How to get an accurate speedometer using GPS

[Editor's Note: This article originally ran in the May 2017 issue of Classic Motorsports.]

One of the trickiest facets of a restoration is getting the speedometer and odometer to read correctly. Old car speedometers are notoriously inaccurate. And we compound this problem when, in an effort to increase performance or drivability, we change tire sizes, rear-end ratios and transmissions. Our Triumph TR6, for example, has been fitted with a Toyota five-speed conversion. If only there were a way to get accurate readings from our original speedometer.

Then we heard about Classic Speed, a patent-pending, GPS-based conversion kit from Classic Automotive Innovations that simply connects via cable to the original speedometer. We assumed it was too good to be true. Still, we had to try one. 

Like a smartphone app, Classic Speed uses GPS satellites to accurately determine your speed. It picks up that signal with a small, amplified antenna that can still get a signal despite tall buildings, tree canopies and other interferences. 

Fitting this antenna is easy, too. It contains a magnet, one strong enough to even stick to steel covered with a light piece of vinyl. Attach it to your dashboard and you’re done. 

The GPS signal is sent to the unit’s control box, which measures roughly 3x6x2 inches and mounts inside the engine bay. Then, simply find the speedometer cable, disconnect it from the transmission, and reconnect it to the control box. Now the GPS signal tells the speedometer what to do. 

The Classic Speed kit sells for $275 and includes a cap so transmission fluid won’t drip from the now-unused speedometer cable port. An extra $37 adds a custom speedometer cable that’s as long or as short as your new setup requires.

Classic Speed is designed to work with any pre-1985 car running a 12-volt, negative-ground electrical system. Most of our classics have standardized speedometer fittings at each end, and the folks at Classic Automotive Innovations tell us they’ve yet to find a car that won’t accept the kit. Our Triumph TR6 with Toyota transmission seemed like the perfect test bed, especially since we have struggled to fit a traditional speedometer cable. 

  

The kit arrived promptly, the directions were clear and simple, and the customer service from company owners Brian and Pam Induni has been over the top. 

Installation was even easier than we anticipated. Since our speedometer cable was broken, we just left it hooked to the transmission. Then we unhooked the other end from the back of the speedometer. 

We attached the Classic Speed control box to the top of the footwell, right by the master cylinder. To insulate the control box, we sandwiched a piece of foam between it and the car. Then we ran a new 12-inch cable from the speedometer to the control unit. 

From there we hooked up power and ground per the instructions, then went through a quick procedure to acquire a GPS signal. Voilà, the speedometer came to life. 

When we compared the GPS-controlled speedometer to our phone app, the speedometer seemed to read a little high. So we used the calibration dongle that comes with the kit. This involved simply running the TR6 at different speeds, stopping for a minute to plug the tool into the box, and adjusting the computer a couple of clicks. Within a couple of tries, we had the speedometer within 1 mile per hour of our phone’s app. We figured that’s close enough and, of course, assumes that our phone app is 100-percent accurate. 

How much do we love this new product? We’ve already ordered another unit for our Edsel wagon, whose speedometer is at least 10 mph off at highway speeds.

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