Speedometer Ratio Box

I changed the differential ratio of my E-Type about two years ago.    My car was first delivered in the USA, so it came standard with  3.54 ratio.    Most of the 4.2 cars were standard with 3.07.    When the car was restored in the 90’s, it was converted to 3.07.   However, the previous owner converted it back to 3.54.   My conversion put it to 3.07 again.    The car still has its original speedometer, which means both the speed and odometer were incorrect.   This page lists the different Jaguar E-Type differential ratios.

There are generally two ways of accounting for the differential ratio in the speedometer.   American cars have an easily changeable speedometer gear in the output of the transmission.   This allows them to have the same speedometer regardless of the differential ratio.   This approach simplifies things for their high production volumes.   It also simplifies modifications, although I doubt this is the aim.    European cars generally had a different speedometer for each ratio.      If the ratio is changed, then either the speedometer needs to be changed, or an inline Speedometer ratio box needs to be installed.

As Sydney is infested with speed cameras, I had the cable removed from the car and sent to a speedometer shop.    The original plan was to modify my cable to accept a ratio box.   However, my cable wasn’t able to be modified – it was too thick and the right fittings are no longer made for such a thick cable.    Therefore, a new cable and ratio box was made up.    Having the new cable made up was more expensive than I was hoping, but If I avoid two low range speeding tickets, I am ahead.

Helpfully, the Smiths speedometer actually lists how many revolutions per mile it expects.   In the case of an E-Type with a 3.54 ratio, this is 1312.    For a 3.07 ratio, it is 1120.   These ratios allowed the speedometer shop to correctly build the speedometer ratio box.    For reference, the 3.31 ratio should be 1216 revolutions per mile, and 2.88 is 1058.

The new cable was installed today and a small bracket fabricated to hold the speedometer ratio box.

I have not yet compared the speedometer and odometer to a GPS, but it was reasonably accurate before the differential ratio change, so I expect it to be ok.   This is a timely change as the NSW Government have just announced the speed camera warning signs are to be removed.