Speedo calibration

[Watsonic]

Hi Guys

Thanks for all you help in previous topics  This next call for help is doing my head in 

I 'm running an 18" wheel and speedo drive from an old 'G' Reg Suzuki GSXR750 with 120 90 18 rubber.....& wait for it..... A speedo from a 535 virago cos its chrome and looks good.
The overall diameter of the 120 90 18 tyre is 674mm
The overall diameter of the Virago tyre which was a 19" was 641mm
A difference of 33mm about 1 1/4 "
Any ideas if this will make the speedo faster or slower, & if it is greatly inaccurate how can it be rectified
Please help...I'm pulling my hair out

[toad]

in theory it should read slower. there is somthing on the market called a speed healer not sure if it will be of any use? bit like a kmh to mph converter failing that get someone to set a set speed then mark the speedo glass

[tbone]

or have a word with draken1 and see if you can strike a deal on a new lcd speedo

[Manky Monkey]

"Speedy Cables" -their link's in the Links for Commercial Purposes bit of the forum. Give them the relevant info & they'll make you a recalibration unit. I've got to get one for Taz's trike as the speedo will be driven off the gearbox & it'll have much bigger rear tyres than standard. Last time I spoke to them they were about 30 quid. It's a reworked Tachograph unit & is about the size of a matchbox. Contains a couple of cogs, which, in the right combination, will slow down or speed up the cable rotation. They'll also make up any other cable you need to any spec you need. 

[Watsonic]

Spoken to the team at Speedy Cables they were unable to help, They put me in touch with a guy who deals in this sort of thing but only on Brit Bikes. However he did suggest using Sat Nav as a bench mark to check how far out my speedo is as that sat nav gives an indication of speed.

[TwistedPatience]

Use an electronic bicycle speedo good enough for up to about 70MPH and cheap too.

[Manky Monkey]

Speedy Cables unable to help? That's unusual.
Yeah, loads of peole use bicycle speedos. Only problem is that they're supposed to be illuminated for the MOT. Few are. Depends how dilligent your tester is I guess.

[TwistedPatience]

Ok for calibration tho' 

[Manky Monkey]

True. You type in the circumference of your wheel & it does the maths for you.

[Watsonic]

Bought a Cycle speedo and I've fitted it, Works a treat thanks for the idea

[TwistedPatience]

Your welcome