I have 2002 1500 quad cab short bed with 4.7 engine auto trannie.
Tag on door says 17" tires. It has 20" on it. I don't know or know where to find my rear ratio. How do I tell if speedometer has been calibrated for the 20" tires? Anyone have any idea what I speed should be at, say, 2000 rpm?

 

Try checking your speed with a GPS unit or just have someone follow you.
Just a couple of ideas that could work.

 

The door tag is probably going to say 245 width 17" tires regardless of how the truck was equipped. That was the minimum spec from the factory. I have 275-70-17 as oem, and 20s were an option as well. You'll have to check with a gps or phone app. "Speedview" is a freebie on the android market. My speedo is dead on.

 

That speedview sounds good to me. I googled it and found it. I don't know how to get it on my phone. I gon't have web access on my phone, can I use it? It also talked about android, what is that?

 

Android is the operating system used on Droid smartphones.
Just ask someone you know that has a portable GPS if you can take it for a spin with you.
Chances are, you will love having turn by turn direction and you might even buy one.

 

You could also use a Pedometer while running alonside the truck and some math

haha

Recomend the Garmin or TomTom for purchase

 

What makes you think it is incorrect. Drive by a construction zone with the speed scanner

 

I do not think it is wrong, I just don't know and would like to know for sure. I thought the 20" wheels were added by previous and thought if so and not calibrated it would be wrong. Now I find that it says 17" in door regardless which size came from factory.
Also in doing research I find that the 20" would get poorer mpg and performance. I can understand poorer performance since that would be a little higher ratio. Can't understand poorer mpg since you would be going a little farther at a given rpm.

 

The 20 inch wheel and tire combo weighs a lot more and has rotational mass vs a 17.

 

I would just invest in a cheap GPS like Garmin or get a programmer and enter your current tire size and wallah..

Overall tire diameter plays more than just rim size. In regards to your gear ratio, are you the original owner cause if not then who knows what your truck has been through over 10 years so what is on the build sheet may not be what it has now.

Prob' better to jack up the rear, mark the drive shaft and do a full rotation of the tire while counting how many revolutions the drive shaft does. Should give you an idea to start with