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Tire ratio effecting speedo reading?

chuck hahn

chuck hahn

Forum LongTimer
Past Site Supporter
Currenty have a 100/90 on the front of the Cooley and I passed one of those radar booth things at a construction site and the reading said I was doing 30..but the speedo said 35.

So this got me to thinking that would this be to the tires circumference throwing the speedo off?? If so, what would the correct tire be to make it all read relatively close to right????
 
If my thinking is right, a smaller tire would spin the hub faster, thus making the needle read above actual speed..right? Maybe the speedo is just that far off and I gotta adjust.
 
Glad to hear you are back in the saddle ! Maybe you just need "go fast air" in those tires.
 
Chuck, what bike are you asking about and does your phone have GPS or do you have a navigation device of some sort?

Compare the GPS reading to the indicated speed on the speedometer, over a range of vehicle speeds, and let me know what they are.
 
My experience has been that those portable radar speed signs are almost never correct. I think they're supposed to be calibrated from time to time or something. I've seen them 0-5 mph off in either direction, doesn't seem to be any consistency.
 
No got a GPS available Dale. If i recall correctly the stock tire is a 100/90 and the back is a 120/90??? The speed booth thing got my eye and now my curiosity going.
 
Not sure about your 1000, but the smaller bikes came with 90/90-19s on the front and were rather optimistic on the speedo readings. I have put 100/90s on all our bikes and have gotten the readings quite a bit closer to actual speeds. Now, an indicated 60 shows up as 57 to 59 on my GPS, depending on the bike. That little bit of error I can live with. :o

In order to slow down your speedo, you would have to fit a 110/90, not sure if that would fit. :-k

.
 
But on say a 100/90 -19 tire isn't the 100 the width? Why would increasing the width (say from a 90/90-19 to a 100/90-19) affect the speedo?
 
70% is sidewall and 30% is tread width. So the 100 would be slightly taller/larger diameter than a 90 tire.
 
the second number "90" is the aspect ratio, or percentage of the width. so 90% of 100 would be greater than 90% of 90, which is the height, or overall circumference of the tyre.
 
greasyjonny said:
But on say a 100/90 -19 tire isn't the 100 the width? Why would increasing the width (say from a 90/90-19 to a 100/90-19) affect the speedo?
Click to expand...
Because a wider tire is also a taller tire. A taller tire will roll a bit farther on each revolution, which will slow down the speedo.


GSX1000E said:
70% is sidewall and 30% is tread width. So the 100 would be slightly taller/larger diameter than a 90 tire.
Click to expand...
I agree with the 100 being larger than the 90, but where the heck did that 70/30 thing come from? :-k

.
 
Ok I got it. Makes sense, I knew increasing the diameter would affect it but I was ignorant to he fact that increasing width also increased the height. Maybe i should have bought those 100-90/19 Avons instead of the 90/90s.
 
Steve said:
Because a wider tire is also a taller tire. A taller tire will roll a bit farther on each revolution, which will slow down the speedo.

I agree with the 100 being larger than the 90, but where the heck did that 70/30 thing come from? :-k
.
Click to expand...

Ask themess (sp?) he worked at making tires.
 
Hey Chuck .
The speedo on my 'S reads a bit "fast".
I got Deb to run at 100kph in our car (a reasonably late model Toyota) on a straight bit of road and checked my speedo against that . It was reading 110kph .
I use that and the tacho (4,500 rpm @ 100kph in top gear) to get my speed right .
Surprisingly , my odometer reads fairly accurately .
It didn't freak me out enough to pull my speedo apart to get it perzactly correct , I just adjusted me .

Don't spin out mate , it's not a perfect world (and working out tyre sizes and $hit will do your head in).

Cheers , Simon .
 
Im not freaked about it Simon. I just want to know how much off it is so I can also adjust myself to what it reads. Dang ticket is not what I need. If its 5 over then i can adjust from that point.
 
simon kuether said:
Hey Chuck. ... I got Deb to run at 100kph in our car (a reasonably late model Toyota) on a straight bit of road and checked my speedo against that. ...
Click to expand...
I still would not use that as a 'true' speed. :-k

We have two Toyotas that we consider to be "reasonably late model" (they are only 9 and 12 years old). When compared to the speed shown on a GPS, her indicated 70 mph is actually 67 on the GPS. When mine is showing 70, the GPS is, too.

Yes, sometimes they are correct, but other times they are not.

By the way, both of them are still wearing original-size tires, so that is not a problem.

.
 
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