Amazon: New (to me) 1963 Volvo 122

[Othen]

Originally Posted by Derek UK

It was to make you think. It's much cheaper to live with the speedo error and get so your brain tells you whether you are speeding or not. At this point I don't think you really know what the error is under road conditions?
As has been mentioned if you have no inhibitor switch and select reverse and drive backwards with the OD engaged you will wreck the box and/or the OD.

Thank you Derek UK,

We (meaning myself and the other correspondents to this thread) do a lot of thinking here, all these points have been discussed above - and indeed you have contributed to many of those discussions (and I'm grateful for those contributions). In this bit you told me that no other speedometer drive pinion would fit in place of a 19t one:

https://www.volvoforums.org.uk/showp...&postcount=531

... and we know GAM has a 19t speedometer pinion, because the teeth may be counted in a photo in this post:

https://www.volvoforums.org.uk/showp...&postcount=641

You will know very well (because you contributed to it) about the speedometer drive pinion issue, and the conclusion that it was not worth trying to fix. Another way to change the relationship between the speedometer reading (which is proportional to the rate at which the type D OD is rotating, and nothing else) and the speed of the car along the road is to change the size of the tyres (I have to say that you were just plain wrong in claiming smaller tyres would not change that ratio - but I'm still assuming you were just pulling my leg about that part).

GAM's spare wheels/tyres development hasn't been specifically about trying to fix the under reporting speedometer. I'd acquired a good set of spare wheels very cheaply and thought I'd use them in the winters. They came with (scrap) 175 section tyres which looked quite good on the wheels, so I bought (remarkably cheaply I thought) a set of new 175/60 M&S tyres and have fitted them myself (thus gaining a new skill - I like doing that). A welcome side effect of the 175/60 tyres is that they will lower the overall gearing by about 7% (I'm right about this one Derek UK, it is simple physics and there is no argument). So there we have it: the aim (well discussed above) was to acquire some winter wheels and tyres, in doing so I'm trying a slightly different look with GAM and coincidentally may end up with a speedometer less inaccurate that I had previously.

If I get time today I may well try out the silver wheel set with 175/60 tyres to see what they look like, check the motor car steers and handles well with them on and test the speedometer accuracy (against a GPS measurement on my cell phone). If the wheels/tyres don't work out then I have not lost much and will have pursued another avenue on the GAM experiment (which I enjoy).

All of this had been well thought through in the thread above.

Your point about the inhibitor switch has been well discussed in the above thread also (indeed I'm pretty sure you have contributed to those discussions), that is why I have just brought it forward again. Reversing with OD engaged would not wreck the M41 gearbox of course, but it would not be any good at all for the Type D overdrive. I've known about this issue since I bought GAM and as you well know, I intend to fix it once I've thought it through.

I'm really enjoying the GAM project (as I have the RB project) and I find it invaluable to think through issues via this thread. I welcome contributions (and I think many correspondents enjoy commenting also). I hope you will continue to contribute to the thread Derek UK.

:-)

PS. Bobwalkin time :-)

PPS. I have just found this lovely 1960 P1800 brochure on the internet:

https://www.pyhimyskerho.com/@Bin/50...chure-1960.pdf

... isn't the internet wonderful. The specification sheet explains the issue with GAM's speedometer drive perfectly:

Rear axle: Hypoid type rear axle. Ratio: 4.1 :1
or 4.56:1 when overdrive fitted

... hence GAM's speedometer reading 11% slow - einfach.