proper tire diameter for exact speedometer calibration

[Chuck78]

In searching for aftermarket speedometer & tach, I've read that Suzuki's generally use 2240rpm=60mph @ speedo, & tachometers are 4:1 ratio.

I was wondering if all units generally have same front tire outer diameter, therefor all using the same speedometer gears in the hub?????

I have a 77 GS750B with a 19" front tire. I believe the stock size was a 3.25 X 19, which in an Avon AM26 would equate to a 26.1" diameter tire. I was wondering if anyone had a formula with the gearing (if not 1:1) that would tell us what size the speedos were calibrated precisely for. I am basing my tire selection somewhat on diameter closest to a 3.25x19 (110/90/18 for the extra width is closest in a wm4 18" rim). May go 120/80/18 in the avon if smaller is closer to stock Avons seem to run wider for the listed size than comparables, so I wanted to do more research.

Any formulas or tire/speedo specs???

[SqDancerLynn1]

usually the speedo is fast. to make it more accurate a larger tire is needed.. stock gs850 tire 100/90/19 going to 110/90/19 makes the speedo more accurate

[Big T]

Why not the 3.25 Avon and the 4.00 Avon?

Problem solved

[Chuck78]

He was saying that with the stock sizes, the speedometer still reads a little bit too fast

[Chuck78]

I assume that maybe not all GS speedo gears are the same? Do most models have a 19" front? My wife's drum rear 77 GS550 has a 16 in back and 18 in front. My 77 GS750B has 18"x2.15" rear disc wheel and 19"x1.85" front wheel. If those two bikes have the same speedo gear in the hub, then I can see they had very low tolerances on the speedo accuracy!

So a 26.1" or so diameter tire (Avon brand in stock front size 3.25x19) is a little smaller than what the speedo is actually calibrated for?

I have a Kenda 100/90/19 on front. I will make a point of it to drive past one of those radar clocking "slow down" mobile trailer signs on my GS750 next time I see one to see how close mine is.

Also, if you have lower air pressure or are a heavier rider, the tire will squish more, so the speedo will then read faster.

I have a few front tire options ranging from 25.8", 26.1", 26.4. 26.8, etc so I am trying to decide 18 or 19, more options in 18, but taller profile, so more squishiness in cornering. 19 would limit my width to something narrower but height would be lower profile.

[Steve]

I have checked the speeds on all five of our GSes with a GPS and found that they are remarkable ACCURATE.

All of them are running 100/90-19 tires on the front, but not all the same brand.

Can't remember which one is the least accurate, but seems that when the speedo reads 70, the GPS is showing 68.
All the other bikes are showing 69 or 70 on the GPS. Close enough for us.

My Wing, on the other hand, is also running a stock size tire. At an indicated 70, the GPS is showing 71.

.

[Guest]

Tires always slip. Always. Differences in road surface texture, inflation pressure, vehical speed, tire, road and air temperature, and cross winds all contribute to slippage. Getting within 1% requires tested engineering fudge factors.

Using a bicycle speedometer allows each rider to adjust the fudge factor to what works best for him.