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Okay I did not want to have to type this out and I still dont. But the idea that increasing wheel size doesn't effect gear ratio and only increasing tire size does is flat-out wrong. Anytime you alter the ratio of the circle of the ratio an ecm is suppose to spend it effects how it performs . This is why when sports cars get larger rims they decrease the size of the tire. Increasing wheel size or tire size has the same effect. If the new tire size + the new wheel size is greater than the old wheel size + the old tire size the gearing is gonna be off
Yes this is true - When Ford offers an 18 inch wheel you get 265/65/18 tires which are 31.6 inches and with the 20 inch wheel you get 275/55/20 which is 31.9 inches. Since they change the wheel size, they change the tire to accommodate and make the overall "circle" of the wheel & tire near the same height. Changing the wheel size is rather irrelevant to re-gearing, it's more focused on the overall tire height, 33's, 35's etc.
Okay I did not want to have to type this out and I still dont. But the idea that increasing wheel size doesn't effect gear ratio and only increasing tire size does is flat-out wrong. Anytime you alter the ratio of the circle of the ratio an ecm is suppose to spend it effects how it performs . This is why when sports cars get larger rims they decrease the size of the tire. Increasing wheel size or tire size has the same effect. If the new tire size + the new wheel size is greater than the old wheel size + the old tire size the gearing is gonna be off
When you say tire size, are you talking about the sidewall height? When I read "decrease the tire size", I think of a going to smaller diameter tire.
Example:
Which tire size is bigger? 265/75/16 or 265/70/17?
They are the same size. Both are 10.4" wide and 31.6" tall.
Bottom line is, determine what tire size you want run with the 20" wheels, then you'll know if a regear will be necessary (Unless you are going to 35s or bigger, or have very high axle gears, you probably won't need/want to regear)
When you say tire size, are you talking about the sidewall height? When I read "decrease the tire size", I think of a going to smaller diameter tire.
Example:
Which tire size is bigger? 265/75/16 or 265/70/17?
They are the same size. Both are 10.4" wide and 31.6" tall.
Bottom line is, determine what tire size you want run with the 20" wheels, then you'll know if a regear will be necessary (Unless you are going to 35s or bigger, or have very high axle gears, you probably won't need/want to regear)
Believe it or not this are not the same size tire. The radius of a tire actually increases the height as well. I know I didn't think so at first either but go here and just test this theory with any same size different rims example you like : https://tiresize.com/calculator/
but I do think we are misunderstanding each other tho. This is why I am saying changing the wheel equally effects the gears and size of the circle spinning . Example, if I go with 285 75 17s , that's a lot smaller than going with the same size on a bigger rim, say 285 75 20s ,
It's hard to believe but that's the way the ratio of the circle works
Believe it or not this are not the same size tire. The radius of a tire actually increases the height as well. I know I didn't think so at first either but go here and just test this theory with any same size different rims example you like : https://tiresize.com/calculator/
The sizes in my example are the same. 265/75/16 vs 265/70/17. Your screenshot shows a 265/75/16 vs a 265/75/17:
but I do think we are misunderstanding each other tho. This is why I am saying changing the wheel equally effects the gears and size of the circle spinning . Example, if I go with 285 75 17s , that's a lot smaller than going with the same size on a bigger rim, say 285 75 20s ,
It's apparent that you understand the larger tire is ultimately what's important. It seems to me like the confusion/misunderstanding is coming from what we each consider the "size" of the tire. I personally wouldn't call a 285/75/20 the "same size on a bigger rim" than a 285/75/17. To me, saying the it's the same size but on a bigger rim implies the overall tire height/width are the same between the two (i.e. a 33x12.50x17 vs a 33x12.50x20).
But you still can't make the generalization that a larger wheel always equals a taller diameter tire. Just think of a car tire on 20" wheels vs a truck tire on a 17" wheel.
So, in an effort to circle back to original question/concern...are you trying to figure out if you need to regear for your new (or soon to be) wheels/tires?
Last edited by jp360cj; 06-21-2017 at 04:34 PM.
Reason: added size comparison pic
The sizes in my example are the same. 265/75/16 vs 265/70/17. Your screenshot shows a 265/75/16 vs a 265/75/17:
It's apparent that you understand the larger tire is ultimately what's important. It seems to me like the confusion/misunderstanding is coming from what we each consider the "size" of the tire. I personally wouldn't call a 285/75/20 the "same size on a bigger rim" than a 285/75/17. To me, saying the it's the same size but on a bigger rim implies the overall tire height/width are the same between the two (i.e. a 33x12.50x17 vs a 33x12.50x20).
But you still can't make the generalization that a larger wheel always equals a taller diameter tire. Just think of a car tire on 20" wheels vs a truck tire on a 17" wheel.
So, in an effort to circle back to original question/concern...are you trying to figure out if you need to regear for your new (or soon to be) wheels/tires?
Well I was gonna run 35s until I heard alot of people on here saying it turned their truck into a slug and/or blew their transmission. If that's not true please preach to me brother,, so I was gonna settle for 33s if I knew that that would be okay with the current gears. However if it gonna break it anyway than f it bring the 35s on...
If you go from 18" wheels to 20" wheels, you down size the sidewall height, the middle portion, and adjust accordingly the width, the first number, which is the ratio.
So if yours came stock with 18" wheels, the stock tire size is 265/60R18, with an overall diameter of 30.5", to upgrade to 20" wheels, the tire size would be 275/55R20, with an overall diameter of 30.8".
Regardless of gearing, the difference of .3 diameter is hardly noticed, maybe 1 or 2 MPH difference. 18" is 681 RPM while the 20" is 674 RPM.
Now if you are NOT going stock size and put larger tires on 20 " rims, then gearing will come into play. If you are running 3.73 gears, then the effective gearing gets taller, down to say 3.55 gears, so if you are looking to put larger tires on, and plan to tow/haul weight, then a regear to shorter gears, 4.10 or lower is called for.
As far as tearing the gears or trans apart, thats a myth, what could cause trans/gear damage is putting a larger heavy tire on and hot rodding it. Spinning the tires from a dead stop, hopping the rear end, etc, can tear the gears and trans apart, but normal driving wont.
If you are going oversize on tires and want to also maintain low gears for towing, use a gear calculator to find out what the new ratio should be.
06-21-2017, 11:25 AM
