Tire size and gearing

[LasPulgas]

I'm looking at a 1999 F250 4WD with a 7.3L diesel. The factory tire spec calls for 31.7" diameter tires. Would 35" tires warrant a differential gear change? Thanks!

 

[Dirtman]

Not with the 7.3. It's got enough torque you wont even notice the difference.

 

[RonD]

Go here: https://therangerstation.com/tech_library/Gear_Tire_RatioChange.shtml

Fill in the blanks

But as dirtman said diesel engines torque is much higher than gasoline engine, but its really up to you
speedo will be off

Drivers door label has Original tire size and the axle code number for current ratio, seen here: https://www.blueovaltrucks.com/tech-articles/axles/identify-your-ford-truck-axle-from-the-door-sticker/

Use Original size tires, if different than current, for the calculation

If you have 3.73 ratio and calculation shows "New Ratio", with larger tires, as say 4.14 then a 4.10 ratio(closest to 4.14) would make ratio 3.73 again using the larger tires, so speedo would be correct, lol, and driving and towing would be same as before the tire change

Problem with 4x4s is that you have to change BOTH front and rear ring and pinions when changing axle ratios