mph to revs cb750

[Bitsa (Ralph Wright - RIP)]

Hi
Can someone tell me with standard gearing in 5Th gear what mph is at 4000 revs is?
I am messing about with clocks both kph and mph.Say at 50 mph the wind is whistling around my head but at 90 kph its nothing at all.I know that 70 mph is 110 kph
but it all seems strange to me.You continental bikers should be able to help me on this.I have been through a stationary radar trap no photos type and at 50 kph seems fine but as revs go up all my speedos are like me bloody slow.
Cheers
Bitsa

[Green1]

Good Question Bitsa been meaning to ask the very same thing for ages.
My speedo has been out of whack ever since it's bump on the nose.
I think 30mph is 2000rpm and 70mph is 4000rpm my speedo reads about 10mph fast as far as I can tell and jumps like a good en.
   

[fisjon]

My CB750K1 came on the cam at 5300 revs, exactly at 70mph.
Quite revy in contrast with my GT750K which purred at 90mph at only 4000 revs, mind you it did come with a 'chips' highway patrol front sprocket, (one extra tooth).

[Lobo]

I guess Bitsa, if you want the most accurate calibration grab a small GPS, a straight road & constant mph to check / calibrate your speedo through the various speed ranges.
Just a thought...., and yet another excuse to take it out!
Lobo

[steff750]

 

    1st Gear Ratio   2.5/1     2nd Gear Ratio   1.708/1        3rd Gear Ratio 1.333/1    4th Gear Ratio 1.097        5th Gear Ratio   0.939

the gearing is 18 teeth by 48 teeth and shows that in 1st gear the front sprocket is going two and a half revolutions to one at the rear sprocket and 5th gear is more like an overdrive so on my bikes the speedo and rpm rise together at the same rate meaning that in top gear they are in exactly the same positions on both gauges I think its about  70 mph at 4000 rpm and the top speed of 125 mph should be at 8500 rpm in theory anyway

[Bitsa (Ralph Wright - RIP)]

Thanks Guys,
I have checked the speedo against non flashing cameras and it seems to be about right up to 50k at least.I have a car garmin if I try that, that should do it or am I wrong?
Cheers
Bitsa

[Lobo]

Any modern GPS will be more accurate than a '70s speedo.... assuming you give it a few seconds at a constant mph/kph (or revs) to settle down to a true reading. And best of all you won't have to worry about whether your bike has standard wheels, gearings, inaccurate rev counter....or '"doin' the math" etc.
Cheers,
Lobo