I am wondering if the data stored in the PCM is read directly by this tuner,
or does the tuner extrapolate based upon the particular model.

I was going to configure a custom tune, and the tuner displayed 3.31
as my gear ratio and had a tire size of 2250mm.

My truck came from the factory with a 3.55 rear end and 265 70 17 tires.
(I come up with a tire size of 2550mm).

I bought the truck used, so changes could have been made, but it
has the stock tires and the tag on the door says H9 (3.55) and
the tag on the rear end says 3L55.

Any ideas?

 

The tuner will read your vin and computer code (four digit code on PCM/door). From the calibration code, it will give you the stock options.

 

Accorrding to the EVO 1 manual (from internet):
(I actually have the newer Evolution but assume they operate
simularly in this regard).

 

I just installed an EEF2100 today (Sept 9) on my 99 Expedition and the custom tune had a gear ratio of 3.31 instead of what the door tag says as 3.73.

Does anyone have any idea why the Evolution is showing a different gear ratio?

Thanks!

 

This can be a confusing issue, so I'll try to explain it for everyone.

First, the PCM has 2 different places that it stores the info for what gear ratio and tire size is used on the vehicle for purposes of speedometer & odometer calibration, and a "soft" switch to select between the two in the PCM's software (well, firmware actually).

Now from the factory, Ford uses a *different* location in the PCM than aftermarket tuners do - and especially in the F-series (including Expeditions, F-150's, 250's & 350's with gas engines), very rarely does the location that aftermarket tuners use actually show the correct original factory gear ratio & tire size - usually it's just a default gear ratio and tire size.

What this means is, in most cases, when you change tire size for example, you also need to know the correct gear ratio your vehicle actually has so that you can plug that in, too - see, once the internal "soft switch" in the software of the PCM is turned off, which makes the PCM take it's speedo calibration info from the area that aftermarket tuners access, then you have to have *both* the correct gear ratio and tire size entered for the speedometer to be correct, in most cases.

So it's actually quite simple, really - you just have to understand that there are 2 different places in the PCM where this can be controlled, that Ford uses a different place than aftermarket tuners do, and last, that the default info in the areas that aftermarket tuners use is generally wrong to begin with -= and it doesn't matter, because it's not being used until you use an aftermarket tuning device.

One last point - the area that Ford uses can be changed at a Ford dealership (called the VID block), but it can be changed ONLY to axle ratios & tire sizes that were offered by the factory on that vehicle in that model year - which is why most of the time, yo need an aftermarket tuner to correctly calibrate the speedo for gear ratio and/or tire size changes.

I hope that helps everyone to understand this a bit better!

 

I would like to thank Troyer Perforance for explaning that there are two addresses that hold the gear ratios and tire sizes. This was really bugging the crap out of me.