I agree with your findings.

I have played around with several different combinations of sprockets for the Z and the stock final drive ratio seems to be the best.

I have tried -1, -1 +1, -1 +2, +2 and +1.


Every combination, while they "felt" like it was accelerating faster, they weren't. They accelerated at about the same rate as a stock gear bike. Just with a lower top speed.

On big bikes with a lot more hp going -1 was an amazing increase in acceleration that was evident when going against a stock geared bike. But with the Z, it made no difference besides decreasing the top speed.

 

Most people don't really talk about their real reason for changing gearing - they want to wheelie easier.

Over the years I've come to hate bigger rear sprockets or smaller fronts. Gains in speed are offset by the need to shift more often.

 

Bottom Line ..................

If your a newbie to bikes or want to save yourself some time and money and are not looking to get involved with all the talk about which sprockets are best and are just looking for the bottom line.

It looks like the bottom line for best performance with a totally stock bike is to keep the stock sprockets as well as the with BBK builds

But everyone can have different needs out of there bike that can be very different one person to the next .

If you need more bottom end (lower gearing like -1/+2 ) due to using your bike for stunts and wheeling or needing to pull more of a load due to being heavier rider and/or carrying a passenger , or typically riding on steep mountains roads . Then your particular needs outweighs the loss from the best performance range .
.
If you need more Top end (higher gearing like +1/nc or nc/-3) due to using your bike more for commuting at higher speeds and wanting better gas mileage . Then again your particular needs outweighs the loss from the best performance range . but it also looks like due to the small HP engine it might not go any faster or climb any better due to the loss from the best performance range unless your on flat ground and a lighter rider .but it should always get better gas mileage on flat ground if your not pushing it hard.
.
So Again, this is just a simple guide line , and just my .02

 

I went with a 15 tooth front on my Z and even with 143 big bore kit with head and cam you could feel the loss of power up hills so I went back to the stock 14.

 

.
I see it's been 6 months but with the 15T and all your engine mod's do you remember if it increase top end speed 5-10 mphs on Flat Ground, no wind, perfect conditions .
.
And/Or anyone here with just the 15T and the BBK's ?
.

 

Top End Results

 

Hey Kawee I went back to the 15tooth sprocket about a month ago just to drop the revs a little and It doesn't
Go as good up hills but it's not like it's bad.
The bike still hits the rev limiter in all gears but as for what speed I'm doing in top hitting the rev limiter I haven't even checked it.
But I'm supposed to be going for a ride with my son on the weekend so I will check it out and report back.

 

We didn't get to go riding on the week end DOH! But soon I hope 😀

 

good read... I've been running a 15t front on my stock z, and while 4th is more lethargic, it gives me a few more mph in the lower gears and I can always click down to 3rd for inclines, and keep up with traffic.
My GPS indicates 68.6mph on the limiter, but I have to get in a full tuck to get there on level ground.

 

Thinking I should put this "Speedometer reading errors n corrections" link here on this thread due to talking about earlier at post #7 and is related to the same subject
.
https://www.z125owners.com/forum/ka...aki-z125-general-discussion/14058-speedometer-reading-errors-n-corrections.html
.

Wanting to know what difference my 15t front sprocket made on my speedometer, I re-searched Goggle and YouTube, and this seems to be the average errors for stock , and the most common gear ratio changes
.

Z 125 factory speedometer error with "stock tire sizes"

With the stock 14t front and a 30t rear sprocket
the speedometer will read +3 mph faster than your really going.
.
With a 13t front and a 30t rear sprocket (-1/nc)
the speedometer will read +6 mph faster than your really going.
.
With a 13t front and a 32t rear (-1/+2) sprocket
the speedometer will read +12 mph faster than you are really going.
.
With a 15t front and a 30t rear sprockets (+1/nc)
the speedometer will read "the actual speed your really going"
Due to the stock setting is purposely set high.
.
The stock speedometer reading is purposely set to read higher than you are actually going because it helps prevents car and motorcycle manufacturers from lawsuits. this keeps you a little under the speed limit so you cant say the vehicles speedometer is off and that's why you got a ticket.
.
"GPS isn't perfect, but it's a lot more accurate than a vehicle speedometer,
.
So the best way to know your actual speed is to use a phone/gps against your actual speedometer reading and compensate according next time you ride, so you dont have to mount your expensive phone out where it is excess able to damage


just my .02 again / anyone with different actual results post'em up

 

Stock gearing and a 120/80 rear tire, the speedometer will show actual speed.

 

Kind of an old thread but I thought I’d share some findings with my new Z’s. Both stock (2018 and 2019’s) showed 2mph faster on the speedo vs actual GPS speed. Totally stock.
With a 130/70 on the rear of my 2019 it’s 1mph fast still
With a 140/70 rear on the 2018 it’s now accurate with GPS.

Both seem to have about the same top speed (i have exhaust on both now). I’ve noticed that as it gets colder here in the evening with the air being so dense they top out about 55-56mph around 50 degrees with high humidity. That’s flat land, no tuck, mild wind. With it warmer during the day I can hit 60mph in the same run.

Now get some cars in front of you and it’ll run right up to the limiter. I did a GPS today and hit 65mph, no tuck, 130/70 rear tire with a bit of light traffic ahead of me to break up some of the wind.

 

I have personally found that GPS using an iPhone app is itself not at all reliable - even keeping the bike at a steady speedometer speed and engine rpm, the GPS indicated speed varies by up to 4 kph = 2.5 mph.

A university study on GPS accuracy that I have mentioned before found that GPS is NOT accurate enough to be a comparison standard, varying by several percent at normal speeds, and becoming as bad as over 20% off at low speeds, especially when the road is not dead straight. The study is old now, but there have been no more recent studies, and the basic math behind GPS has not changed.

My own limited testing seems to consistently show that 55 to 58 true mph is the REAL top speed for an untucked average to larger rider on a stock Z125, if the road is TRULY flat, there is no appreciable wind (tail or headwind), and no drafting effects via other vehicles in close proximity. But top speed also falls very dramatically, and increases dramatically, on uphills and downhills respectively, because of the awful weight to power ratio for a stock Z125.

Jim G

 

Increasing Top Speed Options

Unlike most automobiles and because we have a top speed rev limiter point you can add all the aftermarket horsepower parts you want and it will only get you to the same rev limiter point faster, You will still top out at the same mph's with the Stock gearing, Stock size rear tire, and Stock rev limiter setting... 63 to 64mph is the actual GPS top speed the bike will go
(stock speedometer reads +3mph off)

To raise your top speed with stock gearing you can use a taller rear tire and/or raise the rev limiter by using fuel controllers that have raising the rev limit capabilities
"Provided" you have enough horsepower to still hit the rev limiter in 4th gear in most all riding conditions

Or you can start raising your top speed with taller gearing
"Provided" you keep adding enough horsepower so you can still hit the rev limiter in 4th gear in most all riding conditions

You can also do any combination of any/all of the above.
''Provided'' you have enough $$$$
.