I got a new rear tire today, size 140/70/17. Feels a lot different! I think it's looks nicer, too :icon_mrgreen: My front tire is a 110/80/17.
With my old rear tire, a 130/70/17, and a 15T sprocket, I would be doing about 6.8K rpm at (an indicated) 70mph. With my new tire it says about 6.1K @ 70mph. The change in RPMs in relation to speed make it feel a lot different to me, but I'm wondering how much of a change there actually was, or if the new tire sizes have my gauges all out of whack. I have no idea how to go about doing the math for this, so can any gurus on here enlighten me??
The speedometer runs off of the front tire. It will only be affected by changing the front tire size.
measure the diameter of the FRONT tire and compare it to a STOCK front tire
simple math new tire diameter / old tire diam * indicated speed = not very much
and you think a speedo is accurate..... :o
There is a gearing calculator on www.evilpumpkinracing.com (http://www.evilpumpkinracing.com)
Since changing the tire diameter has the same effect as changing sprocket sizes, A larger rear tire is basically the same as a smaller rear sprocket or a larger front sprocket. Play with that spreadsheet you will understand.
Ah shoot... So putting on the bigger tire has slowed me down a bit, then, eh? Well... It's not too much bigger. At least I'll be getting a bit better mileage now.
Tires all vary a little, but comparing the BT45's for example; the diameters are 110/70 = 23.2, the 110/80 = 24.0, the 130/70 = 24.4, and the 140/70 = 24.7".
For the speedo, at the same actual road speed an indicated 70 mph would become 23.2/24.0 x 70 = 67.8 mph. The GS speedo is actually a little more accurate with the 80 profile front tires.
For the tach reading a 6800 rpm reading at 70 mph should drop to about 24.4/24.7 x 6800 = 6717 at the same actual speed.
Actually a stock GS should be turning about 4700 rpm at 60 mph which would be about 5480 at 70 mph. Change to a 15T would give 5480 x 16/15 = 5850 approx at 70 mph. Correcting for a 10% speedo error would come close to your 6.8k figure considering there is also a possible tach error of maybe 10% to consider.
Your 6.8k to 6.1k rpm change for the 130/70 to 140/70 tire change seems a bit more than it should be. Of course the 130 was worn out and the 140 new and your actual new 140 may be larger than a BT45 in dia. Complicated isn't it? :laugh:
Hey, that's the tire I Got, a BT45. I don't remember the front brand, but I remember seeing "BT45R" on the sidewall and thinking "Heyyy, they talk about that brand on GS twins... Cool!"
So my gauges are pretty much estimates, eh? That's not too comforting.
But, it can get you out of a lot of tickets...except fix-it tickets. :icon_mrgreen:
That had occured to me :icon_twisted: I usually just go only slightly faster than all the cages :icon_mrgreen: