I have used electronize speedos in my two wheel drive version of red dave. Didnt have an issue with going fowards, just make sure its all set up well and that.
Possibly a slight issue with one side of the drive? One drill motor might be more nackered then the other. I find my two wheel drive robots will veer off to one side if there is a slight issue with one side of the drive.
Not an issue with 4wd robots.