Gearing changes from tire size

[mwc]

Just picked up a set of 17s for the RMX, and I'm trying to figure out what to expect as far as how the change will affect the final drive ratio.

Is the change in tire size in percentage affect the final drive by the same percentage?

 

[kurth83]

I think you are on the right track, they are proportional as far as I know.

 

[mwc]

That makes sense to me but that doesn't always mean much :teeth

I dropped 10 teeth on the rear to make up for the tire size change and to gain some top end on top of that. We'll see how it does.

 

[bpw]

Don't forget to use actual tire diameter not wheel size.

 

[OldFatGuy]

I think you'll find that the rear tire diameter is going to be fairly close to the stock diameter. I think dropping 10 teeth on the rear sprocket may be a little excessive. But you'll have to try it to know for sure.

Does your bike measure speed from the front wheel? If so, your speedo will probably be 15-20% off (it will read fast).

 

[kurth83]

So are those tires bigger or smaller than stock ones?

 

[auntiebling]

You Need to share:
-Stock tire size
-new tire size
-Stock front/rear sprocket sizes (tooth count)
-new sprocket sizes

Without that info, pissing in the wind will be more informative

 

[mwc]

I think you'll find that the rear tire diameter is going to be fairly close to the stock diameter. I think dropping 10 teeth on the rear sprocket may be a little excessive. But you'll have to try it to know for sure.

Does your bike measure speed from the front wheel? If so, your speedo will probably be 15-20% off (it will read fast).

The tire is going to be a little more than an inch smaller than the stock, a little over 6% different. The gearing now is the same I have on my dirt only RM, 13/50. The fastest I've had the RMX is 65, and that's the top of 5th. 10 teeth might be too much but I definitely want some more top end for the occasional quick jump onto the freeway or whatever

Speedo is a Trailtech Vapor, I can adjust tire size so it'll read accurate.

So are those tires bigger or smaller than stock ones?

Smaller by just over an inch, from 100/100/18 to 150/50/17

You Need to share:
-Stock tire size
-new tire size
-Stock front/rear sprocket sizes (tooth count)
-new sprocket sizes

Without that info, pissing in the wind will be more informative

Going from 100/100/18 to 150/50/17, and from 13/50 to 13/40, but I'm not looking for my numbers, just asking if a change in wheel size in % is the same change in gearing in %

 

[auntiebling]

, just asking if a change in wheel size in % is the same change in gearing in %

Aaaah
Essentially Yes. A wheel is a direct driven gear. It usually cancels out of the equation since most gearing changes do not alter tire size

 

[mwc]

I know most gear changes don't alter tire size, but all rear tire diameter changes affect gearing and that's the info I'm after.

 

[bpw]

Measure diameter/radius of both tires, use that number to get circumference of each tire.

Then divide circumference of current tire by new tire. This should give you percent difference.

 

[mwc]

I know the % difference in the two sizes, but I'm not sure if that percentage is the same percentage as the gearing change resulting from the tire change.

 

[OldFatGuy]

I know the % difference in the two sizes, but I'm not sure if that percentage is the same percentage as the gearing change resulting from the tire change.

Yes, it is.

 

[kxmike]

most guys run a 14 or 15 front and around 41 rear so you'd be in the ballpark but might want to change the front too. My DRZ will do 105 at Thunderhill with 15/41 gearing.