I've read a lot of posts on this forum from people looking for answers and ideas related to
increasing the tire size of their Dodge trucks. In February, I bought a used 2007 Dodge RAM
1500 SLT QC with 17k miles, Hemi MDS, 17" Stock Aluminum wheels...yadda...yadda...yadda.
Through my neighbor, I met a guy who was selling 20" stock tires with about 5,000 miles
(look almost brand new) so I bought them.

It seems there is more than one "school of thought" on what changes happen to your truck
when increasing the tire size; gas mileage, speedometer, etc. I know it's all true, my
speedometer was off, gas mileage was reduced, but I still had a feeling it wasn't as bad as
it appeared or some were saying. So, I went to my local Dodge dealership and asked them; my
primary concern was gas mileage. The Dodge Service Department didn't seem to have clear-cut
answer either as to why gas mileage reduced when larger tires are put on a vehicle. I don't
believe the BS that heavier tires cause this, especially with a V8 Hemi. I admit my logic
has some flaws (depends on the tire size, type, etc.) but I can't believe adding wheels and
tires that are the 3" larger can weigh much more weight than one or two additional people in
the cab. Anyway, the dealership, for $66.50, connected my truck to a computer and made some
adjustments to the ABS, which included the brakes, speedometer, MPG LED, etc. My gas mileage
only reduced by a little more than 1mph and the speedometer is correct, but I also noticed a
difference in how the truck handles. So, those of you who want to upgrade or have upgraded
your tire size, I suggest getting the ABS adjusted to the truck's (new) tire size at the
dealership or with a mechanic...it will make a noticeable difference.

Also, I found a website that has a calculations tool to compare the differences between one
tire and another, based on your input. No I'm not promoting buying tires or anything else
from this site, but the tool provides an excellent resource for showing the differences in
tire size, revolutions per mile, speedometer readout, and more:

www.discounttire.com/dtcs/infoTireMath.dos

[IMG]local://upfiles/93996/23D808F7C2E544EE873FCA9E1261C471.jpg[/IMG]

 

The 20s do weigh a bit more. It's un-sprung weight, so that difference will be more significant than just adding the increase in weight to the bed of the truck, such as with cargo.

 

Multiple posts, because I get errors when I post more than 1 or 2 lines. Also, the 20" tires are a lot wider, which increases rolling resistance.

 

Then finally, the taller tires lower the engine RPM, which lugs the motor.

 

You need to re-gear to 3.92s or better

 

i changed to 20s from 17s recently and there is no difference in the height if the tires but there is a difference in width... does your theory still hold?

 

when rolling you have to take in to account inertia. Rolling inertia increases with the square of the radius. Making some assumptions and doing some math says that 20" have about 40% more inertia than 17"s just by the increase in size, I ignored the increase in weight completely. That's a big difference, especially with the increase in weight it'll be worse.

 

I purchased a set of new truck take off 20s this springto replace of the factory 17s. The local dealer wanted a half hour labor for a 3 minute computer connection to correct the speedometer. I declined since I plan to run the 17s in the winter with the more aggressive tread design. The only setting they were going to do is the wheel height which corrects the speedometer. There was no additional settings for the abs since I believe it is controlled by an index wheel at each corner that calculates the different wheel speed. The overhead mpg is a calculation by the computer counting the injector pulses against the mileage driven as recorded by the odometer.

Observations: The 275/60r20 is only 2cm (3/4 in) wider than the 265/70r17. The larger tire has a less agressive tread design than the all terrain 17s offsetting any change in rolling resistence. I find my mileage is about the same, slightly worse in town, slightly better on the highway. There is a increase in road feel, but less road noise. The speedometer is off about 3 1/2%, 70 indicated, 73.5actual and I can live with that. I am also getting 500 miles of extra warranty for each 10,000 miles driven. Because the odometer is off, the overhead mpg is closer to reality. Again, this is comparing the stock 17s and 20s.

 

Ivaskaj said it all..unsprung weight is a killer and isn't BS. Also all the dealer did was put in your new tire and wheel size which calibrates the speedo...they didn't do anything to your abs/brakes/mpg led. They told ya that to give you a warm and fuzzy...

 

I know for every one pound taken off the wheels (for my bike) its another four horsepower....changing to lightweight aluminum wheels and new lightweight front and back tires took 10lbs off; for the thousands I spent on the motor, this was the biggest and a cheaper gain...now I dont know the specs for our trucks, but I know my buddy lost over a second in the 1/4 with his lightning from switching his stock aluminum 18s to chrome 20s