|The MGA With An Attitude
CHANGING SPEEDOMETER GEAR RATIOS - ST-203
This course is about optional ways to re-gear the speedometer and odometer to give the proper readings. You would need to do this after changing the final drive gear ratio in the rear axle, or maybe after installing a non-stock transmission.
The original MGA odometer is geared to turn 1450 revolutions per mile (25x58 internal gear reduction). For the MGA 1600 and the Twin Cam which used slightly larger tires, the odometer ratio was 1440 turns per mile (32x45). The resulting cable speed after the 3.9 rear gear change would be about 1318 turns per mile, but there are no reasonable gears available to do that in the odometer drive. A rebuilder once told me that the primary gear (driven by the input worm gear) will be either 25 or 30 teeth, nothing else. But for sure some (if not all) Jaeger 1440-tpm speedometers have 32x45 rearing for the odometers. I think the factory supplied speedometer for 3.909:1 final drive would have been 1325 turns per mile (25x53). There is a real factory part number for that speedometer unit, but they are exceedingly rare. Considering the current public demand for this unit, it is unlikely that you would ever find an original one.
Some speedometer repair shops might have the parts to modify a stock 1450 unit to the 1325 ratio, as all it required is to change the 58 tooth final odometer drive sprockets to 53 tooth (standard and available parts). You might expect them to charge over $100 for the service (if they can do it), and then you would be faced with the future maintenance problems for the rare or non-standard part. This was in fact my final resolution to the problem. I sent the speedometer to Mo Ma, and they changed the final odometer drive sprockets and also remagnetized the spinning armature (about 10% increase in magnet strength) to bring the speed indication back up to normal. The new 1325 ratio number is written on the back of the unit, and the old 1450 number on the face has been blacked out (just to avoid confusion).
Another possible solution is to install a small speed change gearbox in line with the speedometer cable. So far I find these units available for 7/8" and 5/8" threaded cable nuts, generally intended to be attached to the gearbox before attaching the cable. They are available in a wide assortment of either increase or reduction gear ratio in increments of less than 1%. You would be looking for a 1.100:1 speed increase. Available ratios are 1.0961 or 1.1038. Which one you choose could depend on the tire size you use. The cost of this small gearbox is about $70 list. Unfortunately the cable fitting thread on the MGA gearbox is 3/4" (or 19mm), and the one on the speedometer is 12mm, and so far I haven't found a gear reducer with either of those thread sizes. The standard solution in that case is to install a 5/8" threaded reducer somewhere in the middle of the cable (using two shorter cables), add a small cost for any custom made speedometer cables. You may find these small gearbox units supplied by Speedograph Richfield Ltd in England.
Another solution recently used by a couple of local club members was to replace the original cable driven speedometer with an electronic speedometer, including an electrical transducer which gets mounted on the gearbox. These are generally adjustable to provide any desired drive ratio in small increments by setting a series of binary rocker switches on the back of the speedometer. Off the shelf units have different face plate appearance than the original MGA instruments, but for a fee they might be custom painted to look much like the original unit. Translation: $$$$.