I'm a little curious about this because the speedo seems to be calibrated fine, but I've got huge wheels for a 4.7

Would there be any benefit to changing the gearing in the rear end? If the wheel size has changed, then shouldn't the gears also be changed?

 

You should be fine with the stock gearing. Technically you are correct, when you change wheel size gearing should also be changed but, as I understand it, that refers more to when you are lifting up the truck and adding larger tires not wheels. When switching from 17"s to 20"s there is not as much change in overall wheel height, the 20"s will just have lower profile tires to compensate for the larger rim. My truck also came with stock 17"s but I have swapped them out for 20"s. I use 20" chromes during the summer and then switch back the the 17"s during the winter. When I first swapped them out the speedo was off by 5 mph or so (example: speedo would read 50 but I was really going 55). I bought a Flash Paq programmer though and now can adjust the speedo based on which rim/ tire combo I have on the truck. I am by no means an expert in gearing but this is what I have been told and know from experience.

 

Someone changed them out.
Stock 17's have 31.5" diameter
Stock 20's have 33" diameter
The speedo will be off, but only slightly.
The person who swapped them out could have had the dealer reflash for 20" wheels which is one way to correct for tire size. A programmer is another. You need to be traveling at highway speeds to really notice the few mph difference.

20" wheels reduces your overall towing capacity by more than 1klbs.
With the 4.7L being not so good tower, and if you have 3:55's, the 20's are gonna make your truck work very hard.

 

I'm not really towing anything anyway, This is a daily driver with the ability to do stuff

I have a flashpaq 3865 so I can adjust the speedo with that based on tire size so I'll have to look on my tire walls and find out what size they are and adjust accordingly.

I'm a little disappointed with the heavier wheels and extra work the engine has to do but unless I can get 17" rims free I'm stuck with the 20's.

My wheels are solid too, unlike my dads 04 which has the same rims except the "chrome" is a plastic cover.

 

which oem 20's?

 

Where you from? It's a long shot but I have stock 17's I'd be willing to part with up here in Canada

 

London ontario......long drive.