Ok, before the boos and hisses begin. I am new to car forums and I am not a typical car guy, but I figured you all would know what is up.

So I bought a 2007 Ford Mustang GT Shelby. The speedometer is way off at 40mph (off by 7), at 60mph (off by 10), at 80 mph (off by 15). I have taken it to the Ford dealer four times and they can't seem to get it right.

Also there seems to be a standard air filter installed and the another air intake is in the trunk. From my research it should be the one in the trunk installed. Could that lead to the issues?

Sorry for the newby crap, but I am at a loss.

 

Moving to tech..

I'm guessing the car has steeper gears installed (ring and pinion) which explains the speedometer discrepancies.

That's weird about the intakes though.

 

Your number 1 course of action should be to get a tuner... but check where the spare tire is to see if the previous owner left you a tuner, sometimes mustang owners will hide a tuner there so that it makes it with the car.

It sounds like mostly either your tires or rear gear need to be tuned for.
Is the speedo higher or lower than the actual speed?
here's a handy calculator that you can use to try to find out what rear gear you have installed. as a note, 4th gear (transmission) is a 1:1 gear ratio, so get out and drive the car in 4th at a given RPM, say 3000, and take a note of the speedo speed as well as actual speed and play with the axle ratio until it matches, note that the normal axle ratios for our cars are 3.31, 3.55, 3.73, 3.90, 4.10
http://www.tremec.com/calculadora.php

 

Welcome to MF!

Changing your tire/wheel size will also affect your speedo reading. There are a number of calculators on the net, but here's a simple one: http://www.gordon-glasgow.org/tirecalc.html

Once you've identified your gear ratio and tire size, you can plug these values into a tuner to correct your speedo.

The intake in your trunk wouldn't have an effect on your speedo reading, though it's odd that it's not installed. It's easy to install yourself, but getting it tuned properly would be the tricky part since it sounds like your Ford dealer doesn't know what they're doing.

 

I will try those tips.

It is registering higher that actual speed.

 

That's about the difference between what a 3.31 and a 4.1 gear would be... meaning it may be programmed for 3.31 when what's actually installed is 4.1
assuming that when the speedo shows 80 the car is actually going 65.

 

Wouldn't doubt it the previous owner took off the air intake and tuned it back to stock to sell the tuner and didn't think about the gearing.

 

Thats what I am thinking!

 

The stock intake though should be the one in the trunk for the Shelby GT, as that was one thing to help boost power to 325. Thats strange.

 

+1 on the gears being changed... AND possibly the tire size also coming into play if the tires on the car are a different O.D. than the OE tires. You'll need a hand held tuner to correct the value in the PCM for both the gears and the tire revs/mile.

The intake issue has me confused since I'm not sure which was supposed to be on the car but if the box shown in the engine compartment is not throwing a code, then I'd be leery to install the CAI that is in the trunk. The CAI typically requires some different tune parameters than the box so swapping could result in throwing a lean code...