought new tires the other day, and it's about the same 800 revs/mile. I wonder if my Mustang has a different gear ratio? Is there a popular gear ratio that would require 800 revs/mile? The diff sticker says they're 3.55. Also, my speedo needle rests inbetween 0 & 5 Mph about 3 Mph. Could this cause the 800 vs the normal 740?
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Old 07-12-2012, 10:45 PM   #2
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Re: Speedometer Calibration

I question the reliability of tuners when it comes to calculating "actual" MPH based on revolutions per mile.

I installed 245/45-17 wheels and tires (I replaced 15" factory wheels) and in order to get an accurate Rev per Mile figure I went to Tire Dimensions Made Simple - Discount Tire I input 245/45-17 and it said I was at 785 revs per mile.

When I input 785 into my SCT tuner my speedometer was off by a significant amount when compared to GPS (I detected 16 satellites, but was only was using 11 satellites at the time).

Through trial and error, I ended up setting my tuner to 815 revs per mile before the speedometer and the GPS agreed with each other. At 95-100 MPH on the interstate, my speedometer is accurate within 1 mph.

Discount Tire's calculator is WAY WAY off... or my SCT Tuner is WAY WAY off. Either way, I was dealing with serious inaccuracy.

What I learned from this is that you may have aftermarket gears which would throw off the speedo as you suggested, or, the other alternative is that your (and mine as well) GPS is wrong.
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Old 07-12-2012, 11:21 PM   #3
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I never have believed a gps when it comes to mph.
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Old 07-12-2012, 11:32 PM   #4
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On my Android I had an app that showed my current speed based on GPS. It was pretty accurate. The thing with GPS is it lags behind a bit.
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Old 07-12-2012, 11:43 PM   #5
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Re: Speedometer Calibration

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Originally Posted by 99ford View Post
On my Android I had an app that showed my current speed based on GPS. It was pretty accurate. The thing with GPS is it lags behind a bit.
Yeah, I was using an Android app myself. It has been accurate in everything I have tested it on, until of course I swapped wheels/tires on my Mustang.
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Old 07-13-2012, 02:52 AM   #6
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There is a track GPS iPhone camera app... Harry's something can't remember it's the coolest thing on a phone I've ever seen this will for sure fix ur problem but it's pricey...
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Old 07-13-2012, 07:57 AM   #7
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Don't go by the posted revs per mile. Calculate it yourself. Mine was off and used the stated rate and was still wrong. You have to measure from the ground to the center of the wheel and double it to get the diameter and account for the flat area of the tire against the road. There are apps for calculating revs per mile based on the diameter. I think my actual calculation came to almost 790. It fixed my speedo problem.
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