I own a 2005 Dodge Ram Quad Cab with 17 inch rims. If I was to put 20 inch rims on my truck, would this be a plus or a negative? Would it affect my gas mileage?

 

20" is for looks and handling. The rotating mass and tire width is what plays a role in reduced gas mileage.
If you go with super heavy 20" rims and 35' superswampers, yeah, your gas will diminish.
Overall, it's negligible. What extra the rim weighs, the lack of rubber on the tire makes up for but you'll still be around 5-8lbs each corner heavier. You may see .5-1mpg difference on a SIMILAR tire tread. If you go to an A/T, that will reduce more as it would even on 17's.
Please see tire/wheel fitment for proper size and offset.

 

It has more to do with the weight and size of the wheels than the rim diameter. Also, the attitude of the truck would make a difference. I drove mine stock for many years with the 20" OEM wheels and 33" Goodyear HP's and got what I considered very good mileage for the size engine and type of vehicle - also compared to my friends that drove chevy's and fords. A few years ago, I put a leveling kit on it and dropped to 17" after market rims with 34" tires and noticed a decrease in the mileage. It wasn't huge. But with the front end higher and taller, wider, and heavier (Load Range E) tires, it did make a difference. Now I have replaced the 34's with 35" mud tires and notice even more of a decrease in the mileage - but that was to be expected.

 

the 20's will have a harsher ride since you wont have as much side wall to obsorb impacts from the road.

 

While that may be true, relatively speaking, when my truck was stock with stock 20" wheels and 33" Goodyear HP's, I was always happy about the ride. I rode/drove in Fords and Chevys that were stock and similar in age, and none rode as good as mine. That was one of the main reasons I bought it. I went to the dealer to pick up a part for my older Ram, test drove the new one, and ended up buying it that day. It was also the reason I waited so long to level it and put different wheels/tires on it - but even after finally doing that, I was still very happy with the ride.

 

I agree Greg. My stock 20's feel great. It's all about the front suspension.

SJ03

 

i didnt say it was a horrible ride.

my trucks rides fine with the stock 20's but for some people it might ride a little rough for a half ton truck.

 

Maybe just a difference in terminology. What one may call a rougher ride, someone else may consider it a tighter ride. What you may notice giving up in softness, you'll gain in handling. With the 20's, it handles corners amazing for a half ton pickup. Now with the 17's and 35's, it still rides pretty good but handles nowhere near as good as it did. Off-road, however, no question that 17 or 18" rims with more sidewall on the tires will be better going over bumps and such.