Did They REALLY install my 4.10's? PLEASE HELP
 

I have a 2006 Mustang GT and my stock gearing is 3.55

I had 4.10's installed today. Or at least I think I did.
I may not have though.

I was driving home and my dad was in a different car driving.
He was in a Toyota SUV.

We lined up and set our cruise controls

Well.... he had his cruise control set on 61-63 mph.
I had my cruise control set to 70.

But he was ahead of me and slowly getting further away.

I was wondering.... did they not tune my car correctly and the speedometer is off.... OR did they not really put 4.10's in like they said?

What would be the difference that I would see if
1. They really did not install the gears but put a tune for 4.10's on.
2. They did put the 4.10's in but did not adjust the speedometer on the tune.

Would the results be the same?
Or would they have different results depending on which one was done?

I talked to someone and asked them what my RPM's should be in 5th gear
going 70MPH. He said 2400. That is EXACTLY what I was at.
Does this mean for SURE that they DID install both the gears AND adjust the tune properly? Or could the RMPs and speed match what 4.10's are supposed to correspond at... BUT SOMEHOW they set it up that way and really did not install the gears OR did not install the tune right and thus the reason why this happened with our two cars?

I Really need some input on this and why my car said it was going faster than my fathers yet he was pulling further away from me.
Thanks

Oh yeah and I had my stock wheels/tires changed from 18.x8.5's to 18x10's all the way around and got a bigger tire.. not sure how much bigger but they are 275/40/18 if that matters.

if you got 275/40/18s then you have a smaller tire actually.

and it soulds like you have the correct gears and tune installed

I wonder why the difference with our two cars?
any input?

I mean difference with the speedo on my dad's and my car.

Who did the install and tuning. It sounds like you have the right gears, but the tuning did not happen. I remember when I went from 3:55 to 3:73 gears my speedo was way off until I got my Xcal2 tune adjusted. When I first did the gears, the Ford dealer said that they could adjust the PCM so I let them try and they came back and said that the only options they had for gears were 3:31 and 3:55 and I would need an aftermarket tuner.

+1...

It was custom tuned on a dyno.
I asked him before I took the car home what he had to do to adjust the odometer and he said all he had to do was select 4.10's.

He gave me backups of the custom tune and under axel ratio it says stock.
I asked him why it said that and he said it was because HE made the tune and it was his stock tune so the SCT called it a stock tune and it did not mean the cars stock tune.
He told me that if I selected 4.10's manually it could mess up the speedo.

I look on my Brenspeed tune and it said stock too under the axel ratio of course i WAS stock then.
But does anyone esle have a Brenspeed tune and have stock gears and see if theirs actually says stock or has the gear ratio with a check mark by it?

Also... if the tune was not there wouldn't my RPMS me messed up in coordination to my actual speed?
If the RPMS for 5th gear at 70 are supposed to be 2400 doesnt that mean the tune has to be right?
Or would the RPMS be correct with the speed still but the actual speed be off?

OR what about this?
Again... could the tune be correct then for 4.10's but the gears not really be 4.10's?
I am confused.

OH yeah...

It was not tuned by a ford dealer or anything.
It was a custom shop and was tuned on a dyno.

I am not gonna mention the name unless I find out it is certainly off.
The place has a good rep and I do not want to talk bad about them if it is I that am in the wrong.

Try comparing your odometer to the mile markers on the highway - it sounds like your car is set to the stock specifications though and so your speedometer and odometer are off.