I tried asking this in my thread in the build section but no responses yet. I noticed that when I travel above 40mph, the speedo tends to be off by about 6-9mph. I'm not running big rims or anything. Just some FD wheels and 225/45 or 55 tires. Any ideas? Thinking it might be the speedo cable, but I wanna be sure it's that before I go ahead and replace it. Thanks. It's an '89 Turbo if it makes any difference.

 

So the first thing I will say is that speedometers.. especially those that are older.. are not going to get more accurate as you increase speed. Quite the opppsite. Second, do you know the OD of your wheel tire combination and whether or not you've stayed within the specs of the factory calibration ? The Infamous miata tire calculator would be perfect for this, but I certainly won't do it for you! :-)

 

especially when you are working with cars of unknown history. could be a manual swapped auto car with the lower geared differential and then add in the wheels are 1" larger diameter which is going to throw off the tire outer diameter.

supposedly there is an adjustment screw on the back of the speedo to recalibrate it, though i haven't needed to find it since my speed is only 1-2 mph off at 75mph(according to my GPS).

 

They have an adjustment in the back?? Never knew this... I'll give that a look.

I'd have to check when I get home in a couple weeks. I mean, the wheels are 16"... Shouldn't be off by that much. But yeah I'll look up that miata tire calc in a minute. Thank you

 

Stock tire size is 205/55-16. A 225/50-16 is closest to factory spec in overall diameter. If you're running a 45 or 55 aspect ratio, your speedometer is definitely off, assuming of course everything else is stock.

 

So do I look for the adjustment thing in the back of the gauge cluster like Evolution mentioned?

 

I've never looked for it. I have had my speedometer out, but never knew such an adjustment existed and thus didn't notice one was there.

 

satch mentioned it being there, perhaps he knows where to look for it. i looked at the back of a cluster and it isn't there so it would have to be on the speedo unit removed from the cluster itself.