Re: Speedometer correction

Posted by BH On 2009/11/26 17:38:19

I've been able to select radials that have an outside diameter that is relatively close to the OEM tires on my V8s - with less than 5MPH deviation at highway speeds. However, I believe this is an error of percentage rather than absolutes - that is, the deviation is smaller at lower speeds. Of course, I would rather have a speedo read a little fast than slow; most law enforcement won't bother you for going only 5MPH over the posted limit.

In years past, when swapping to different size tires on late model cars (typically, just one size larger), it wasn't too hard to determine what alternate driven (aka - pencil) gear to install - provided you knew the tooth count of the both speedo gears in your tranny and that of the available and compatible replacement gears and could do the math. In fact, this situation was common enough that I kept a stock of new driven gears of just about every tooth count when I first worked in a GM dealership, 25+ years ago. I also kept an "exchange" inventory of take-offs to swap out or just try for customers. However, availability of replacement gears these days for 50+ year old cars could be a problem.

Occasionally, when we couldn't get close enough by swapping pencil gears, we turned to a ratio adapter for correction. Visually, the unit was a small, squarish, die-cast housing with fittings on each end that allowed it to be mounted, inline, at the tailshaft - between the cable and the sleeve/adapter for the driven gear. These adapters could be ordered in specific ratios, but usually in small increments - intended for fine tuning. If you search the 'net for "speedometer ratio adapters", you should find suppliers of custom-built units, but be forewarned that they are more expensive than a pencil gear.

Swapping gears or installing an adapter isn't difficult work, but you better take good notes on observed vs. actual readings over a range of speeds and get your math right to ensure that you get the right part.

Yet, before you go to all this trouble, make sure you have a tire size that is as close as possible to original for your car or a factory optional size (provided you have the correct gear(s) for the latter) and it's a size that you plan on staying with for a long time.

Also, the above corrections assume that the speedo head itself is in good working order. For example, a speedo head the exhibits roughly the same MPH deviation over the normal working range will NOT be corrected by swapping gears or installing a ratio adapter.