Tyre size Vs speedo reading?

**Damage88** · 25th May 2010, 15:20

I recently purchased a bike and I've noticed the speedo is out by 15-20%. The bikes a Hyosung gt250r which runs a digital speedo, I've emailed Hyo NZ and they say it cant be adjusted.

So, what I want to know is will changing the rear tyre to a larger size give me a more accurate reading, currently riding at 50kmh actual speed will read ~60kph.

The other thing is the tyre is still in really good nic, its a Pirelli Sport Demon 150 / 70 - 17 M/C 69H with about 5mm of tread all round and I have heard nothing bad about this tyre.

What would you do? Learn what reading equals what speed and keep the tyre? Get a new tyre/wheel? Or do something completely different?

Help would be appreciated,
Casey.

**slofox** · 25th May 2010, 15:24

Learn how what the dial says relates to reality. All standard speedos are "optimistic"...keeps the hoons happy y'know

**onearmedbandit** · 25th May 2010, 15:36

15-20% is high though. How have you measured it?

**Damage88** · 25th May 2010, 15:50

I've heard most bikes are out but seeing 120 when your ticking along at the speed limit is slightly nerve inducing

Originally Posted by onearmedbandit

15-20% is high though. How have you measured it?

I got the old man to drive in front of me at 50, 60 and 100k's so I knew what the actual speed was.

**SPP** · 25th May 2010, 15:55

Sounds about right
I had one of those. I had a Garmin and found the Speedo to be out 10kmh at 100kmh as well. You'll either have to live with it, install a Speedo healer, or play with sprockets.

I don't think it’s feasible to fix by changing tyres:
Stock: 150/70
Required tyre size for 10kmh at 100km with current gearing: 150/90 ... good luck with that.

You could do it with a sprocket change:
Stock: 14/46
Required sprockets for +10kmh at 100km: 15/45 (+1,-1)

If you ride m'ways a lot, then the bike could benefit for the higher gearing since you'd end up riding around at lower indicated speed/rpm to stay with traffic.

**Damage88** · 25th May 2010, 16:02

Ok, so a change of sprockets would help raise the actual speed toward the higher displayed speed, what else would that change? More/less top end low end?

**Ronin** · 25th May 2010, 16:03

Originally Posted by Damage88

I've heard most bikes are out but seeing 120 when your ticking along at the speed limit is slightly nerve inducing

I got the old man to drive in front of me at 50, 60 and 100k's so I knew what the actual speed was.

Without being smart, your assuming the other speedo was correct. My suzuki is around 10% high @ 100 k's

**Damage88** · 25th May 2010, 16:12

Originally Posted by Ronin

Without being smart, your assuming the other speedo was correct. My suzuki is around 10% high @ 100 k's

I can be pretty sure Its on or at least close enough to the actual reading not to matter. Its a late model Hilux so I'd expect it to be pretty accurate. I plan on taking my gps out though and seeing exactly where it sits.

**Ronin** · 25th May 2010, 16:18

Originally Posted by Damage88

I can be pretty sure Its on or at least close enough to the actual reading not to matter. Its a late model Hilux so I'd expect it to be pretty accurate. I plan on taking my gps out though and seeing exactly where it sits.

I can't find the threads but it has been done before on here and from what I can remember, most manufacturers allow for +/- 10% accuracy on speedos. If your serious about it, you can have it calibrated but the GPS is a good idea.

**slofox** · 25th May 2010, 16:28

Originally Posted by Damage88

I can be pretty sure Its on or at least close enough to the actual reading not to matter. Its a late model Hilux so I'd expect it to be pretty accurate. I plan on taking my gps out though and seeing exactly where it sits.

I wouldn't assume that. My late model Subaru wagon over-reads by 4% at 100km/hr - timed with stopwatch over measured 1km at steady speed. 104 on clock = 100 according to elapsed time...

I'd borrow a GPS if I was you...

**TimeOut** · 25th May 2010, 17:09

Originally Posted by slofox

I wouldn't assume that. My late model Subaru wagon over-reads by 4% at 100km/hr - timed with stopwatch over measured 1km at steady speed. 104 on clock = 100 according to elapsed time...

I'd borrow a GPS if I was you...

Yep new Nissan ute out 5% last Toyota 8%

Get a GPS

**FJRider** · 25th May 2010, 17:16

Most BIKE speedo's run off the FRONT wheel ... the rear wheel tyre size cannot change speedo readings. A new rear tyre effectivily gears it up though ... slightly ...

Just find out what it reads at (actual) 100 km/hr

**firefighter** · 25th May 2010, 17:17

Get a speedohealer. All you need to do is take it off when you get a new bike, yep you can take it with you, it's all plug and play. The calculator is all online and easy, even taking into account sprocket and tyre changes.

When you get a new bike, just buy a new harness and it's good to go. Easy.

They are'nt that expensive considering it should last forever really. Mines already been through two bikes and probably another one this year/soon.

**Damage88** · 25th May 2010, 19:15

Originally Posted by firefighter

Get a speedohealer. All you need to do is take it off when you get a new bike, yep you can take it with you, it's all plug and play. The calculator is all online and easy, even taking into account sprocket and tyre changes.

When you get a new bike, just buy a new harness and it's good to go. Easy.

They are'nt that expensive considering it should last forever really. Mines already been through two bikes and probably another one this year/soon.

Just googled them, seems like the perfect fix. Will have to check em out, thanks guys.