tach question

[dzbowtie]

just installed an auto gage 2301 in my 70 c10 with hei. the tach is set to read 8 cyl. the wires are connected properly, with the ground attached to the bottom of the metal dash (is this a bad location?) when the truck is in park, the tach seems to read correctly, idles approx 500, and when i hit the accelerator, the tach will raise to over 4000 as expected. however, when the truck is in gear, the tach will raise up to around 2500, but seems to slow down, topping out around 32-3400, when it should read 4000+. what would cause this to only occur when driving, as opposed to in park? the tach is mounted at a slightly upward angle (nothing drastic), surely this would not effect operations? this is not the first tach that has done this, which makes me believe that i did something incorrectly (power to key on source, green to hei port), or there could be some issue while the truck is driving. any advise would be greatly appreciated. could there be too many grounds in one location?