tire size help

[hottwheels04]

I bought my 95 mustang with 265 45 zr17 inch tires on it out back with 17 inch rims with stock transmission and 3.73 gears, I changed out to 255/35 zr18 inch tires outback and now have a aftermarket tremec tko500 with 3.73 gears, So I did a measurement and thats a 25'' inch tall tire? with 3.73 gears what does that = to ? I read there is a plastic little speedo tooth gear that is diff for gears, when the dealer installed my transmission I'm thinking they might of installed the gear for stock gears for my year which would be 2.73's or 3.08's. and I run aftermarket 3.73's. I'm trying to see how far off my speedo is and how to fix it or test? and if the tire size and gear ratio is good.



Reason I ask I feel the faster I go my speedo is way off when I have my GPS on the speeds dont match up , and what the speedo says and how fast i feel im going dont add up. I kno tire height / gear size etc makes a huge diff esp if you change it. I know the newer cars with digital odometers they sell a "speed cal" to adjust this problem, but are older years have the mecanical speedo so what can I do to make it accurate, and with that tire size what is the gears = to . I might step up my rear tire size being I need new tires soon, I might buy some 265 or 275 rear tires.

 

[Addermk2]

Got a lot of info there... Let's start from the top.

Your car should have had 245/45/17's. Which is a 25.6" tire.
Your 255/35/18's are a smidge undersized at 25".
Your TKO swap probably had a 6 tooth or 7 tooth speedo gear in it. This is what you REALLY need to know.

Crawl under your car and pull out the vehicle speed sensor, take a flashlight and shine it into the trans. Tell us what color the gear is INSIDE the transmission.

Report back and we'll get to figuring out what gear you need on the sensor.