The speedo on my GT is not correct. I have verified it by GPS, stopwatch timing the mile markers on the highway, and having a police buddy radar me.

When the gauge says '40' I am actually doing 45 and by the time the speedo reads '60' I am traveling at 70. Needless to say this is not good. I have taken it to the dealer and they did a BCE diagnostic which showed no problem and a cluster check which also checked out OK. I had them contact Ford and I guess all that they relayed to the company was that I had checked it with GPS since Ford's reply was that GPS is not accurate enough, completely ignoring the other two verification methods I used.

Has anybody else with one of the late models had an issue with their speedometer reading?

 

Are u on stock rear axel gearing?, did you tune?

 

Any mods?

 

Bought it used, so I can't address those questions with certainty, but IIRC, for the speedo to be slow should mean the PO had installed taller gears and I can't imagine him putting taller gears in an automatic trans Mustang. Tune - dunno about that either.

No mods since I've owned it. Stock everything to my knowledge.

 

tire size?

 

Factory wheels & tires.

 

So does the dealership think it's correct or not?

 

They drank FoMoCo's Kool-Aid. I'm planning on taking it to another dealer next week and tell them to reflash the PCM to stock. If that doesn't do it, all I can think to do is pull the diff cover and count the gear theeth.

 

Interestingly, the error is not consistent. Were you in the same gear at both 45 and 70? The variation at 45 is .125 while at 70 it is .167. If you were in the same gear the variation should be the same.

 

I suspect that when it read '40' and was actually doing 45, that I was in 5th gear maybe just in 4th. When it reads '60' and is doing 70, I am definitely in 6th.

 

Definitely take it to another dealership. They never should've had to ask anyone at Ford Corporate for anything just to figure out if the speedo is off...It doesn't take a genius to figure out if it's correct or not.

 

Have only read about minor errors the other way with the newer models. Mine reads 72 & I'm actually doing 70, so the error is minor & nowhere near the error you are seeing.
Like the other comments, verify tire size. Otherwise it would almost have to be a tune or ring & pinion change.
Ring & pinion you could check on by posting your RPM's @ a certain speed compared to similar cars. Much easier than pulling the diff cover.

 

True enough. If I have your RPMs at 70 MPH in 6th gear, your 6th gear ratio, and your rear tire specifics (brand, model, size), I can back into your rear axle ratio.

 

RPMs @ (actual) 70MPH in 6th gear = 2,000
Transmission 6th gear ratio = 0.69/1
Tires = Pirelli PZero Nero All-Season 235/50ZR18
Axle ratio = 3.15/1

 

My calculations indicate you should be doing 72.17 MPH in 6th at 2,000 RPM with a .69 gear ratio and a 3.15 axle ratio. According to TireRack, your tires make 765 rotations in a mile. Given analog instruments, your 2,000 RPM could be off just a bit and that would account for the slightly higher calculated speed. The bottom line is that your assumed 3.15 axle ratio is almost certainly unchanged. The next step in either direction would have yielded a greater variation than that. So we can conclude that your speedometer is, in fact, off. It looks like your speedo is calibrated as if you had a 3.73 rear axle ratio.
Sounds like you need to visit a better dealer.

 

Thanks for the input. This is in line with what I had figured, also. We'll see what the next dealer's service department says when I take it to them, hopefully next week.

 

Let me escalate this to the customer service manager of your area, rriddle3. Please, make an appointment with any Ford dealer and PM me with your VIN, dealer, mileage, full name, and best daytime number so I can help.

Deysha

 

Thanks, PM sent.

 

UPDATE:

Took it to another dealer. They verified the rear axle gear ratio, the tire and wheel size, and contacted Ford engineering hotline who told them to reflash the PCM. The tune was stock, but somehow things got corrupted, causing the speedo error. Got it back and checked it with GPS and at 60mph on the gauge it reads dead-nuts-on. Everything was done under warranty. Yea!

And, Deysha, thanks for the connections. Jason with FoMoCo talked with the dealership and then with me today. I'll let him know tomorrow that things got sorted out.

 

Good to hear. That's how simple it should've been in the first place.

I just noticed that you're local...I'm curious which dealerships you took it to?