Spitfire Speedometer Calibration

[docarhamilton]

One of the main reasons why I have joined this group was that I'm a member of a Facebook Triumph Spitfire forum and one of the members was asking about how he could recalibrate his speedometer, which was reading 10% high. I could recall, many years ago. seeing a magazine article about how to do this, so I got online to see if I could find anything about the subject, only to find numerous threads on various sites with people saying how they had no idea how to do this. Looking into the subject in more detail I eventually got the information I'd seen years ago, so I thought I'd pass it on, since this seems to be a fairly common problem with our cars, since many have been modified, different ratio Differenials fitted, gearboxes changed, overdrives added, larger wheels fitted, etc., all of which would contribute to inaccuracies in speedometer readings.

Basically, it seems that what is required to recalibrate a mechanical speedometer is to make sure the gears in the speedo are of the correct ratio. There's a worm type gear which is driven by the square end on the inner speedometer cable and this drives a gear on the back of the needle, which indicates speed through a slipping magnetic setup.

In order to work out the gearing required a speedometer specialist will require certain information. I contacted a company called "Speedy Cables" in the UK to ask about this and they have been very helpful. They don't supply the gears, but will fix your speedometer if you sent it to them with the required information they need for correct calibration, which is as follows:

Guidelines for the calibration of instruments and gauges​

CALIBRATION

Speedometers cannot just be changed from one vehicle to another. They only read accurately when used on the unmodified vehicle for which they were originally calibrated.

Even when used on the original vehicle, any changes that may have been made to the wheels, tyres, gearbox or other related parts can seriously affect the speedometer readings to the extent that the vehicle will fail to pass an SVA test or may result in a speeding fine. In this case, the speedometer will need to be re-calibrated.
To ensure that your speedometer reads accurately when fitted to your particular

vehicle, we will need to calculate the required "revolutions per mile" >>>>

For calibration of a new instrument, or re-calibration of an existing instrument, we will need you to provide the following information from your vehicle...

(1) Take the measurement from the centre of the hub of a drive wheel to the ground, with the tyre pumped to normal pressure.

ANSWER1)__________ distance in Inches

(2) Put a chalk mark at the bottom of the measured wheel also marking on the ground where it meets. Push the vehicle forward one revolution of the chalk mark and record the distance travelled

Answer2) __________ distance in Inches

(3) Now disconnect the speedometer and place a cardboard arrow on the end of the protruding inner speedometer cable.

Put a chalk mark at the bottom of the measured wheel and then push the car straight forward (with gear in neutral), counting exactly 6 revolutions of the wheel, whilst a partner counts the number of times that the arrow on the cable revolves.

ANSWER3)__________ number of turns of the cable. (N.B. include parts of a turn e.g. seven and a quarter turns)

DO NOT DO THIS BY JACKING UP THE CAR AS THIS WILL GIVE COMPLETELY FALSE RESULTS.

Now send us the values that you recorded for Answer (1), Answer (2) and Answer (3) above.

I would hope that there are also speedometer/Instrument specialists in the US who would ba able to do this work or, possibly even send you the required gears to fit yourself.

If anyone knows of such a specialist please feel free to post this information here.

One final though is that I'm quite used to the speedometer in ANY car I drive reading around 5% above actual speed as measured by satnav. My feeling is that this keeps me safe from speed cameras, so I'm happy to live with it. If, however, you're doing 66mph when your speedo says 60 that's much more of a problem and would be worth getting fixed.

 

[CJD]

Before the turn of the millennium, when all speedos were mechanical, every town had 2-3 speedo shops. Now everything is electronic, and all the shops disappeared. They went the way of the wooden wheel repair shops...

 

[ed_h]

Speedometer and tachometer calibration can be done as a DIY job by anyone with decent mechanical skills. Here is a process for GT6 instruments--certainly the came as the Spit units. Calibration starts near the middle of the page.

GT6 Restoration: Speedometer & Tachometer

Ed

 

[docarhamilton]

Speedometer and tachometer calibration can be done as a DIY job by anyone with decent mechanical skills. Here is a process for GT6 instruments--certainly the came as the Spit units. Calibration starts near the middle of the page.

GT6 Restoration: Speedometer & Tachometer

Ed

Wow, you've done an amazing job in refurbishing your instruments! I have read that there's a fine spring somewhere, which is easily damaged and difficult to get back into position once removed. Did you find this? The only thing I'd suggest is using silicine grease instead of petroleum jelly as lubricant for the seals.

 

[ed_h]

Wow, you've done an amazing job in refurbishing your instruments! I have read that there's a fine spring somewhere, which is easily damaged and difficult to get back into position once removed. Did you find this? The only thing I'd suggest is using silicine grease instead of petroleum jelly as lubricant for the seals.

Thanks for the nice comment.

Yes, there is a fine spiral "clock" spring that biases the pointer counterclockwise. You do have to be careful, but it is actually fairly well protected.

My practice on lubricating rubber is to use silicone on organic rubbers, and organic grease on silicone rubbers. I read this recommendation years ago, and I believe it has something to do with the dissimilar chemistries reducing the likelihood of degradation. I can't necessarily vouch for it, but it's what I do.

 

[docarhamilton]

Thanks for the nice comment.

Yes, there is a fine spiral "clock" spring that biases the pointer counterclockwise. You do have to be careful, but it is actually fairly well protected.

My practice on lubricating rubber is to use silicone on organic rubbers, and organic grease on silicone rubbers. I read this recommendation years ago, and I believe it has something to do with the dissimilar chemistries reducing the likelihood of degradation. I can't necessarily vouch for it, but it's what I do.

The information I have differs and says that petroleum jelly will degrade silicone rubber (and ordinary rubber) and that a silicone lubricant should be used for all types, though it also seems to be the case that some types of rubber ARE compatible with petroleum jelly.