Speedometer accuracy

[conedia]

My few months old V40 D2 SE Lux Powershift (with 15k miles on clock and I think on the original tyres) has a surprisingly optimistic speedometer. I have the optional 18" wheels with lower profile tyres. I can set the cruise control to 75 mph knowing that the car will run a true 69-70 mph (according to my satnav). Even at low speeds, a true 30 mph will indicate 34. What is typical for a V40? Does this wheel/tyre combination make a difference? (Only recent comparison is with old C5, which was never more than 2 mph out, even at motorway speed.)

[chiptivo]

On my old S60, the tyre/rim diameter was alterable via vida (Dice) and this setting changed the MPH and Speedo ratios.

[Ninja59]

Originally Posted by conedia

My few months old V40 D2 SE Lux Powershift (with 15k miles on clock and I think on the original tyres) has a surprisingly optimistic speedometer. I have the optional 18" wheels with lower profile tyres. I can set the cruise control to 75 mph knowing that the car will run a true 69-70 mph (according to my satnav). Even at low speeds, a true 30 mph will indicate 34. What is typical for a V40? Does this wheel/tyre combination make a difference? (Only recent comparison is with old C5, which was never more than 2 mph out, even at motorway speed.)

Whilst is more accurate you should not really use your GPS as they cannot accurately measure on inclines and when there is any kind of cover.

The worst it will be is 10% which is the legal maximum.

[bradams]

I haven't been on the forum for a while, so am backreading some threads of interest.

When I first got my V40, I checked out the accuracy of my speedo digital readout against my Road Angel GPS (which constantly gives the same reading as roadside speed displays as I pass them). At 30mph true, the car displays 32mph and at 60mph it is 63mph. I also set my cruise control at 73mph on open roads with fixed cameras and have never had a ticket. That has been the case for 6000 miles now, so I am happy that these readings are reliable.

[RSVolvo]

I have the same issue in my S60. In the modern electronic age I find it annoying Volvo don’t produce an accurate spedo. I do a lot of motorway driving and it’s now very common to have roadworks with average speed camera and even ‘smart’ motorways with variable speed limits. In these areas I use the speed limiter and cruise control a lot. The way Volvo have designed it it’s easy to set to a 0 or a 5 but if you set it at 50 you have a HGV up your backside so you have to do what Bradams does and set it a couple of mph higher. Do-able, but fiddly. An accurate spedo would be so much simpler...

[Bonefishblues]

The issue is that manufacturers can be fined for any speed under-reading, whereas there's a wide tolerance for over-reading.

Guess which side they (all) err on?

[Westyman]

When I took my Westfield I had built for SVA the speedo under-read which was a fail. Removing the speedo and altering some DIP switches sorted it on the day.

[Ninja59]

Originally Posted by RSVolvo

I have the same issue in my S60. In the modern electronic age I find it annoying Volvo don’t produce an accurate spedo. I do a lot of motorway driving and it’s now very common to have roadworks with average speed camera and even ‘smart’ motorways with variable speed limits. In these areas I use the speed limiter and cruise control a lot. The way Volvo have designed it it’s easy to set to a 0 or a 5 but if you set it at 50 you have a HGV up your backside so you have to do what Bradams does and set it a couple of mph higher. Do-able, but fiddly. An accurate spedo would be so much simpler...

You hold it to increase by 1, press to increase by 5.

In honesty just let the trucks by more easily in roadworks.

I just sit at 45.