Yamaha TRX850 Enthusiasts Forum

Coming onto the back straight speedo's reading 140kph and braking at 190kph but this now is with a 120/70 17 dunlop slick.
I looked at the gear commander but that doesn't give you speedo difference on the front rim with tyre profiles..
Anyone got an idea of speedo reading difference on average between a 60 and 70 profile.
Obviously I'm doin more than 140 and 190 kph
Hignsight should have checked the taco and worked it from there.

I was reading on the speedo 140kph exiting turn 2 onto the back straight and 190kph before braking at the end of the straight. This was with a 120/70 dunlop slick not the std 120/60 .
Does anyone know the difference between 60 and 70 profile to calculate the diffence in the speedo reading.
I looked at the gear commander but that's only on the rear not the front where the speedo works from. In hignsight i should have read the rev counter and worked it from the gear commander.
I know I was doin more than 140 and 190 kph

Hi Cobba

A 60 tyre is 60% 0f it's width tall, it's called the aspect ratio. So a 120/60 would be about 72mm in profile.

Similarly, a 70 tyre is 70% of it's width or about 84mm in profile.

I don't know how much this varies from maker to maker or profile to profile or even tyre pressures and rim size, but I believe that's the basic formula.

Cheers, Andy.

I was hoping to find out maybe a common speed...100kph and the rpm difference after someone has changed from std 60 to a 70 profile and the gearing they are running.
Either that or two trx's riding beside each other with different profiles at a common speed and the difference in reading..
What about a digitial speedo??? do you have to dial in the tyre profile??? if so if you dialed in a 70 profile size even with a 60 fitted ,would this give the answer????

cobbadiggabuddyblooo wrote:Coming onto the back straight speedo's reading 140kph and braking at 190kph but this now is with a 120/70 17 dunlop slick.
I looked at the gear commander but that doesn't give you speedo difference on the front rim with tyre profiles..
Anyone got an idea of speedo reading difference on average between a 60 and 70 profile.
Obviously I'm doin more than 140 and 190 kph
Hignsight should have checked the taco and worked it from there.

http://pw1.netcom.com/~sgalaba/tiresize.htm#ustires

Cobba,
I'm very tired(nodded off several times reading posts) so I hope this makes sense the way I write it.

To calculate the difference accurately you need to measure the roll out of the two tyres in question. This will give you the tyre's development. Then you can see the difference in development from the 120/60 to the 120/70 and you can calculate the speed adjustment.

eg.
Say tyre 1 is you oem fitment, hence your control, as you know it to be correct.
(using simple numbers not specific tyre measures to make the example easy)
If tyre 1 (OEM sizing calibrated to speedo) had a roll out of 100cm and then tyre 2 has a roll out of say 110cm then the difference in development is +10%.
So if you were riding tyre 2 and the oem speedo said 100km/h you would actually be doing 110km/h.

I think I have both sized tyres in the shed, I'll go and do a quick measure for more realistic figures.

ok, have woken up a bit now thanks to the cold air outside
120/60 development of 1810mm.
120/70 development of 1860mm.

Differential of roughly + 2.8%.
When I ran a 120/70 front with oem speedo drive, I had had a tyre boffin run the calculation for me and the result was the same. They have software for this so that they don't fit a tyre more than a certain % outside of oem spec and be liable for speedo variations.

Keep in mind manufacturers differences as well as profiles and tyre pressures could make a bigger difference between tyres, hence the need to measure yours.

I've merged two threads together. Cobba, your first posts are almost identical but I've left them both as they are.

If you're speedo's is anything like mine, you'll need an indicated 110 or so for a true 100km/h...

So the higher profile would 2.8% higher still?

Cheers Stu.. The 70 series slick actually has a higher centre profile than a road tyre so I'll roll that out and measure it and I have the std michelin 60 series road tyres on other rims . I'm pretty sure my speedo's on the money if previous fines are any indication

And thanks Jak..

Hi Cobba,
easy job is to tape a tom-tom (or even your fancy phone with google maps or something) to your dash and calibrate off the GPS. Very accurate, very cheap.
Wear on the back tyre and gearing makes a difference to the tacho/ground speed.

Tom tom I aint no drummer mate.. (bit hard when the 70 series is a slick) and $50 telstra phones for me.Good idea tho bagmaker .
I've measured the two tyres on the rims, 1805 and 1875mm.... so time to pull out pen and paper and use the brain, work out a % and bingo

cobbadiggabuddyblooo wrote:I've measured the two tyres on the rims, 1805 and 1875mm.... so time to pull out pen and paper and use the brain, work out a % and bingo

That'll be +3.88% if the pen and paper 'aint working mate .
That will give you the correct figure vs your indicated speed.
Remember though, for absolute correct speed you would have to know the calibration variance of your speedo as oem they are allowed to be +/- 10% but usually over read to avoid liability claims for speeding issues. If you get the opportunity though check it by GPS as bagmaker suggested, much easier than any further calculations.
Just by chance I happened to GPS check the correctness of my wife's car speedo today. At an indicated speed of 100km/h, GPS said 94 km/h.

3.8781163434903045%

so 140kmp + 5.3018kph= 145+kph
190kph + 7.1953kph= 197+kph
I know I was still braking early and as Stank also noticed I was on the throttle early and still not scrubbing down to the full potentual of the tyres.. After inspecting the boots and toescrapers worn away I have to change my style a little more to get a little more lean angle....

cobbadiggabuddyblooo wrote: so time to pull out pen and paper and use the brain, work out a % and bingo
3.8781163434903045%

so 140kmp + 5.3018kph= 145+kph
190kph + 7.1953kph= 197+kph

That's impressive maths for pen and paper. You sure you didn't have a calculator?