What is the best tool to track US positions from the UK?

[Webrevolve]

We are currently tracking the rankings of a US-based client (we are UK-based) and it appears we are seeing some considerable differences in positions.

To check ranks we often use Fat Rank which can be set to a US-based search and a manual search on Google.com. However, our American client is receiving different results (actually higher positions than we have recorded!) and is claiming to have used both a manual check on Google.com and the following tool.
http://smallseotools.com/keyword-position/

Would anybody be able to tell me how accurate this tool is regarding US-based positions?
We’ve since made use of the tool too and it adheres to the rankings that the US-based client is claiming – rather than our checks using Fat Rank and Google.com.

[jesse-landry]

Well... I'd use the Moz Rank Tracker but that may be too obvious seeing as how we're on the Moz forums and all. But just in case: http://moz.com/researchtools/rank-tracker

[dotfly]

searchmetrics & sistrix are international tracking tools used in Germany... but not for free

[BlueprintMarketing]

I agree with Jesse you are already a Pro member and the ability to track keywords in the SERPS comes with your complete package.

I would use

http://moz.com/researchtools/rank-tracker

or

This is a fantastic tool it allows you to check almost every country on the map except for North Korea and even get as granular as by zip code

This link will take you to the free check however you will have to enter it manually and you must sign up for the trial and then pay for the software after 30 days to continue tracking rankings all over the world.

https://serps.com/tools/rank_checker this is free

You can also use the trial versions and keep in mind you must pay after 30 days both of the tools here

https://serps.com/
http://authoritylabs.com/
can give you the zip code level searching. I utilize Moz, SERPS & AuthorityLabs I find it very comforting to have that ability and honestly most of it was from interest in which was better at what and what made them different. I would recommend connecting Google analytics to all of them
The only time I would not use Moz is an extremely rare occasion where local matters a lot for instance you want the zip code of whatever country or state has the answer you're looking for lies in whatever countries Providence, broken into zip code via state or Providence

I would then consider using

SERPS.com they have a free tool as well that will allow you to check your zip code rankings in any country for free however you must sign up and use the trial than pay if you wish to track them.

I also agree search metrics & sistrix are outstanding tools and unfortunately is not free but will do the job as well.

Great answers everyone.

Sincerely,
Thomas