got a quick gearing question

Okay so my bike has a 150/60 17 on the rear.....I believe this isnt stock...stock is 140/70 17 right? okay

how will this throw off my speedo.....?
will my speedo read high or read low?

also I just picked up a 12T front sprocket, havnt installed it yet.

any one running this set up? still have the stock 43T in the back.
from your guys experience how far positive or negative will my speedo read with this set up?


I will be getting a speedohealer soon, when i have more money!! hahaha, spent all my money on the bike !!FML!!

thanks mucho

Evan

I don't think just the rear tire change will have a big effect on speedometer reading.
The smaller front sprocket will, as the speedometer will then read faster than actual speed.

On the 2008 WR-250X I had, I used that size rear tire (because that was the smallest Michelin Pilot Powers came in) along with a 13/49 sprocket setup.
Bike was great like this for sport riding, and still did 80 mph and cruised at 60 mph just fine.

A speedo healer-type of device was definately needed - I used the 12o'clock labs unit.

http://www.gearingcommander.com/ <-- go here


You can tag in sprocket combinations and tire sizes and see what that does for your speeds.

You can also use the calculator on the 12 o'clock labs web site for finding out how much the effects of each have on your speedo readings.

Okay thanks guys!

Ok guys - so at present that 150/60 rear tyre is just noticably smaller in overall circumferance than the stock size, almost 10mm. So obviously this has the effect of slightly lowering the final gear ratio. At the calculated rate it has the same effect on the final drive sprocket ratio of approx 1 tooth bigger on the rear. So basically just with the change in tyre size alone would give the equivalent ratio of 13/44. ( I know as i've just gone thru all the calculations for my WRR to go from the stock 120/80/18 rear to a 130/80 rear. This has the same difference in overall diameter as per your 17inch set up - only my change will be to a slightly taller tyre, so it will have the effect of gearing up to the equivelent of one tooth on the rear ).


Ok, so now add in the new smaller 12t front sprocket and with your current tyre size & standard rear 43t rear sprocket, overall this final gear ratio would be approx the same as a 13/47 set up. So quite a decent % in terms of a lower final drive ratio.


So in regards to the displayed speed with your current set up - basically it will be so far from reality that by looking at the speedo you will believe that you have the fastest 250cc 4s D/S in the world, lol.... Many say that the stock speedo is approx 7~10% too optimistic - but in my case my speedo in stock form was almost 15% too generous, doh ! ( All GPS tested & confirmed ).


In short there is no point in worrying about what the sprocket / tyre size changes will make to your speedo UNTIL, as others here have also advised, you fit a speedo correction unit. I have the 12 o'clock labs unit & can highly recommend it for price, quality, adjustability & ease of use.....


Have you had a few runs to peg WOT long enough to see what your best MAX displayed speed was ? If so please let us know what the magic number was, and we can then further tell you by how much your current speedo lies,


Cheers, Gavin.

On my speedo today to work I pegged 107 lmao

150/60 tire 12/43 gearing

Ok - 107 mph hey, .....so in reality with your gearing & tyre size i'd suspect that a genuine 85 +/- mph would be about tops....

Anyway others here with the WRX version & similar gearing + calibrated speedo / GPS may be able to chime in with what their MAX speed is.

So apart from a false sense of speed, you are also actually clocking excessive "extra" miles onto your ODO.... I'd highly recommend to fit a speedo correction unit - but 1st take a photo of your tripple figure display, cause you will never see those numbers again,

Cheers, Gavin.

Oh yeah next pay check I'm buying one ! Lol

^^ Good plan mate - once dialed in, there's nothing better than knowing that your speedo display is proberly way more accurate than that of the cop's radar unit that's zooming in on ya,

Haha yeah it will be nice :)