Stock 15T/45T to 15T/47T and stock 140/60-R17 to 140/70-R17

[AkasH5ky]

Hello there, I am from Bangladesh and I have a motorbike called "Suzuki Gixxer 155cc 2014".

My bike has a ground clearance of 160mm. Problem is I often get pounded at the bottom part of my bike in many uneven speed breaker in my country due to this very short ground clearance.
Many in our country has overcame this issue by simply upgrading the rear tire size from 140/60-R17 to 140/70-R17. But I know this will cause another problem, false reading in the speedometer.
There are very few resources in my country, so I can't calibrate the speedometer or make any complex customization to the speedometer to make the reading right. So I came with another plan.

My stock engine sprocket has 15T and drive sprocket has 45T. Stock rear tire is 140/60-R17. I have my "speed reader"(I don't know the exact name of it) at the engine sprocket.
So, for stock rear tire the overall diameter is 599.8mm with a circumference of 1884.327mm.
If I change the tire size from 140/60-R17 to 140/70-R17, then the overall diameter is 627.8mm with a circumference of 1972.292mm.

By the unitary rules--
• Circumference ---> Engine Sprocket(front) ---> Drive Sprocket(rear)
• 1884.327mm ---> 15T ---> 45T
• 1mm ---> 15T ---> (45T/1884.327mm)
• 1972.292mm ---> 15T ---> (45T X 1972.292mm)/1884.327mm = 47.100T ≈ 47T

I know the power distribution will be low. I don't care about the top speed either. Just want to make sure the speedometer read the correct distance I am travelling.
So, if I change the stock chain sprocket from 15T/45T to 15T/47T and stock rear tire from 140/60-R17 to 140/70-R17, will it show the correct reading in the speedometer?

[weasley]

It depends where the speedometer signal is taken from. If it’s from the front wheel then changing the sprockets will make no difference to the speedometer reading.
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Yamaha XJ600 | Yamaha YZF600R Thundercat | KTM 990 SMT | BMW F900XR TE

[stinkwheel]

It looks like your speedo drive is mounted on the front sprocket.

So yes. To within a very small margin of error, that will correct the reading (less than 1% error).

That said, we are only talking about a (roughly) 4.5% error anyway, which is probably below the acceptable margin of error on the speedo anyway.

So if you do nothing to the speedo and just change the tyre, it'll read 29mph when you are doing 30mph and 57 when you're doing 60mph. Speedos can have as much as a 10% over-read as standard anyway. That's before you take tyre wear into consideration.
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“Rule one: Always stick around for one more drink. That's when things happen. That's when you find out everything you want to know.”
I did the 2010 Round Britain Rally on my 350 Bullet. 89 landmarks, 3 months, 9,500 miles.

[AkasH5ky]