Just wondering if I really do have 3.55 gears. Tag on the rear diff starts with a 55 and some other numbers but I don't know what that actually means. Really got me thinking when I was driving 70 the rpms were at 2100. When I had my 04 dodge automatic it had 3.55 gears and ran at about 1900 at 70mph. They're both the 4spd OD do I'm confused as to why this truck is different. Stock tires and wheels if that matters.

 

Look at the sticker in your door jam and Google the axle code.

 

idk what gears i have but around 70mph im just under 2 grand like 1800,1900

 

ya I have 3.55LS with factory tires and run about 1900 at 70, are you 2 or 4wd?

 

The door jam ael code does indicate 3.55 gears. I just don't know if the previous owner might have changed them

 

Going off my sticker I have 3.55 gears. At 70 i'm around 1750 rpms. I have 305/70/17's not sure how much in either direction that would change it.

 

Bigger tires= less rotations it takes to go each mile, so lower RPMS per mph. @OP if you have a 2wd you will have smaller stock tires which means you will be at higher RPMS then a 4x4 with stock tires

 

i do have a 2wd and the dodge was a 4x4 with slightly larger tires. so that makes sense then

 

I have 3.55 gears as well and I also do around 2,100 rpms at 70. Mine is 2wd. According to the door sticker its supposed to have 255/70r16 tires but the previous owner put factory 17's with 235/70r17 tires so I don't know if that makes any difference. According to gps my speedo reads 3mph fast.

 

That'd be the smaller tires. Rim size doesn't affect mpg. It's all tire size.