Speedo reading wrong - '93 corolla

[jayn5]

Hi all

I've been having speedo issues since I got my car, it's reading about 10-15mph above what I'm actually doing, and when it's really cold outside it can easily read twice the amount, apparently I was doing 110mph on the motorway at one point....

The odometer/trip seems to be reading correctly, even a smidge fast, but I haven't tested it thoroughly yet. I've done some research but haven't found anything concrete, I've heard it could be the speed sensor in the trans, the needle head is gone, or it could be wiring related. Any ideas what it could be or how I could diagnose what's causing it? I don't have access to specialist tools though. It's only got 45k on the clock and always well serviced, and would be surprised if it's a trans problem. Thanks!

[FROSTYBALLS]

Have the wheels/tyres been changed from the standard spec? If so, use a tyre checker website to check the effect on the speedo - http://www.autopartuk.com/tyre-calculator

[jayn5]

2 minutes ago, FROSTYBALLS said:

Have the wheels/tyres been changed from the standard spec?

I've currently got 15in alloys on it, but the speedo was also reading wrong on the OEM 14in steelies 

[FROSTYBALLS]

Legally the speedo has to be optimistic.

In the UK a speedometer should never under read. The indicated speed must not be more than 110 percent of the true speed and for all actual speeds between 25 mph and 70 mph (or the vehicles' maximum speed if it is lower than this), the indicated speed must not exceed 110% of the actual speed, plus 6.25 mph. For example, if the vehicle is actually traveling at 50 mph, the speedometer must not show more than 61.25 mph or less than 50 mph.

[RumbleO]

I have a similar problem on my 1999 Corolla. @Jayn5 ... Does your speedo start at zero, or does it show 10-15 mph when you are stopped? Myspeedo reads 15 mph when the ignition is switched on and not moving. It then treats that as virtual zero , so when I am doing 30 mph it shows 45, etc. (I haven't checked if it drops when reversing.) I suspect it could be a faulty resistor, if the speedo is electronic and activated by Wheatstone Bridge type arrangement. Has anyone else had this issue, and how do we resolve it?