Readers’sagas
Dodgy speedo readings on an MGA cause a motorway go-slow.
My sincerest thanks must go to all those who joined the impromptu funeral procession that marked the death of Frisky’s speedometer. Your attendance, although involuntary, was greatly appreciated, because without the presence of each of you who was forced into an inescapable convoy on a stretch of motorway (reduced to one lane because of roadworks) it may have taken a lot longer for me to realise that FSK’S cable-driven Jaeger speedometer had failed in its sole purpose in life.
I’d had had my suspicions, but it was an unusually long queue of traffic behind us that led me to voice my concern. ‘Er, Daaaad,’ I said. He returned what remained of the cream cake he’d been eating to a cardboard box in the passenger footwell. ‘I think Frisky’s speedometer is on the blink. I mean, I know I’m not one for particularly nippy driving, but something’s not right.’
Now, there are few things that come between my dad and his food, but as a mechanic it’s in his DNA to engage geek mode when a problem with the MGA presents itself. He downloaded a Gpsenabled app called ‘Speedmeter’ on his phone and set about obtaining an estimation of our speed: 41mph, but the needle said 50, and further investigation recorded an even larger 10-15mph discrepancy. Back at Vowden HQ, I removed the driver’s seat base to make room for a mission into the footwell and under the dash panel. Wedged uncomfortably in position with a torch, I inspected the tacho cable for kinks, damage or corrosion, but found nothing. It was going to have to come out.
Time for more tools…
After disconnecting the tacho cable from the centre of the unit, I persuaded the headlamp illumination and dial illumination bulbs to pop out from their positions. The bracket holding the speedometer in place isn’t part of the dash assembly so I unscrewed each of the two knurled
nuts, losing a washer down the collar of my overalls in the process. As I loosened and removed the bracket, I took great care because I didn’t want to put any unnecessary strain on the unit without its support. Following an inelegant exit from the footwell, I repositioned myself with a driver’s view of the dash, and applied pressure to the back of the speedometer. I was able to wriggle it free, but the sealing ring didn’t survive. A sticker on the back of the unit told me that it was last repaired in 2004. Before sending the speedometer off to the experts at Speedy Cables in Swansea, we had to determine the required revolutions per mile for them to carry out the repair and recalibration accurately. This is when the real lesson began.
A speedometer can only read accurately when it’s used on the vehicle it was originally calibrated for – if it hasn’t been modified. Any changes made to the wheels, tyres, gearbox or other related parts can interfere with the reading.
‘We’re running heavy-duty tyres,’ said Dad, ‘so we need to make a decision. Do we revert back to the smaller ones (165/SR15) that Dodo ran on the car, or stick to the adventure-spec tyres (195/65R15 95H) that have a larger diameter, and get the speedometer recalibrated to these?’
Dodo, if you’re wondering, is my late grandfather, and we decided to stick with the larger tyres.
With them pumped to their normal pressure, we measured from the centre of the wheel to the ground – 11¾ inches. Then, to establish the distance travelled by the same tyre in one revolution, we marked the bottom of it with chalk, and the spot on the ground beneath it. With Frisky in neutral, using the chalk markers as guides, dad pushed her far enough to complete one revolution – the distance covered was 75½ inches.
To obtain the third and final measurement, dad fashioned a cardboard arrow and fixed it to the end of the protruding inner speedometer cable. Using the same chalk marks as reference points, he pushed Frisky forward, counting six revolutions of the wheel, while I counted the number of times the arrow on the cable revolved: 10¾.
After being carefully wrapped, it was shipped off for some specialist TLC at Speedy Cables, and a month later the speedometer made its return to me. With it nestled back where it belongs, we called upon a recent purchase, a Usb-powered GPS speedometer with a 4.5in screen, to test its accuracy. I’m pleased to report, Frisky’s speedometer needle is now entirely on point.