Speedometer

Cable Replacement

Standing at the front of the car, locate the cable from the where it comes out of the firewall and follow it with your hand all the way down to its mounting on the transmission. Its a simple 1 inch knurled nut on the cable that most of the time can be removed by hand. You're sort of doing it blind so just make sure your turning the correct way. I used a mirror to help see what I'm doing. If it won't come undone by hand you might try using channel lock pliers. More likely you'll find it easier to undo one housing bolt (think its 7/16") and remove the entire pickup gear assembly. Then you can work on removing the cable once pulled up to where you can see it. Make sure the new cable is located as far away as possible from the exhaust headers as heat is their enemy. Put a tie strap on the speedo end to prevent that ends nut from sliding down to the trans.

Electronic (and/or Retrofit) Speedometers

You may wish to fit an 80m or 100mm speepometer to your Mini to replace the standard item for cosmetic reasons or due to instrument failure

There are a number of issues you must be aware of:

1) To get the replacement MECHANICAL SPEEDO to work 100% will probably require calibration to match the number of revs per mile of your cable to the speedo device, suppliers such as Speedy Cables always have hints and tips on their website as to how to establish the "magic numbers" (this could be number of turns of the cable to 6 turns of the road wheel as the vehicle is pushed along the road... or it may be something else they need)

you may also need a custom cable with a Mini engine end and a custom end to match the retro fit speedo

2) To get a replacement ELECTRONIC SPEEDO to work will require the collection of pulses to trigger it, with a typical ratio of 1280 turns per mile the cable only turns at around 35 revs per second (35hz) at 100MPH ... this may seem high but one well priced device (RaceTech) requires a trigger pulse of 833hz to read 100mph

There does not appear to be a pulse generator that can be fitted on the passenger end of the speedo cable that will deliver this frequency. TerraTrip produce a nice sensor that WILL fit in the cable but it only generates around 70hz at 100mph (2 pulses per rev).

Alternative and proven applications use direct sensor measurement from a rotating part such as a CV joint, driveshaft or slotted disc mounter thereon.. however these sensors are exposed to the elements and are not easily accessible to be adjusted

I am investigating producing a pulse multiplier (x8 to x12 with +/- 10% adjustment) to combine with the VERY reasonably priced combination of a TerraTrip speedo cable sensor and RaceTech 80mm Electronic Speedometer. This will also have to run on 12v but output at 1-1.5v

Added by FB 13-10-04 with birthday money to spend!

Calibration

See also Final Drive

I half remember seeing a table with part numbers and ratios for alternative speedo drive gears. This morning while in the 'reading room' I found it!

March '98 Mini Mag. One of the FAQ's on this list is regarding correction of speedo indicated speed after changing tyres or gearbox (diff) ratio's. Now, there's no simple answer for this cuz there's too many variables. I did a wee bit of work this morning and here's what I found:

Now, obviously changing to larger rolling diameter tyres will SLOW DOWN the speedo reading. Here's another table (I hope you can switch to fixed pitch font!) which shows what happens at the speedo head by changing tyres and leaving the diff ratio constant. On top of each column is the diff ratio (the most common ones) and under is the number of turns per mile of the
speedo cable:

Note that stock speedo heads for 3.76 and 3.44 diffs and 10" wheels are 1408 and 1280 T/mile respectively, which are pretty close to the first entries above. You can read your speedo ratio right off the face of the instrument. It's the number just below the odometer window. If you change diff ratio, the above table can be read backwards. For example, if you have a stock 998 Mini (3.44 diff, 145x10 tyres and 1280 speedo ratio) and switch in an Austin America lump without switching the diff from its 4.13, your speedo would be reading 1517/1280=18% too high (the least of your problems). Actually I think 4.13:1 is the Austin 1100 ratio and 3.65 was the AA. Not important to this discussion (but it would explain the presence of 2 sets of 3.65 gears under the bench!). What the Mini Mag article made me think about was if speedo errors could be corrected by replacing the speedo drive gears. Here's what's available:

So, as an example, if all you changed in your 998/3.44 diff Mini is to move up to 175/50-13 tyres, you could retain your original speedo by changing the speedo drive gears to the 16/6 gear set (within 2%). Of course, a 998/3.44 drive train would loose substantially in the performance department since this boot change is the same as jumping down to a 3.27:1 diff but that's
another discussion.

What this all means is that with

• 11 different tyre diameters

• 7 different differential ratios

• 5 speedo drive gear ratios and

• who knows how many speedo head ratios

you have too many variables to have someone tell you what will work and what will not. You have to figure it out for yourself. Here's how you do it:

(wheel turns/mile) x (diff ratio) x (speedo gear ratio) = speedo turns/mile

examples:

• 145x10 tyres

• 3.44 diff (18/62 teeth)

• 17/6 speedo gears

• 1040 x 62/18 x 6/17

• 1264 turns/mile at the speedo head, reads 1.2 % low
   

• 175-50x13 tyres

• 3.44 diff

• 16/6 speedo gears

• 1013 x 62/18 x 6/16 = 1308 turns/mile at the speedo head reads 2% high
   

• 175-70x12 tyres

• 3.76 diff

• 1280 speedo

• 925 x 64/17 = 3482 / 1280 = 2.72

• 16/6 speedo drive gears (2.666:1)will give an error of only 2%.

Of course, changing the speedo drive gears is an engine-out procedure, so the actual usefulness of this information is questionable, but it does give you some options when designing your custom Mini project.