Speedo recalibration problem...need some clarification

I want to upgrade from stock P235/75R15 tires to 31x10.50s on my 1995 Sport. The tire store said it was no possible to recalibrate the speedometer. I called the Ford dealer and they said the same thing. I searched on here, and saw the stickied thread at the top that mentions a speedo gear is needed for my truck...but it seemed like if that was the case the dealer would have told me. That leaves the Superlift TruSpeed, which is expensive, but a speeding ticket is more expensive. I'd like to get a performance chip for the Ex, so are there any out there that could make the correction for me? My Ex is OBDI, not OBDII...

Also, how far off will my speedometer be with the change in tire size?

 

[FROADER]

You can correct it, just like the stickied thread says. For now, until you do correct it check this out.

http://4lo.com/calc/gearspeedo.htm

 

[TequilaSnrs087]

Alright. Looks like the guy on the phone just didn't know about changing that little gear out. I may just get 30" tires since they are $110 cheaper and the speedometer will only be off by about 3 MPH at 75 MPH.