I'm new to the truck game. Been a long time coming for a alabama boy like me. I'm trying to figure out what speedo gear I have. I have a 1997 4x4 lariat with the 4.6L. This is my door jam sticker. I want to put 33s on my truck (285/70r17). I need it to figure out how many teeth the new gear will need. I would go up to 35s but with the 4.6 I'd want to change my 3.55 to something beefier. Don't feel like doing that quite yet. Thanks in advance guys!

 

Pull your current one tell me what color it is. Located on the driver side transmission tailshft. 12mm socket to take nut off, then it will slide straight out.

 

I had a 19 tooth and went to 21 tooth and 33.25" tires! M/T2

 

you dont really need to change speedo gear to go from 265 to 285 tire.
thats roughly 31.4 " to 32.8 " dia = 4.4% or 2.5 mph at 60 mph. WHEN new. Wear 1/2" off the tread (50%)and its 2.7% or 1.6 mph at 60 mph.

My speedo reads about 2mph slow compared to gps My calibration in PCM is for the 265 tire.

Just remember to account for the difference in calculating fuel mileage, because the odo reading is low. My truck has almost 10,000 more miles on it probably than the odo indicates due to this.

I have 5.4 with Pi heads, and 3.73 gears. I would say lt285 is just doable performance wise. With a 4.6 and 3.55....expect a dog in acceleration

 

Try this....

http://www.tciauto.com/tc/speedometer-gear-calculator/