Smiths Speedo Calibration

[Jez Morton]

Can anyone using an electronic speedo confirm the following for me.

 

Rolling circumference of wheel (RHSC 15") 1791mm, giving 894.4rev/mile. (measured wheel centre to road for rolling radius of 285mm)

 

I have 3.92:1 diff, giving at prop 3521.7 revs/mile.

 

Speed tranducer 4 pulses/rev, total 14087 pulses/mile.

 

Firstly does this sound about right.

 

Second I am assuming the speedo cable matches prop revolutions 1:1 can someone confirm thanks.

 

Jez

[Snapperpaul]

Don,t now about the type 9 ( i have gone electronic with magnets on the prop and a compensation chart from the speedo supplier) but most gearboxes i have come across have a gear ratio wheel that goes in the gearbox then the cable plugs in to that, this gear ratio wheel allows for different diff ratios and wheel sizes, the propshaft rotation is the same it's when it gets to the diff and then to the wheels it all changes.

Email me your email i have a very usefull diff gearbox wheel size program ( Thanks again to Nigel)

What size tyres? 195/50/15

[Guest elwe]

The speedo cable does 1000/revs per mile. The speedo sender gear is matched to the diff ratio and wheel size.

 

Your logic seems about right, I haven't checked the numbers.

[Guest micky bigthumbs]

Jez anyone with a road angel or RA2 can check the speedo just stick it in the car the GPS gives a spot on reading ...I use mine as a speedo as the siera one is 7mph out

 

Mick

[Guest ady]

Has anybody fitted a GPS as there only speedo, would this pass an SVA? It would be a lot easyer than working out dif ratios etc.

[DAVE CASTLE]

No it wont pass SVA as the speedo is checked on a rolling

road so your car don`t move therefore your satilites

cant work.

Dave

[Guest elwe]

That seems to be more of a problem with the SVA test than a real reason why you can't use a GPS speedo. As far as I know you could have a 4 wheel drive system with limited slip/lock diffs that prevents the rear wheels from turning without the front wheels also turning. Or you could cause problems by going along with 10mm ground clearance so they can't drop the back wheels in the rolling road. I don't think either is illegal but I don't know what would happen. You haven't failed anything but they can't use their test equipment to pass you.

 

I think a GPS speedo would be considered inadequate because it won't work in tunnels or very built up places.

 

On a simular line I wonder what would happen if someone turned up for a driving test in a bike engined car without a reverse gear. I can't think of any of standard manovers that don't involve reverse. If you get out without the handbrake on a push I think they would have to fail you. If you let the car roll in a manover thats a fail. I can only think that the driver will have to stay seated and a passenger will have to provide reverse power. In a 2 seater that means the instructor! I might have to ask the DVLA.

 

Going back to the speedo, what if you modified the optical sensor from a computer mouse to look at the road and tell you your speed? Fit it near the non driven axel and the rolling road test won't work.

[Guest Alan_builder]

Why don't you phone Malcolm @ SVA testing station Yeading and ask?

 

 

I my RH

[Snapperpaul]

Don't know whether it would be legal on a car but i did see push bike speedo used on a low rider bike and a trike, it would be small, unobtrusive and removable.

Could be worth a try.

[Jez Morton]

Thanks for your thoughts guys.

 

Elwe,

1000 rev per mile on the speedo cable rings a bell, will make cal. easy 4000 pulses per mile. Only thing is do you have anything that could confirm this.

 

Thanks

Jez

[Guest elwe]

1000 rev per mile on the speedo cable rings a bell, will make cal. easy 4000 pulses per mile. Only thing is do you have anything that could confirm this.

Have a look at aftermarket mechnical speedos and try and find one that isn't 1000 revs/mile. I very much doubt you will.

 

For instance have a look at CAI who make Smiths Instruments guages: http://www.caigauge.com/Smiths_Classic_Instruments.htm. They state all their mechnical speedos are 1000 rev/mile.