Basically. Unless you spend some cash on light weight fordged 20's then you shoudn't notice much differance.

 

you could always take an angle grinder to em or put helium in the tires? lol jk. Quarter times are effected too by the 20's. My buddy has a ccsb with 20's and that's one slow SOB even with a tune. him and I add about 500 lbs to the truck when we're in it though haha.

 

I know it didn't make much of a difference on the diameter but I wanted to post it up to be accurate. I did my own testing with the same truck and same driving habbits and got at least a 2.5 mpg gain by switching back to the stockers. I think it's great to have a XCAB Z71 get 19+ mpg on the hwy.

 

3 weeks ago, I went from the stock 255/70/16s to (rear)305/50/20 (I have 285/50/20 on the front). The 20s are quite a bit heavier (I would agree 20lbs+ each) and I increased the tire size from 30" to 32" on the rear (30" to 31" on the front). It worked out to about a 300RPM change.

I can say without a doubt that even though I picked up 2" in diameter on the drive wheels, I lost about 2mpg average and A **** LOAD of acceleration. A 300RPM drop can not compete with the addition of close to 100lbs in rotating mass.

IF the tire/rim combination remained the same weight, then there would be a average gain in MPG, provided we are talking a relatively steady state condition (i.e.: highway driving across Kansas). If we are talking about city driving (or heavy hill climbing/towing were torque is needed) then there is a possibility that we would still see a drop in mpg as a larger tire will take more fuel to accelerate off a light/due to down shifting to provide torque (again, A LOT of variables in this one).

On the surface, bigger tires will provide better highway fuel economy. But once you get past the surface and look at the entire system (weight/torque/driving conditions/etc), it isn’t always the case.


As a side note, I love my 20s.....the truck finally looks good.....and hell, now I have an excuse to build more HP (damn...truck got slower with 20s....needs more power...blower? )

 

Effective interia of a wheel, simplified.
I = pi * d^4 / 64

Yes, that's to the fourth!

for 20's: 125660
for 17s: 4100

there is more to it, stuff like tire diameter and wheel design and whatnot

 

ok so far i have this. i did a 20 minute run on my truck which has 20s on it and another extra 200 pounds of weight in it. I averaged about 23mpg. This was done last weekend.

Now next week end i will pick up another truck from my brother and do the same test, with the same amount of weight, same distance, and same location.


Now this was done on 5.3L Extra cab NBS. Im gonna get the same truck but with only 17's on it.


Numbers dont lie.

Drivers do. If you make sure that you fallow the same process on both vehicles. Then i should work.


For the people that say they lost a few mpg when they went with 20's from there 18's that can be explained in driver error.


Less time to accelerate means more gas for the heavy foots out there. When you are use to the acceleration of the 18's then when you get the 20's on there you are going to try and get the same acceleration.

 

That's a very inaccurate statement.

 

noo...this is...

..... I just did a round trip of 300miles, 99% of which was highway and have the MPG drop to show that heavy 20" rims hurt fuel economy, all of this was done with the cruse set so my "heavy foot" had nothing to do with it. I have previously done this same trip and seen better economy with the old 16s.

 

Man you crack me up, the funny thing is that both of the quotes above are from this thread....

 

unless you have to go up and down hills, then you're fighting the acceleration of gravity which gets ya
and RandomHero this isn't xbox live so show some respect, even if you are a 43 on ur other account