Hey all, Did some work on trying to dial in my speedometer with recent gear changes. Speedometer reads perfectly (compared to phone speed-o app) but by the time you get to 60-ish it begins to lose its accuracy, upwards of 5 mph as you go faster. It seems to plateau at ~5 off, but unsure as I couldn't get much over 90. Any ideas? 72 Skylark Custom.. I know the apps aren't 100% accurate but if it matches on lower speed, unsure why it would slip on higher speeds? Mark
Speedo cable could be binding or your GPS could be off. Plus or minus 2 mph is about as close as you can get with these old analog speedos, but 5 could get you a ticket. I was able to get mine -2 mph regardless of overall speed. I plugged all my numbers into one of those speedo gear calculator things to get the correct gear replacements (trans was a replacement I put in and it initially was running about 10 mph off, regardless of speed). Speedo read correct for a while, then started getting crazy off before I figured out the gears in the cruise control transducer were slipping. Replaced the transducer and it went back to reading correct.
The error is a ratio, not a set amount so it will double with the speed doubling. Mine is off about 10%. At 55MPH it shows 50MPH but at 30MPH, it doesn't really get noticed. At 100MPH I'd be showing 90MPH but I don't go that fast on public roads. That's my story and I'm sticking with it. One other thing, does speedo needle bounce at lower speeds? I think that is a sign of a kinked cable.
Hey guys, thanks for the info so far. No bouncing, I did the gear chnageout in the trans using the calculator - hence me being pretty much dead on at lower speeds. Agreed it's a ratio / proportional to speed increase. No binding of cable, either (I'll still double check that one). I was wondering if the speedo was in need of a rebuild...
Now that I am thinking about it, I had a heck of a time getting my speedo correct. There is an adjustment in the speedo itself for tightening/loosening the spring. After I had gotten all the gears and such correct, it was still off and I found out the speedo itself wasn't any good. Replaced that (along with all the other stuff) before it finally worked correctly. It did give me a car with lower mileage
