|
Here's a tip on calculating speedometer error:
http://www.vclassics.com/archive/speedotip.htm
Also did a search on J-type laycock speedometer and got this site:
http://www.triumphowners.com/collection.cgi?task=SI&parentID=103§ionID=111030
Calibrating your speedometer on the cheap.
------------------------------------------
Depending on the source of your J-Type, your speedometer will need calibrating after all your hard work. If the error is small enough, you can ignore it, or recalibrate the speedometer itself to get the indicated speed correct.
The recalibration of the spedometer itself only goes so far, as this will render the odometer and trip meter inaccurate. The odometer will not reflect any adjustments made to the indicated speed, as it is gear driven and cannot be adjusted.
Instead, the author used the following method to calibrate his speedometer after installing the overdrive:
1.The Volvo speedometer pinion gear was pressed off its shaft, likewise for the original TR gearbox pinion gear. The gearbox pinion shaft was trimmed to the same length as the Volvo pinion shaft using a Dremel, and the Volvo pinion gear was pressed onto the shortened shaft using a vise. The number of teeth on the Volvo gear was counted (21 teeth) and the assembly was installed into the OD. This procedure was necessary to permit the pinion to grip the speedometer cable used by Triumph, as the Volvo pinion shaft will not grip the end the TR cable.
2.Using highway mileage markers and a watch with a sweep second hand, it was determined that the speedometer indicated 51 MPH when elapsed time between markers was 60 seconds (i.e. a true 60 MPH).
3.A little algebra yielded the following formula:
Number of teeth needed = (51 * 21) / 60
4.The pinion gear needed to correct the speedometer required 17.85 teeth. A quick call to Quantum Mechanics, and an 18-tooth pinion gear from a Spitfire J-Type overdrive was on the way, which put the speedometer within 2%.
5.It is possible that you will find your speedometer off by up to 40%, depending on whether you have the earlier (1972 and earlier?) speedometer in your car. In this case, it is difficult or impossible to calibrate the speedo just using a pinion gear. You have two options:
a. Replace both the pinion and the drive gear with parts from
Quantum Mechanics or other suppliers.
b. Consider installing the speedometer from a later model
year, which can then be calibrated using the Spitfire OD
pinion gear. Speedometers are available on eBay, breakers, or
swap meets. The later speedometers used a different drive
speed, and can be identified by 4 tiny digits on the bottom of
the speedometer face. Later speedometers are marked with the
digits '1120'.
Parts Sources:
--------------
The Roadster Factory http://www.the-roadster-factory.com/
Moss Motors http://www.mossmotors.com/
Quantum Mechanics http://www.quantumechanics.com/
Victoria British http://www.victoriabritish.com/
eBay http://www.ebay.com/
British Autoworks http://www.britishautoworks.com/
|
