Smiths PA gearing

[Howard Wright]

Hi All

Sorry another PA speedo question.

The speedo now has new balls in it’s bearings, it has been lightly oiled and works almost without a quivering needle ....... but it reads slow.  About 30% according to my mobile phone app.  I’ve verified this with my cordless drill that has a 1500 rev/min maximum speed.  Attached to the speedo it reads about 27mph.

The excellent chart by Ron Burchett suggests that the A7 speedo has 35mph at 12 O Clock on the dial, which mine has. The chart also states that there should be 2240 revolutions per mile.  If my calculations are right (and I’d welcome them being checked) that means 1500 r/min on the speedo input should read 40 mph. So it is about 30% under allowing for the drill not delivering max power.

I attach a photo of the back of the speedo.  Can anyone please identify if it is for an Austin Seven (44 to 9 diff and 19” wheels)? Or is it from a different car?

I guess the best option is a 1.3 to 1 gearbox? Simply refacing the dial will mean the odometer is out.

Cheers

Howard

[bob46320]

This is the one on my '32 RN saloon

[Howard Wright]

Thanks Bob

I wonder what the punch dots in the compass mean.  Perhaps it’s the speedo gearing?

Cheers

Howard

PS Bingo. The dots on your speedo read 2040. On mine it reads 4011. 20 mph would give 1337 revs /minute on mine and 40 mph gives 1500 revs on yours. Conversion box it is!

[Neil Morrison]

You perhaps already know the turns per mile that the speedo drive on the gearbox is giving but if not this website gives intructions for determining it,  ignore the magnetic ref. it applies to governor types as well.  There is also other useful info' elsewhere on the site.
www.magnetic-speedometer-repair.com/Howtocheck.php

Neil.

[Howard Wright]

Hi Neil

Thanks

I had seen this site and you are correct it is full of good info.  I’ve now established that my speedo is about 1.97 times slow so a 2 :1 correction box will only be 1.5% out.  Apparently most governor type speedos can be 10% out  .

Strange...almost exactly the same calculations going on on another thread!

Cheers

Howard