XJ6 & XJ12 Series I, II & III 1968-1992

Speedometer Error

Old 09-08-2014, 07:53 AM
Dale Konkol's Avatar
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Default Speedometer Error

My speedo seems to be showing slower speeds than actual. The trip computer seems to be right on as far as indicating actual speed. I hold the reset button on the t/c while reading "average speed" while traveling at set speed thru one of those police speed trap/indicators and the t/c is right on. Speedo reads 30 mph, t/c shows 32. Speedo 50, t/c 53.6. Speedo 70, t/c 76mph.
I was thinking it might have something to do with the new 16" lattice and tires I just put on. But they are the same diameter within .3" (taller) of the original oem size tires. P225/60/16 vs. P215/70/15.
I wonder why the trip computer is right on but not the speedo. And I wonder if it hasn't always been this way but I just hadn't noticed. I would think I would have noticed since owning the car since '93. Never a speeding ticket in that car and I do tend to push the 5-10 mph window the cops will allow you.
Is there a fix to this, or should I just live with it?
Old 09-08-2014, 08:11 AM
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The speedo and the TC share the same speed sensor and most of the same wiring. A tire size change would affect both equally, it seems.

Normally a speedometer reads too high...in which case your slightly taller tires would bring the reading downward....towards a more accurate number.

But that's not happening in your case.

I'd say the problem is actually inside the speedo itself

Old 09-08-2014, 10:34 AM
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I'd find a better data source than the speed trap indicators. Almost all GPS units will display vehicle speed. By law, actual vehicles speed must be the same or lower than gauge indication, but never higher.

Incorrect tire size is the most common source of error.
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