My speedometer was off!
 

I had a gear change earlier this month (3.31 to 3.73) and thought I'd check the speedometer with my GPS.

I have a SCT Strategy Flash that I bought last year. (It's an XCal2 with preloaded tunes from SCT.) I'm using the latest firmware and updates for the device. After the gear changed I select "3.73" from the "Gear Ratio" menu. I thought that would correct the speedometer.

I was surprised to find that it was reading about 5% too high! The speedometer would read 70 mph and the speed was really 64 or 65 mph as measured by my GPS. I've tested the GPS in other vehicles and it's always dead on.

I corrected the "Revs per Mile" to 770 for my stock 17 inch P Zero MS tires, which should have corrected the problem. I was STILL off by about 3 mph. The speedometer would read 70 mph and the actual speed was really 67 mph!

Bottom line. I seemed to have fixed the problem by increasing the revs per mile to 805, although, I'll need to do a few more tests to see if that number is dead on.

I don't know if the problem exists with all XCal2 or just my SCT Strategy Flash. I urge people with the later to check your speedometer if you've had a gear change, either by GPS or by driving a known distance and using the trip meter.

RE: My speedometer was off!
 

What size tire you have on there now?

RE: My speedometer was off!
 

Sorry, I should have made that more clear.

I have stock tires and wheels. 17 inch P Zero Nero M&S tires, 235/55 ZR17 that came with the car.

http://www.tirerack.com/tires/Spec.j...irePageLocQty=

RE: My speedometer was off!
 

yeah the stock revs per mile is 765.5 so that is a big difference. I am not sure why correcting the gears didnt take car of it. Maybe it never read correctly to begin with!:eek:

RE: My speedometer was off!
 

Mine is purposely off;)

RE: My speedometer was off!
 

it is normal for the gps to read a little bit different than your speedo as its more accurate. dunno bout 5% though id say 2-3% is acceptable

RE: My speedometer was off!
 

Yeah, it was definitely a big difference. I wasn't sure about the GPS, so I tested it using the trip meter.

The distance from the stop light at work to my house was 12.5 miles as measured by my Focus (stock). It was 13.1 as measured by the Mustang from the same spot to my house originally after I thought I had corrected the speedometer. It went down to 12.7 after I adjusted it to 770 with a difference of 3 mph on the highway as measured by the GPS. Hopefully now it will be dead on.

The problem is, setting the revs per mile to the manufacturer's number clearly didn't fix the problem. For that reason, I"m having to go to these unusual lengths to get it right.

RE: My speedometer was off!
 

I have the same wheels and tires. This weekend for the first time I took a trip with my GPS in the car. It's a laptop running Delorme software and GPS receiver. At 80 MPH on the GPS, the speedo read 85. That's 6.25% high. That's even worse that yours.

I don't have a tuner. I'm wondering if this is covered by warranty.

RE: My speedometer was off!
 

The GPS might be wrong...ford would have to test and see if it is off by a good amount.....but they should fix it if off by 5+ assuming no mods are done affecting this.

RE: My speedometer was off!
 

Forgot to mention - I have an automatic and the stock rear end.