Hey everyone, I'm having a problem with my Ford Mustang and I'm trying to isolate it. I have changed the gears and tires, but have a handheld Diablo programmer to re-write the ECU with the new gear and tire sizes. Still, the speedometer reports a different speed.

Basically, if I'm going 60mph the speedo shows 70. I got the true speed from the Z1 by changing the map mode, but I'm not sure if it's getting it from the vehicle speed sense wire or by tracking the GPS satellites....

I have a new gauge cluster on order, but I really don't think that's the problem.

Oh yeah, the reason I know the Z1 is right and the speedo isn't, was I got pulled over in a 65 for doing 75. The cop was cool and only wrote me a warning. He suggested the speedo may be off. I asked if he could get on the freeway and do 65 for a few miles in front of me, and he agreed. I drove behind him for about 5 minutes while he did 65. My speedo said 75 the entire time...

well the z1 if connected to the vss reads that signal to give you the speed. if that signal is being read before the computers adjustments are made to it (with your programmer for the ecu) then it is going to be off. Depending on the vehicle (digital speedometer/analog) Id suppose the calibration with your programmer would have to be done differently.. of course depending on how/where it gets the signal from.

Funnny, I was just dealing with this problem.

My speedometer/avic differ by about 7% '(speedo reports faster). Connecting the VSS didn't have any effect. I've checked it with other handheld gps and they agree with the avic.

Then, a couple of days ago, I passed one of those radar signs ("you are going xx mph and the speed limit is yy mph"). The speed it showed me was identical to the speed reported by avic (slower than the speedometer).

So I drove the car through a long tunnel, making sure that the gps had no signals, to ensure that the avic was only using the vss, and checked the speeds. Same 7% difference.

This leads me to beleive that the spedometer is getting a correct signal, but interpreting it incorrectly.

So, I guess I'm going slower than I thought I was (which explains the old ladies giving me the finger when they pass me ). Downside: My car is probably reading 7% more mileage than it actually has. Upside: Aside from the fact that I'm less likely to get a speeding ticket, I'm probably getting better gas mileage than what I thought it was.

I drove behind him for about 5 minutes while he did 65. My speedo said 75 the entire time...

Then you were really lucky you didn't get a ticket 'cuz if you were doing 75, you must have thought you were doing 85!

There are lots of speedometers which are 10% off. I would believe the GPS, but the speedos, not so much.

Yeah, I'm wondering if it's not done on purpose. This way the car manufactures don't have to deal with drivers suing them for speeding tickets. I mean if a radio manufacturer can get the correct speed from the VSS, why can't a car manufacturer?

BTW, I have a 2000 Pathfinder, in case anyone else wants to compare.

I think it's partially that and also to make people "feel" they are going fast. And the needle gets up there "quicker". Motorcycles do that too, some are even digital tachs and they have RPM numbers higher than actual cause it's a big thing how many RPM's you can rev your engine. The system knows exactly how fast it's going but some bike tachs have been found to register +20% on some bikes.

My speedo wasn't this inaccurate when I bought the car. In fact, I had a TomTom GPS (no speed sense wire) and it was 100% accurate with the speedo. I'm not really sure when/what mod caused it to be off, but I've gone over the rear gear ratio and tire revs per mile over and over to make sure they're right in my programmer. I don't know what else it could be.

Not sure what others are reading, but my Z1 is reading speed from the GPS, and the VSS wire is connected. This was comfirmed via the use of two other GPS units. My Garmin 60csx and a friend's Lowerance. My speedo is off, and always has been off, and not due to a change in gearing or tires, but just off from the factory.

At least I know the correct speed with the Z1.... I'm an avid member of many Mustang forums, and I have yet to see anyone who has the same issue as me. I've seen a few speedos off, but they were so low that they were disputed.... like the GPS saying 60 and the speedo saying 62-63.

My Maxima is dead on with the GPS speed, My Tacoma is 2-3 MPH slower. I guess it depends on the manufacturer.

I think it's a combination between the GPS and VSS until the learning process is completed, then relies on the VSS (since I still get a speed rating in tunnels)

It wasnt CState Patrol that pulled you over was it? Cause I remember them to be assholes. What part of the springs was it?