Adjustment

[Skip Lackie]

Ya know, until we all had smart phones with built-in GPS receivers, none of us would have even KNOWN that we had a 6 mph error in our speedometers. In fact, it you go back and read the road tests in 1950s and 60s car magazines, speedometer errors of 10% or even 15% were pretty common.

[mrkstudebaker]

Yeah, I had a few people. Tell me just to leave it alone, and I’m probably gonna do that. When I first got my car, I didn’t have one light that worked in the interior. I had no dash lights no gauges worked, except the oil pressure I think worked. Now I have got everything working, I’ve had my car for three years and I’ve replaced everything from the front of the car to the rear end of the car. I just wanted that Speedometer to read exactly. Thank you guys for all your help have learned so much , And I appreciate all the input.

[r1lark]

Yeah, I had a few people. Tell me just to leave it alone, and I’m probably gonna do that. When I first got my car, I didn’t have one light that worked in the interior. I had no dash lights no gauges worked, except the oil pressure I think worked. Now I have got everything working, I’ve had my car for three years and I’ve replaced everything from the front of the car to the rear end of the car. I just wanted that Speedometer to read exactly. Thank you guys for all your help have learned so much , And I appreciate all the input.

If you really really want the speedo right on the money, you can use what I've always called a speedometer ratio box - it has gears that are selected to correct the speedo on your particular car. Used to be speedometer shops that would run the car on a test stand (kinda like a rear wheel dyno setup), determine the correction required, put the ratio box together, and install it on your car. There probably are still a few of these speedo shops in big cities -- South Bend may have one, and you would think Chicago would still have at least one (maybe). Or now there are places online that will make up an adapter if you supply them with the info on how far out your speedo is.

Go here to see what I'm talking about: https://mrspeedometer.com/products/ratio-adapter

And here is a video showing the guts of one of these adapters: https://www.youtube.com/watch?v=xPf2Z0hVPxA

[enjenjo]

The speedometer works by a magnet that drags the needle around. There is no direct connection between the speedo cable and the needle. If the needle doesn't match the odometer the magnet needs to be tweaked by recharging it. A speedometer shop can do that. If you use a ratio box it might correct the speedometer at some speeds, but likely not at all speeds. And the odometer will no longer read correctly.