interesting problem you have here..!
Do yourself a favour n bite the bullet! Go get yourself some modern gear.. buy a 2.4ghz set up n a cheap speedo and take it from there.. spurious equipment failure is a pain in the butt, n even if you get it working you will always have in your mind the fact that it once didn't work very well!! this is especially amusing if your car is heading straight for a marshal or a brick wall or disappearing into the distance at the time...
2.4ghz radio sets are available at a very reasonable price.. Spektrum 2.4ghz wheel type tx/rx sets on an auction site are going at 40 quid brand new.. you can even get lesser known brands at 30 quid or less..
A new mtronik brushed speedo is less than 20 quid.. or you can pick up some brushless systems for around 50 quid or less..
I mean these comments as helpful nothing more.. I once had a radio problem I thought I had cured and suffered the consequence..
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Don't believe the hype... Think for yourself.. 
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