2000 Expedition 3.55 limited slip

Stock (according to door) 255/70 R 16

When I purchased truck used it came with 265 / 75 R 16

How does the 265's affect that 3.55 axle ratio number? Does the number go up or down with the taller tire?

Its time for new tires. Which size tire will give the best fuel economy?

I have already decided to go with Michelin LTX M/S. Just don't know which size to go with.

 

I am pretty sure, at least using logic that as you get a taller tire the "effective" gear ratio goes down.

I was playing with some model on the ford site, don't recall which one, but it came standard with 17" wheels with a gear of 3.55, if you select the 18" option the gear automatically changes to 3.73. I believe that would be to keep the "effective" torque and fuel mileage about the same.

So from the example you can see the "effective" gear ratio goes down.

Hopefully an expert in gears will come by and let you know for sure.

 

Your effective ratio becomes 3.37. You may want to do a quick test to see if the speedo is correct.
drive a known distance and compare to the odo change.

I've been thinkg about doing the opposite. Going from 255's to 245/65's, then the ratio looks like 3.74

 

Tiger, Thanks for the reply,

Do you think the 3.37 ratio will help my fuel economy vs. the 3.55?
The truck still has plenty of torque to accelerate.


I do a fair amount of towing with the expy. Unloaded, the change works for me. I wonder how I will feel when I have 5-6K of trailer and toys behind me.

 

Question is, what engine do you have? A 4.6L will do ok with 3.55s and 265s tires when pulling. A 5.4L will do a little better. As far as your overall mileage if anything it will be a little worse when empty and it will get worse yet when you're towing. Reason being is that the engine has to lug more to pull the truck and/or trailer along. You may not be running as many rpms going down the road as before, but that doesn't mean better mileage. Oh, your speedemeter will also be off as well unless the previous owner had the computer reset to the larger tire size.