This might seem simple to some, but I'm confused. If you have 3.55 axle ratio with the OE tires (235-70-16) and change to a larger tire, you obviously change the ratio based on tire diameter. For instance, it would be 3.42 if you went to 255-70-16.
My question is: If it is a factory option to have 255-70-16 instead of 235-70-16, and the ratio is listed at 3.55, is it really 3.55 if you option for the larger tires instead of the OE tires?

 

Chuck!
3.55 is the axle ratio. The axle ratio will not change when you change tires...the tire size will affect the overall ratio ( engine to tires ) and performance however.

3.55 is a contraction for 3.55 driveshaft revolutions for every wheel revolution.

 

Chuck:
As shortbox said, stepping up in outside diameter tire size won't change the axle ratio, but it will screw up your speedometer and odometer readings. This is because one tire revolution will now take you a different distance than before, and your truck "thinks" it's still the same as factory. Don't worry, though, a good speedometer shop can correct this for you with the right speedo gear change.

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On order:
2000 XLT Black Sport, Flareside, 2WD, Ext. Cab, 5.4L Auto. Other options include: 3.55, Class III tow, 6-way Captains chairs w/CD changer in console, Remote entry , and Slider window.

 

I think they change speedos with a computer instead of gears in these late model trucks.
Maybe they still use gears...