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Hello
My car is a 1965 Jaguar S-Type.
The speedometer needle is increasingly oscillating depending on the speed. The needle fluctuates between 10 and 20 km/h around the so-called normal value. The so-called normal value is a bit pessimistic compared to the actual speed displayed on the GPS (Google Maps). Could this issue be due to the drive cable, or is it the speedometer itself ? Thank you for your advice.
Gérard
Likely the cable is causing the oscillation. I'd start by replacing it and making sure it is lubricated properly.
There is a small number on the face of the speedometer by the trip odometer, and that is the number of turns per mile it is calibrated for. Some common numbers are 1080, 1152 or 1312. Those changed depending on the rear axle ratio and tire size, so you might not have the correct speedometer. Tires are important too, it's not as easy to get the correct (tall) size anymore. Once you work through the math, you can calculate the correct number of turns for your gear ratio and tires.
If the worst comes to the worst & the speedo needs repair. Come highly recommended & prices are reasonable for an S Type. Orlando from the Register & here had his done & it is rock steady and accurate for his tyre selection. (185VR15 Dunlop SP Sport Aquajet)
What everyone above says about the cable. Mine always had this problem at reasonable cruising speeds. I fixed it by evening out the bends in the cable route. However, the oscillation came back after about a year. Fortunately, the tacho helps: 3000 rev/min = 80 mph in o/d top. As for the calibration, Sir William bought the same type of speedo as Rolls-Royce, but at a fraction of the cost. The result was that Smiths delivered the the accurate ones to R-R and ones that others wouldn't accept to Jaguar. There's a story of a R-R, followed by a Jaguar, followed by a Ford. The R-R driver is sitting precisely at the motorway speed limit plus a couple of mph, the Jaguar driver is thinking that they could be going a few miles an hour faster, and the Ford driver is thinking I can comfortably keep up with these two at 100 mph.
Last edited by Peter3442; May 6, 2024 at 04:52 PM.
. As for the calibration, Sir William bought the same type of speedo as Rolls-Royce, but at a fraction of the cost. The result was that Smiths delivered the the accurate ones to R-R and ones that others wouldn't accept to Jaguar.
Apparently R-R paid Ł3 while Jaguar paid 10/-, Sir William got what he paid for!
All Speedometers were calibrated to be 10% out on the low side. So showing 70 on the speedo was often as low as 63. This was done for a couple of reasons.
If your speedo is reading 70 but you are actually doing 65 it is safer and you are less likely to get a speeding ticket. If it was the other way and on the speedo you were showing 70 but paying the Government every week for doing 77 you would not be amused. Most Police areas in the UK do work on a scale of 10% plus 2 miles an hour before giving a ticket so 79 on a motorway and 35 in a built up area but don't quote me in court when you get a ticket for doing 34.
The second reason is tire wear. As the tire wears down the speed at the wheel axle changes and the speed shown on the speedo would also change in relation to actual across the road speed. The 10% allows for tire wear change of circumference from new to worn tyres which could be as much as 14mm difference.
I have a GPS gadget in my car which is plugged into the cigar lighter socket. Gives me accurate speed all the time on a digital read out. My speedo whilst running 205/70/15s is only a couple of miles out on the low side according to the GPS. Happy days.
Cass has hit the reasons for over reading speedos correctly. There is also the threat of litigation against the vehicle manufacturer if it was proven in court that an "unsafe "speed (over the limit) was because of an under-reading speedo.
I once had a series 2 XJ6 which underread by 10kph at 100 kph. ie only indicated 90kph. I never did find out why.
Strangely my MK1 and MK2 speedos are fairly " spot on" when compared to GPS readouts.
My series 3 XJ6 over reads by 5 kph requiring 105 kph on the speedo for 100 kph GPS.
My X300 is worse still requiring 107 kph for 100 GPS.
By the way for those not familiar with GPS systems they are incredibly accurate, and speeds can be trusted.
Most countries adopted regulations on speedometer accuracy in the 1980s. Typically, they allowed 10 % + 4km/h fast and zero slow, though most manufacturers aimed for between 5% fast and zero slow. Before then, though speedometers were usually obligatory, there were few limits on accuracy. As Bill says, GPS systems are the most reliable (away from extremely steep slopes and tunnels). They are a convenient means for calibrating your car speedo.
I've heard some negative comments about Speedy Cables UK. That makes a difference when you are sending an item all the way from the Southern tip of Africa.
if my review counts, they also were great. One time I asked them about a Blue (bright headlights) jewel because it did not make sense to me that the bright headlights jewel was red, got confused with a low battery or generator not charging red indicator. They sent it to me no charge, when SNG had told me "no such thing".
I've not used either of these companies. I don't know of anyone who has had trouble with either. From what I've read, I've gathered that customer satisfaction, especially with the US speedo restorers, depends a lot on what they are asked to do. Most seem to be able to repair the mileometer and trip, which are often the most difficult areas for the home restorer. They can get the speedometer working and can often put a gear in the drive to match a change in axle ratio. Most seem to fail totally at making smallish corrections to the calibration: if your instrument reads 10% fast or slow, they don't seem to be able to do anything about it. You might imagine they could swap or adjust the hair spring, but apparently most restorers cannot or do not want to try.
Speedometer problem with Jaguar S-Type 1965
