speedo slightly out when towing

[Iceash]

Hi all,

Slight strange one for you all. My speedo reads slower than normal when towing. Fine when not, any ideas why?

Thanks all,

Ash

 

[@udi A3]

Not enough right foot? Did you slow down for the new lower speed limit when towing?

How much slower is it?

 

[Iceash]

I'd say at least 13 percent so if doing 60 GPS was showing round 50mph. Maybe GPS was getting interface from caravan?

 

[aragorn]

GPS should never show faster than the cars speedo?

 

[Iceash]

Every single car I've ever driven regardless whether company/friends/hire the speedo is always slower. As long within 10% all ok.

 

[aragorn]

i suspect your satnav is flakey then.

Car speedos must by law overread.

Speedo is allowed to underread by 0% and overread by 10%

So if your doing 70mph, your speedo in car can show between 70 and 77mph. Most manufacturers seem to aim for the middle to top end of this scale. I've NEVER seen a car speedo indicate a lower figure than a sat nav, but i guess that assumes the satnav itself is accurate, and i guess they might not be. In my A4, 75 on the speedo seems to be around 70 on the GPS.

 

[Artimus]

are you sure your tyres are inflated enough to resist deformation with the extra weight? squashed tyres could result in inaccurate speedo readings

 

[Iceash]

In my A4, 75 on the speedo seems to be around 70 on the GPS.

Sorry lol that way round, I'm shattered lol

 

[Iceash]

are you sure your tyres are inflated enough to resist deformation with the extra weight? squashed tyres could result in inaccurate speedo readings

fair point. Run at 35psi

 

[@udi A3]

Bs, same tyre rolling radius, wouldnt affect speedo

 

[Iceash]

Bs, same tyre rolling radius, wouldnt affect speedo

oh, so what can affect speedo's

 

[@udi A3]

Dont know, can you log anything in vcds? Id guess if the rear wheels were that flat and a different radius the quatro would play up.

 

[@udi A3]

You should still raise pressure though under that load

 

[Iceash]

Only shows a diff where towing the caravan.

 

[Artimus]

Bs, same tyre rolling radius, wouldnt affect speedo

You should still raise pressure though under that load

It's either BS or not - make your mind up.

FYI: if the rear tyres are squashed lower because of the tow weight, it'll have the same effect as having smaller tyres on the back! this is how some TPMS works, different pressures cause different rolling radii. the only question is, which axle feeds the speedo?

 

[Iceash]

Think the sensor is at the front?

 

[Artimus]

I'd increase them to at least 40psi for towing - can't hurt to try.

 

[Iceash]

Front and back?

 

[Artimus]

check your drivers door shut for tyre pressures, as the fronts (on the A3 Sportback at least) are usually higher than the back for standard 3 passenger + luggage driving! you'd need the back to be higher than standard load when towing.

 

[@udi A3]

It's either BS or not - make your mind up.

FYI: if the rear tyres are squashed lower because of the tow weight, it'll have the same effect as having smaller tyres on the back! this is how some TPMS works, different pressures cause different rolling radii. the only question is, which axle feeds the speedo?

So you are saying that the lower pressure/flat tyre, will cover less distance each revolution? I say no. for that to happen the tyre would need to slip on the rim. Otherwise the tyre acts like a tank tread and covers the same distance each revolution.

OVER inflated tyres however COULD affect the speedo reading at the rubber COULD stretch giving a larger circumference.

If the difference was great the 4x4 sytem would throw faults or the abs/tc systems so I highly doubt thats the problem.

My speedo glitched a few days back and was reading faster for a few minutes. Didnt get to the bottom of that either.

 

[Artimus]

So you are saying that the lower pressure/flat tyre, will cover less distance each revolution? I say no. for that to happen the tyre would need to slip on the rim. Otherwise the tyre acts like a tank tread and covers the same distance each revolution.

OVER inflated tyres however COULD affect the speedo reading at the rubber COULD stretch giving a larger circumference.

If the difference was great the 4x4 sytem would throw faults or the abs/tc systems so I highly doubt thats the problem.

My speedo glitched a few days back and was reading faster for a few minutes. Didnt get to the bottom of that either.

The Tyre Pressure Monitoring System I've activated on my A3 via VCDS works by measuring the rotational speed of each wheel, any wheel spinning faster than the other 3 is flagged as being low on pressure! I currently have a slow puncture in one of my tyres and the system has flagged up for as little as a 4psi pressure loss.

Just because YOU SAY NO, doesn't make you right.

I'm not saying this is definitely the cause of the speedo mis read in this case, but it's a possibility which can easily be crossed off the list by increasing the tyre pressures ...and unless the OP has rubber bands for tyres, 35psi will be too low for towing anyway.

 

[aragorn]

tyre pressure depends on the cars weight and the tyres fitted.

Use the data plate on the car.

For "normal use" my avant specs 36psi all round, and the rears go upto 40ish for heavy load.