It's obvious to me that changing the tire size requires the speedo/PCM to be recalibrated to compensate and adjust for the speedometer reading. I see it as a higher RPM so how much would this change the speedometer reading if you went from a 2:73 to a 3:31? or 3:55?

And if it left the factory with a certain rear gearing, Ford would have made the change to the PCM at that time, correct?

 

So what is your actual question?

The cars leave the line with the proper speedo calibration base on wheel/tire size and rear gear correct.

there is a spreadsheet somewhere online where you input the overall diameter and gear ratios to find the correction factor.

You could always do the math yourself

 

If speedometer reading was based on individual wheel sensor, gearing is irrelevant.

 

You answered one (that if it left the factory with a certain rear upgrade, it was compensated for). The other would be how much difference (persentage wise) a gear change would make, going from 2:73 (stock for V6) to 3:55 or 4:11.

I'll find the chart. Thanks for your time.

 

Here is the calcuator you are looking for


http://www.4lo.com/calc/gearratio.php


here are Mustang GT MT82 gear ratios in the trans (you will need this for that calc):

http://www.stingermotorsport.com/car...g_GT_Specs.pdf


Obviously our cars dont have transfer cases so in out that field as 1:1

 

Let's say you replace 2.73's with 3.31's, simple math (3.31/2.73 = 1.21) tells you that if you don't correct your speedometer, it'll read 21% too high. For, example, if you're blazing along at 100 mph, your speedometer will be indicating 121 mph.

 

That's a significant difference. Good enough reason to have the PCM flashed if you change the rear gearing after initial purchase of the car.

 

Bucko - you are correct, and this is where it gets interesting!

Ford wil only do that if you use a gear ratio that they offered as stock or an option; in the V-6 world this means you can only get them to flash 2.73 or 3.31.

If you wish to go to 3.55 or 3.73 (which I did) then you have to make a trip to the aftermarket tuner world, where they can compensate for any ratio. Of course by doing so, you pick up the many advantages of enjoying a tuned car.

Hope this helps!

 

Not true

When I went from 3.31s to 4.10s my dealer was able to adjust my speedometer to within a couple of miles per hour of accurate at highway speed.

Some dealers won't tell tell you this or may not go to the trouble of doing it, but it is possible.

Ultimately I did go with a tuner and now I'm able
to do it myself though I've never had to.

 

When my dealer installed my 3.73 gears on my V-6, they offered to calibrate my speedometer for $50. I told them i have a tuner so i can change it myself

 

I went to 4.56's in my '11 5.0L. Needless to say, an aftermarket tuner was the only answer to correct my speedo. I already owned an SCT tuner, so no problem there.

 

Out of curiosity, what gear are you in when you trip the lights?

 

If he's running a 4.56 ratio, probably somewhere around 13th or 14th gear.

 

 

I'm running 33's on my jeep with 4.56 and it is just about right. I can't imagine this on a Mustang. I can bark it in second with the inline 6 cylinder 4.0. You must shred tires. That said I am also locked with ARBs on both axles.

 

Factory tires were 27" tall. My street tires are 29" tall and I run a 28" tall drag radial, so I lost some effective gearing with the factory gears that I made up (and then some) with the 4.56's. I go through the traps in 5th gear now instead of 4th gear, just one extra gear change in exchange for much harder launches at the dragstrip. In 6th, it only turns 2100 rpm's at 60 mph, so it's not revving like crazy like some people think. 1st through 5th all pull incredibly hard now!

 

Where is the speedometer sensor located, transmission, rear end or at a wheel or wheels?

 

Sensor is at the tail end of the transmission, IIRC. Certainly not at the wheels, that's why a gear swap requires recalibrating the speedo.

 

I take it you meant tail end of the transmission?

The reason I asked is because my 95 F-250's speedometer sensor is in the rear-end housing and senses ring gear speed, so the speedometer isn't affected by rear gear ratios, just tire diameter.

 

Oops, I did mean transmission. Late night after a 12 hour day at work = typo.