I was cruising next to a honda the othernight and happened to glance over and read his digital speedo and it was showing 67. Our speed was matched and myguage was at74-75mph. WTF?? Sure as hell felt like I was cruisin 75 but this isn't the fist time i've noticed my gauges showing faster that a digital read. I pased one of those speed traps along the side of the road andmy guageswas showing about 5mph faster than the trap was showing. I do have a brenspeed tune. Is it possible it threw my guage off.

 

did you mention the tire revs/mile to Brent when he sent you the tune? He might have sent you a tune for stock wheels/tires if you have a different size that'll throw off your speedo after the tune, aslo if you're running a different size gear ratio than stock would do it to after the tune. He's out of the shop till next week but I'm sure they have a record of your tune there and can shoot you an email as to what they put in it for you to load onto your Xcal or Diablo.

 

I looked into this pretty hard last year when I noticed the same thing after buying my GPS. One car's speedometer was dead on with the device, the Mustang was off. I had left the SCT tire rev per mile selection as "stock". I even had stock 17 inch wheels.

Whatever that "stock" value is, mine was really off. After my gear change I found that I was traveling 8 mph slower than what I thought, even when the correct gear ratio was entered. Go look up your rev per mile and input it directly into your SCT, even if you have 17 inch wheels.

After correcting the rev per mile I noticed the speedometer was still off a little, but not nearly as much. I would drive my 2005 Focus one distance and the Mustang the other. Both would register the exact same distance! That means their speedometers should also have been correct.

The Mustang's speedometer was still reading 3 mph high at highway speeds, compared to the Focus which was dead on as measured by the GPS.

Bottomline. The speedometer reads a little high on my Mustang about 3 mph at 70 mph. That's within SAE limits it turns out. Inputting the wrong revs per mile might correct the speedometer by my oddometer and all of my computer readings will be screwed up. (Oh, entering an incorrect revs per mile mile give you the correct reading at one speed and you'll be off at a lower one, I tried that too). Not worth it. Others have noticed the same issue with their speedometers on other boards.

My advice would be to enter the revs per mile manually based on your tire manufacturer's data. (Stock 17 inch Parelli's are 768 revs per mile, I believe, don't quote me) You can find the value on the tire manufacturer's website. Don't use any calculators you find on the internet, they'll be off. Your speedometer may still read a little high, but it won't be off by very much.

 

+1 and good call...that explains A LOT!! lol

 

My speedometer is off by 2mph because i have 28 inch tires in the rear. That was checked using a gps. The funny thing is my aeroforce gauge shows the correct speed while the speedo shows 2mph faster. So either the aeroforce gauge isn't accurate or it calculates speed differently than the stock gauge.

 

Ive noticed the same thing. Around 0-50MPH my speedo is dead on with GPS, but anything above that it reads slightly high. I have stock tires and no tune. Guess its something with the cars.

 

Same exact thing here moosetang. My aeroforce gauge is the accurate one I think. My speedo has always been about 2mph or so off, because of my Mickey Thompsons.

 

interesting......maybe the aeroforce
gauge uses a pulse train, destined for
the odometer and calulates mph from
that.....not sure tho, really