Part 10 Speedometer

There are problems with speedometer accuracy that can occur when converting a TR7 to V8 spec. The axle ratio is changed, usually from 3.90:1 to 3.08:1, and the tyre sizes can cover a wide range. Thus the speedometer could be reading very inaccurately if not re-calibrated.

This problem concerned me during my first V8 conversion in 1983. At that time very few people were converting TR7s to V8 and even if the axle ratio was changed from 3.90 to 3.08 it was generally accepted that the speedometer would be '...somewhere about right...'. In an effort to clarify this I contacted two companies offering V8 conversions, both suggested that I change the DRIVEN cog in the gearbox to the part number they specified. It was not until two years later - and the passing of a number of police cars - that I found out that the speedometer was reading 15.2% slow. As we all know, ignorance is no excuse to the men in blue so I decided to investigate further.

As, at the time, I could not locate the correct part numbers for the TR8 DRIVEN and DRIVING gears, I opted for having the speedometer re-calibrated. The company I found proved to be very helpful and therefore, not surprisingly, is still in business today.
The company is:

Speedy Cables (London) Ltd.
South Wales SA9 1SQ
Tel: 01639 732300

On speaking to them recently, they confirmed that the current cost would be £75.00 (Plus. p&p and VAT,) with a completion time of 10 to 14 days. It is only necessary to perform three simple operations before sending the speedometer to them:

1) Take the measurement from the centre of the hub of a drive wheel to the ground, with the tyre pumped to normal pressure.
Answer (1)........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.
Answer (2)........Distance in inches.

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

4) 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.
Answer (3)........number of turns of the cable (N.B. include parts of a turn e.g. 7 and a quarter turns)

N.B. This operation cannot be carried out by jacking up the rear wheel.