Since I bought the car (1970 S2 FHC - no overdrive) 11 years ago the speedo has been running 1.27x too fast (averaged over the range from 10-70 mph). Gear box is original and has the correct (new) angle drive and car has original rear axle. As far as we can tell it has the original speedometer. We seem to have isolated the problem to speedo calibration and I’m planning to have it recalibrated.
My questions are A) Is it possible for a speedo fault to cause it to run fast - if so what would the fault likely be? (presumably worn parts would make it run slow). B) We’ve not changed the cable, could that be it? (again I’d expect a worn cable to run slow). C) Is there anything else I may have overlooked? Any thoughts appreciated. Thanks.
My first set of in-laws had a Ford Cortina and they couldn’t understand why everybody went past them on the road. Turns out it had the incorrect speedo drive fitted from the factory.
If BOTH the speedometer and odometer are off, then either the speedometer is mis-calibrated AND odometer is mis-geared, OR you have the wrong drive gear in the tranny (not sure that’s even possible), OR the wrong angle drive.
Thanks Andrew. My mechanic reports this: “The speedometer in your car is sn 6332/01A. This indicates a 1312 turns per mile speedo for a 3.54 rear end (which is what your gear casing tag shows, 46/13 gearset ratio).” Sounds like speedo matches diff ratio.
Thanks Ray. 15" wheels. Correct angle drive. Is it uncommon for both speedo AND odometer to be mis-calibrated? I’m curious as to how a speedo becomes mis-calibrated in a way that makes things run faster?
Thanks. My mechanic’s note: “The speedometer in your car is sn 6332/01A. This indicates a 1312 turns per mile speedo for a 3.54 rear end (which is what your gear casing tag shows, 46/13 gearset ratio).”
So when you are going an actual 60 mph as measured by what, GPS, your speedometer indicates a speed of 76 mph? In 4th gear at an actual 60 mph what is the tachometer showing?
A speedometer can easily read too high or too low as parts wear or change. One way is the strength of the hair spring on the drag cup - if that spring weakens the needle will rise higher. It can also happen that the needle slips on its fitting and reads high.
But for the odo to also read high there is a mechanical cause. The angle drive is suspect because the other way the odo calibration changes is by changing the gear inside the head (not as likely).
One way to test this is to undo the cable and snake it out so you can see the end. You use a bit of masking tape to make a little ‘flag’ on the end of the cable so it is easy to see it turn. Then mark off 52’ 9½" on the pavement and set the car at one of the marks. You then roll the car to the other mark and count the number of cable turns over that distance. Much easier with a helper. Count fractions of a turn too. Multiply your count time 100 to get cable turns per mile for your set-up and compare that to the 4 digit number on your speedo.
I just did a quick google search for e type speedometer drive ratios and there are mentions of ratios of 1:1 and 1.25 to 1. As the original post stated that the speedo read 1.27 times high I would guess that there is a good chance that the wrong angle drive is fitted. As others have mentioned that would be the first possibility that I would pursue.
