My speedo is already two mph slower than I'm going haha. But I thought the first number was the width of the tire?
So confused. Thanks for your insight. Ill have to do alot more research

 

Kind of makes sense if you really are running 245's. I am fairly sure the stock size was 265 and that would be what your speedo is calibrated for, so smaller tires would make it off on the slow side, bigger makes it off on the faster side.

I can't say I've totally figured out the tire sizing thing. Seems like it would just be easier to size it height/width/wheel diameter, but I guess that would be too easy. That number is basically the width, but the second number is the aspect ratio of the sidewall relative to the width and the two together determine the height of the tire. If it was strictly width, a 285 would always be the same width. But a 285/70 is wider than a 285/75, but the 285/75 is taller than the 285/70.

Now see what you've done - I need an aspirin. I'm sure I got that all wrong, but it goes to show how confusing it can be. That's why I recommended looking at a size/spec chart to find out what you have as opposed to what you may want, using the height and width columns, and then follow that back to the tire size. You have the knowledge as well that the stock rigs with 20" wheels are running 33" tall tires to start with - so anything at that height or below shouldn't give you a problem in that area, especially with a leveled front.

 

The stock rubber was 245 with optional 265 except for Laramie which automatically came with the 265's. Other packages also includes the 265's so most have never seen the 245's unless you have a lesser equipped truck. The buildsheet and tag on the door jamb should match but I've heard of cases where the jamb said 245 and the truck actually came with 265's. At that point you have to wonder what the speedo is actually calibrated to and if it's one of those cases where they grab whatever's closest on the assembly line.

 

well its the base model. its an of and doesnt even have a cd player. manual windows, locks. no alarm. but i just took a look. the truck came with 245 but the tires are a 265. truck came from florida with brand new tires so i assume somewhere along the line of owning this changed it to 265. which would explain the speed difference. but i think i have it all figured out now. its starting to make sense

 

Bigger tire should make the speedo show a slower speed than you are really going. Maybe that's what you meant and we have been saying the same thing in different ways. You should be able to run LT285/70R/17's on those rims just fine. It's amazing the visual difference that an inch or two makes (at least that's what she said). I just took p245's off my Chevy and put on LT285/75R16's and it looks like a different truck.

 

If the speedo says I'm going 55 I'm actually going 57 or 58. Confirmed with two different gps's. but that's what I'll do. 285's. good bye wallet haha

 

285's on 17's are what I am rolling, to give you a visual but its on a 2500 4x4.

 

Got it. We are on the same page. I compared mine to my Garmin dashtop unit and an app I have on my phone that shows speed based on GPS. On my Chevy, going from 245 to 285, I am about 3mph faster than speedo shows.