Tire Diameter

pitch wrote:
20" tires on a 2013 Ram 1500 have a 32.5 diameter
17" have a 31.7 diameter. What would be the apparent ratio or formula to arrive at such based on 3.55 gears.

The formula is simply:

(orginal_tire_diamater / new_tire_diameter) * gear ratio

It's simply a ratio of change in tire diameter multiplied by the gear ratio.

3.55 means the driveshaft turns 3.55 times each time the tire turns once.

That won't change whether you have 17"s or 20"s.

Your "effective" ratio is only a RELATIVE measure, from where you are now. In other words, you get X performance now with 20" tires and a 3.55 ratio. Switching to 17" tires will give you the same net performance increase as going to a 3.64 ratio with 20" tires.

3.55-3.64 is not much. You probably won't even notice.

pitch wrote:
Thanks. Ram claims 20's are optional, but of the many many I looked at none had 17's. Every salesman told me that they had never seen one either. I kept the 17's that I had on my 2012,mainly to maintain the gear ratio and because I thought the chrome 20's were just ugly.

Up here (Canada) at the local dealership, there are plenty of 2013 RAM 1500's with 17" tires. Currently, almost all the SLT and SXT models on the dealer website have 17" whereas the Bighorn and Laramie models have the optional 20" tires.

When I purchased my 2012 RAM 1500 not quite a year ago, I also recall seeing many 1500's on the lot with 17" tires. My particular truck came with the optional 20" tires. I also went with the optional 3.92 gear ratio which with the 20" tires is fine for my lighter towing (<5000 lbs) needs in west coast mountain terrain.

bguy, I don't understand how it would hurt the tranny either. The engine turns slightly faster, but how is that going to hurt a transmission?
I would really need to see some documentation from Chrysler or the transmission builder to believe that.

By using the above chart, I found that it was not a good idea to put larger (wider) tires on the truck. They look cool but it effectively lowers your gear ratio, not useful for towing.

new gear ratio based on tire diameter

Sorry, heres the other site for rear end ratios

tire sizeGear ratios based on tire size change

The first site will give you the diameter of a tire based upon tire size. The second site determines the new axle ratio as a result of tire diameter size changes.

Thanks. Ram claims 20's are optional, but of the many many I looked at none had 17's. Every salesman told me that they had never seen one either. I kept the 17's that I had on my 2012,mainly to maintain the gear ratio and because I thought the chrome 20's were just ugly.

3.55 in the pinion vs ring gear count. There's a software adjustment to correct the speedo and transmission control. I've been told the putting 17s where there were 20s will make short work of the tranny, which I don't understand, but it came from more than one source unsolicited.

pitch wrote:
20" tires on a 2013 Ram 1500 have a 32.5 diameter
17" have a 31.7 diameter. What would be the apparent ratio or formula to arrive at such based on 3.55 gears.

Changing from the factory 20" tires to the 17" tires would give you an effective ratio of 3.64. Going the other way from factory 17" to 20" tires would make a 3.55 an effective 3.46.

If both of these tire setups are factory options, hard to know what the 3.55 is based on....or if they offer 2 different gear sets based on wheel size so both options result in a 3.55.

I did the math but not sure I answered your question....