Optimistic Speedometer

[StiflerMR2]

Hey all, time to show my total lack of technical knowledge once again

I picked the car up from Murrays yesterday after a service as well as having a 2bular 200 cell cat fitted. I headed out onto the motorway and got up to speed. Only it didn't feel m/way speed and a lot of people were passing me. I pulled off into Kinross services and fired up the sat nav to check my speed.

When I re-joined the m/way the speedometer was showing an indicated 70mph with the associated engine revs but the GPS was showing me at 55mph. This was the pattern all the way home. To hit a GPS indicated 70mph (which felt like the appropriate m/way speed) the speedometer was showing 20 tuts above that.

As mentioned I have pretty much zero techincal knowledge so any help or pointers would be much appreciated. I'll get in touch with Craig on Monday to see what they suggest as well.

Cheers and TIA

[robin]

Is it possible your tyres are flat or that in fact you have the wrong size tyres fitted?

I would expect a small over-reading - worst case 10% at that speed, so an indicated 70 might be as low as 64 mph. The difference between 55 and 64 is 16%. Assuming you are using 225/45/R17 rear tyres then to get such a dramatic drop in rolling circumference you would more or less have to be running on the rims.

I wonder if it's a dash problem - perhaps the speedo unit is faulty/slipping on the spindle or needs to be reset?

Cheers,
Robin

[StiflerMR2]

Hi Robin, thanks for the suggestions.

The tyres are correct sizes and were the same ones it was on when it went in (when the speedometer was pretty close to the GPS). I've double checked the pressures and they were a few psi under recommended (I normally run 26psi rear, 24psi front) but nothing out of the normal.

Looking through the invoice I had forgotten that Murrays also replaced the Multi Function Relay unit and the 02 sensor, everything else was just standard belt service fair (inc. 4 new spark plugs).

Could that be the new MFR unit be the cause?

[kenny]

Try resetting it.

http://www.scottishelises.com/phpbb/vie ... =5&t=15058

[StiflerMR2]

Thanks for the link Kenny, am out all afternoon but will give it a shot when I get back tonight!

[StiflerMR2]

Robin and Kenny you are both stars!

I followed the reset procedure and it's fixed it. When I got in to do the reset I noticed it was sitting at about 8mph with the engine off - can't believe I didn't think to look at that first.

Thanks again chaps, bacon buttie on me next time I see you up at KH

-Mark.

[robin]

Not for me, thanks, I don't eat horse!!

You're most welcome and glad you got it sorted. It was probably a result of battery disconnection to fit MFRU.

Cheers,
Robin

[kenny]

I'll have Robins horseburger if he doesn't want it. No point it going to waste in a Findus lasagne.