Gauge accuracy with non-OE size tires....

[Queso]

I got a new rear tire today, size 140/70/17. Feels a lot different! I think it's looks nicer, too  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??

[Egaeus]

The speedometer runs off of the front tire.  It will only be affected by changing the front tire size.

[werase643]

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.....

[dgyver]

There is a gearing calculator on 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.

[Queso]

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.

[runsilent]

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?   

[Queso]

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.

[scratch]

But, it can get you out of a lot of tickets...except fix-it tickets. 

[Queso]

That had occured to me  I usually just go only slightly faster than all the cages