​​Hello,

I've been trying my hardest to research and gather the information but I just can't seem to figure it out. I recently purchased a 2018 f350 with 3.73 gearing. I am looking to change my tires as the dealership tires are useless. The truck came stock with 275/65/r18 I'm trying to decide between 275/70 or 285/75. With the 285/75 compared to the 275/65/r20 stock option there is less than a full inch difference in total diameter. Why does everything point to me having a way reduced gear ratio keeping the 18 inch rim whereas the 20 inch rim stays at 3.73???

Thank you for anyone that takes the time to enlighten me as I would prefer the 285/75 to close some of the fender gap.

Cheers,

 

Just to clarify semantics, changing tires or wheels does not really change gear ratios, but does change the final drive ratio at the tire-to-surface contact point. Forget the wheel diameter, all the math is based on the rolling diameter of the tire. A 36" rolling diameter on a 18" wheel is the same as a 36" rolling diameter on an 20" wheel. By going with a smaller tire diameter you are effectively lowering your final drive ratio (like changing to a lower ring/pinion ratio). Say a tire is 36" in rolling diameter and you go 1" shorter, then you are changing final drive by 1/36th, or 2.8%. So a 3.73 geared truck would feel more like a 3.83 gear. And a 1" taller tire is like a 3.62 ratio. Not sure that is necessarily a "way reduced" ratio.

 

Make sure you are comparing the same things.

Tire heights (approximate, will vary slightly with tire brand and pressure):
275/65/18 - 32.07"
275/65/20 - 34.07" (6.2% larger)
285/75/18 - 34.83" (8.6% larger than 275/65/18, 2.2% larger than 275/65/20)

So a 285/75/18 tire would be a significant effective final drive change vs your stock 18s (It would feel the same as if you kept the 275/65/18 and swapped the gears to 3.43), but fairly close to how it would drive with stock 20s. So the 'way reduced gear ratio' calculations that you are encountering may be because you are comparing to your stock size, but the actual driveability would be within ~2% of a truck equipped with the 20s. Since Ford sells trucks that way and people don't complain about the 3.73 gears with the 20s, I'd say you are fine going to a 285.

 

Thank you very much guys, you have given me the info I need in the first 2posts, very appreciated

 

If Fords gear is figured with the 245/75/17 base tire, than my 275/70/18 "effective" ratio is 3.54. 265/70/18 is 3.60, 285/75/18 would be 3.37,

 

 

I can tell you that my truck came with the 275/65r18's and I tried the OEM 20's with the 275/65r20 and they really took the power away and I have the 4.30's. I ended up going back to the stock size because I also lost 2 mpg's on average. It just was not wort it for me personally. I am in CO so the altitude really doesn't help either.

 

 

 

My speed is spot on verified with GPS with the 275/65r18's