How to find your Diff ratio with a GPS

From the previous post you may have noticed the dead bike speedo.   I have ordered a replacement VDO A4 and while on the website a Garmin GPS based bike speedo as well, thinking the VDO can just work as an odometer as its always on whereas the GPS one can be mounted on the steering wheel  where it can be seen but needs to be charged before use and is more likely stolen so may only be available intermittently.

I will just backup a bit.  The VDO speedo has a wire to a sensor that on a bike would be mounted on the front forks so it can sense a magnet wizzing past attached to the spokes therefore counting wheel revs, you tell it what the wheel circumference is and it works out how fast you are going. So far so good.

To cut a long story short mounting it like on a bike was not going to work so I stuck the magnet on the tail shaft flange instead.   So with a wheel circumference of 2360mm and a diff ratio of about  4:1 the “virtual” wheel diameter would be 2360/4=590mm, most bike speedos do not go below 1000mm however the VDO does so all should be good.

Took the car out for a test with both speedos and the VDO reads fast so after many stops each time lowering the circumference figure in the speedo eventually I have accuracy at 525mm, so a diff ratio of 2360/525=4.5:1. I have now gone through the paper work for the car and sure enough it has a 12/54 ratio of you guessed it 4.5:1 the usual ratio for a Type 37 is 14/54 or 3.86:1.

So brings me to the Tachometer, on the Bug it runs from a pulley on the end of the camshaft via a flat belt to another pulley .  At 100kph it should be indicating about 3200rpm with my new found gearing… however at 3000 I was passing everyone.  I had reservations about the engine’s ability to rev I now realise I should have had reservations about the Tacho’s ability to indicate those revs.

The wheels.  Luckily with the hub fitment problems, I have not ordered the wheels, imagine how much it would rev with smaller wheels on the back.  So all this maths has saved me money, only two wheels and two tyres needed for the T37 Hotrod.

Quite unrelated to any of the above we went to Noojee for lunch today going the long and interesting way through some of the best country in Victoria.  On the way back I had the opportunity to drive Des Dillon’s Alfonso Hispano Suiza which is sans just about any bodywork so goes quite unlike most Edwardian cars that is to say this one goes, stops and handles.  If I had a garage that would only fit one car, this would have to be it.


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2 Responses to How to find your Diff ratio with a GPS

  1. Geoff says:

    Well chosen, the Alfonso – my choice too: during the Fleurieu rally I respectfully kept a respectful distance from it, not wanting to be responsible for contaminating it with my drooling.

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