I recently upgraded my TomTom One (version 3 1G) maps as I noticed it didn't have newer streets and, more importantly, also lacked many speed limits. This would cause it to assume 30 mph for such roads, so it would head out of the way towards interstates when the best path was almost always a state highway (50 mph in my area). When testing the new setup, I noticed it still was doing so and assumed that info was still missing, but yesterday I was surprised to see it in fact knew all the speed limits for the route I was driving (displaying them as I went), yet was still using 30 mph for my arrival time estimate. The way I went yesterday is 36 miles, which it estimates as taking 72 minutes (30 mpg average speed), whereas Google Maps correctly puts it at 49 minutes. Didn't matter too much in this instance, but longer trips lead to hugely longer planned routes, requiring I force it with waypoints onto a better path.

Does anyone have any idea why it would seemingly be ignoring the speed limit info? Interstates still seem to work properly, as before. Let me know if there is a specific test I can perform to yield more clues.

My unit's info (TomTom Home says I have the latest Navcore updates):

TTGO.BIF:
DeviceName=TomTom ONE
DeviceVersionHW=ONE (v6)
BootLoaderVersion=55001
ApplicationVersionVersionNumber=8010
ApplicationVersion=9369
CurrentMap=USA and Canada 880.3868
CurrentMapVersion=880.3868
TTNAVIGATOR.BIF:
TTGO.BAK:
Not Found...
TTGO.ORI:
Not Found...

Thanks a lot.

I have a similar problem. Travel time calculated as per 33 m/h. Same type TT.

It is an estimate as the name already says.
The road condition, historic traffic, traffic light - all that is TT trying to put in.
A perfect system would tell you the exact arrival time, but nothing is perfect in this world ;)
Dirt- and unmade roads can lift your travel time quite badly. They are calculated for a speed of 25km/h at max.

Their algorithm is basically worthless for route planning, unfortunately.

Might be your opinion for your area, here where I live Garmin is worse when it comes to correct times.

Well, at least it doesn't seem to be something wrong with my unit, if other people are getting the same behavior. All Google Maps seems to do is use the distance divided by the speed limit, and they're pretty much spot on. Traffic and construction would affect that, of course, but here in the sticks we have little of either. Sure glad I was able to save my favorites when I updated, as I've stored a lot of waypoints for longer trips I take. Thanks for the input, everybody.

The results are only as good as the data entered. AKA "Garbage In, Garbage Out!!!!"

Well, to ignore the speed limit data is pretty broken, algorithmically speaking. I may ditch the unit and find a better one, since they're so cheap nowadays.

Have done some digging in regards to the problem:
Tomtom does not use the actual road speed for the calculation, but something called "historic traffic",
which is data collected over the years by users on the devices plus data provided by the local authorities.
On top of that they seem to extend the times of peek hour traffic, according to some rumor that should help with congestion as the traffic would be diverted earlier - as if everone would have a TT in the car....
Other GPS providers use simpler Methods but that does not mean they are more or less accurate.
As an example from a few of my trips in my region:
Mostly travelling over the freeways gives an almost accurate time, differs only by a few minutes.
Country travelling can be accurate but get very inacurate when unmade roads or single lane roads are involved.
City travel is almost always way off as I either arrive a lot earlier or much later depending on the traffic.

Try yourself to check the estimated times in different conditions and you might get similar results.

Few dirt or single lane roads here, so that can't be it. Highways do seem very accurate, but I live pretty far from them. Maybe the traffic data is messed up for the smaller roads? It'd have to be pretty heavy to slow 50 mph down to 30. Maybe if there's been snow recently. :D