Overdrive conversion

[ejmorgan]

I have a 1965 Spitfire, in which an OD conversion was performed (non over drive car originally). Due to the OD conversion, it appears the speedo now runs about 10 MPH slower than the actual speed of the car. I understand this is probably due to the differential not being changed when the OD conversion was completed. Anyone know how to recalibrate the speedo, or whether there is a speedo repair/conversion company who might be able to recalibrate the speedo? Another option would be to get a signal generator, and convert to a programmable speedo. Has anyone performed this before i.e. any info on what signal generator works best with the spitfire? Thanks.

[Clive]

The diff stays the same between od and non od cars (except GT6's)
The speedo is matched to the diff. However it is possible that the wrong speedo drive gear is fitted in the OD, I know there are different ones, and they may be different colours? But that is a very sketchy memory.
There are speedo calibration companies out there, speedy cables is one but there are others. And it is possible to buy a kit to convert to electronic speedo, But I am unable to remember the name of the manufacturer. Not cheap though, I remember that!
Cheers
Clive

[jowettdriver]

Or it may just be that your speedo is old and past its best. Mine has no overdrive and the correct speedo but still reads under all the time. I believe this is due to the magnets on the drive plate becoming weakened with age. I tend to just drive it on the rev counter!!

Cheers

Tim

[neilnaz]

It could also be that a non-standard wheel and tyre combination is in use leading to a different rolling radius......

[Richard B]

405 wrote:

I have a 1965 Spitfire, in which an OD conversion was performed (non over drive car originally). Due to the OD conversion, it appears the speedo now runs about 10 MPH slower than the actual speed of the car.

Do you know whatmodel gearbox & O/D it is?

At a guess it's either a MkIV 3-rail box with D'Type which is calibrated for a 3.89 or a 1500 J'Type single rail box which is calibrated for a 3.63.

If it's a D'Type you can get the replacement pinion from one of the good Triumph specialists or from one of the OD suppliers (they can have a minimum order though)

If it's a 1500 - J'Type I think recalibrating the speedo will be the easiest route.

[neilnaz]

Forgive me for saying this but what difference will the overdrive make to the speedo reading? Doesn't the speedo drive run off the final drive via an angle-drive and it is either the incorrect speedo, faulty speedo or altered differential or rolling radius that will affect the speedo reading. When converting from non-overdrive to overdrive tyou don't change speedo you just make sure you connect to the final drive via an angle-drive don't you?

[Richard B]

Rimmers site lists
J'Type
Speedo Drive Pinion;
For 3.63:1 diff ratio - (1) - AEU1578
For 3.89:1 diff ratio - (1) - NKC49

Gear (speedo drive);
For 3.63:1 diff ratio - (1) - NKC45
For 3.89:1 diff ratio - (1) - NKC46

D'Type
Speedo Drive Pinion - (1) - 506119

Chris Witor lists the number of teeth for the big cars but not the part numbers;  

Non-Spitfire variations,
Drive pinion gears, ‘J’ type overdrive,
17 tooth 2.5S,
18 tooth 2.5 PI, 2500 TC, 2000 to 1975
19 tooth red 2000 1975 onward 3.7


So for the easy life, if you have a Spitfire 1500 Non-OD and fit a 1500 J'Type OD then everything stays the same or if you have a MkIII Spitfire Non O/D and fit a MkI-III Spitfire 3-rail D'Type box.

Hovever if like one of my daughters you have a Herald with a 4.11 diff and a 1500 Spitfire J'Type single rail box your speedo gets a 'bit' confused.

As the D'type only has one pinion listed for Spitfire I - IV and GT6 & Vitesse 2 Litre I presume that the variations are catered for in the Speedo head.

[neilnaz]

A Speedo reads (effectively) rate of prop rotation and the reading is hopefully proportional to road speed. If you only change gearboxes this will not change the speedo accuracy (or lack of accuracy) unless the speedo drive set-up differs between the gearboxes. I hadn't thought of that!!  ::) :-/

[thescrapman]

Richard_B wrote:

As the D'type only has one pinion listed for Spitfire I - IV and GT6 & Vitesse 2 Litre I presume that the variations are catered for in the Speedo head.

It does.

Each diff ratio has a corresponding speedo.

And Neil, one rotation of the prop is not 1 rotation of teh speedo cable. The gear on the main shaft is about 10 teeth, and the output gear varies from 15-odd to 22-odd I think

Or something like that.

Cheers

Colin

[Tryumf]

405 wrote:

the speedo now runs about 10 MPH slower than the actual speed of the car.

If its a constant 10mph why not take the speedo apart and move the pointer round until you get it right..David

[Richard B]

thescrapman wrote:

The gear on the main shaft is about 10 teeth, and the output gear varies from 15-odd to 22-odd I think

Or something like that.

According to CW; on the J'Type there are also two different wheels (to go with the pinions) 5 & 6 starts. changing those however requires a strip down.

My head hurts.....  :'(

[rotoflex]

D type rooolz!

[Richard B]

rotoflex wrote:

D type rooolz!

Well I used to agreed with you, but my 3-rail D'Type is struggling not to slip until the oil has warmed up with my 2.5PI 165bhp (estimated) Spitfire. So I am considering a J'Type conversion.

The box is fine TLD built it a good 15 years ago and it's still going strong, but the OD......  :-/

[ejmorgan]

The engine and OD are from a 1500 i.e. 1500 J'Type single rail box which is calibrated for a 3.63. My understanding is that if the dif is not changed with the OD, you then have an error in your speedo of about 10 MPH (which is what mine is reading). If I understand the responses, my best options are to either re-calibrate the speedo, or upgrade to a programmable speedometer. I can get a signal generator which attaches behind the dash to the speedo cable for about $60, and a new programmable speedo for around $150. Anyone know what a speedo referbishment and calibration might run at a shop? Also, what information would they need to recalibrate? Obviously the dif is a 4:11 rather than a 3.63.

[Bob Bowling]

Due to change of diff my speedo is off by 10% (under reading) so I just use the rev counter and do some mental arithmetic.
Most speedos are off by a fair bit anyway - try and calibrate it using a sat nav.

[BarryH]

Have a looka t this for what is needed to recalibrate a speedo.

http://www.jdo10.pwp.blueyonder.co.uk/page10.html

[don cook1]

I have a 5 speed gearbox and 3:63 diff in my GT6...so I tend to use Tom Tom!

[ejmorgan]

Nice site and thanks for the reply. I have a Garmin, and use it for MPH; however, I like to have things working correctly. The speedo being off will bother me enough that I will need to fix the problem. Looks like a trip to a shop to recalibrate the speedo is in the future. Again, I appreciate the help.

[beefertoo]

there is a place called speedy cables (google) who will recalibrate your speedo, all they need to know is how many turns the cable does per turn of the wheel.
craig

[rhodes]

Assuming there is nothing much wrong with a speedo, this calibration business is a DIY job.  At least it was with an old Mini Smiths speedo I have and I presume the Spitfire is no different.

The turns per mile is written on the face and my 1500 says 1000.  You can easily check this with a hand drill and a bit of counting.  To then check the mph all you need to do is drive it with an electric drill (anticlockwise for a Mini) up to say 70mph or so and time how long it takes to do a mile or two.

If this is wrong, you need to take the speedo apart, grab hold of the aluminium disc on the far end of the needle spindle and then twist the needle round a bit.