Hey guys,

I'm at school right now but just thought of something I noticed over the summer using my gps. It seems my speedometer is about 10% off. When it says I'm going 30 I'm actually doing about 27 which isn't too bad, but in the higher speeds it's kind of ridiculous. Thinking I'm going 70 when I'm actually doing 63 or less, that kind of thing.

I'm not the original owner, but I mean is this normal? I know some manufacturers mess with the speedo from the factory but 10% seems a little much. Is it messing with my mileage?

And yes I know gps's can be off, but it's been accurate in every other car I've driven while at cruising speed.

Thanks in advance

 

First off, Are you sure this car has it's stock rims and gears? The most likely answer that comes to mind is this was flashed back to stock before selling/trading-in, or just one of the above mentioned was done without an actual tuner or having a dealer adjust for what was done and he just lived with the speedometer being off.

The more obscure possiblities are that It's an analog display, someone could have previously pulled the cluster and screwed with it, something with the cluster itself might be having an issue, or the stepper motor itself might be croakin'.

Have a tuner? Plug it in and display the VSS(Vehicle Speed Sensor) readout from the car directly. See if it matches the analog. If they match, then ain't nothin' wrong with the cluster and either the rims, or the gears got upgraded.

 

It's got the stock 17" 5-split spoke (best way I can come up with to describe it) on with the pzero rubber, so I don't think that's the problem. As far as gears I can get nearly 30 mpg on the highway and while I don't remember my exact rpm at certain speeds, I feel as though with gears I could get no where near that. I haven't gone under and spun the axle and tires freely but that would be the way to be sure. But I feel as though it's got the stock gearing on it.

So yeah, still clueless :/ I'll be getting a tuner hopefully during the winter or at the latest early summer, but for now I'm unable to check it through that. Would it be something worth bringing it into the dealer for? I feel like they'd charge me just to plug it in...

Maybe I should just wait to get the tuner? I can then do what you said and possibly mess around with the gear ratio to find out if it makes it accurate. And thank you for the suggestions, btw.

 

like wayne said, this sounds like a car that was upgraded than flashed back to stock. I was going to ask if you were the original owner than just as I was going to I read that you werent. I wouldnt worry about it, and just wait until you get your sct. Once you get that put in the wheel/tire info that currently on the car now and start with the stock gear ratio and go up from there. If say you "have" 3.31 gears and the speedo is off, set it to 3.55 with the sct and see what happens

 

+1 When i picked up 06 GT that had 4.10's in it it was way off... it sounds like you probably have 3.55's

 

Interesting. That was usually a factory option right? Maybe a careless Ford tech flashed it back to "stock" tune with 3.31 gears when it had some work done after it came with 3.55's from the factory. Who knows, I feel like I'm going out on a limb with that one but I doubt I have 3.73's or 4.10's. When I go home I'll check the diff cover for the tag on which gears I'm supposed to have, then mark the rear driveshaft and tires to see what I actually have.

Grabber, I'll follow what you said and just wait for the sct. Makes me even more excited for it now

Thanks for the help guys!

 

PM me with your e-mail address and I'll forward a spreadhseet for engine RPM vs speed where you input your gears and tire size. You can play with it to see what gear ratio gives you that roughly 7 MPH difference at highway speeds.