Question on Gearvendors overdrive . Is it a basic bolt in operation for a

I suspect the lack of fuel efficiency increase after installing an overdrive device be it GV, a 4,5, or 6 speed transmission often can caused by a big lumpy cam that is rather inefficient at low rpm. Running a big overlapping cam that needs high rpm to improve scavenging requires a rear gear ratio of 3.75, 4.11 and on up. While running an engine so equipped at low rpm sends a lot of un-burnt fuel out the tail pipe, and drops below the ideal power & torque curves. Improving fuel economy of an engine equipped with a big cam is not going to happen, it is what it is

Also most of the engines with big cams I have seen are equipped with oversized double pumpers. These carburetors are not generally set up or tuned for economy, coupled with a big cam providing low vacuum while operating at low rpm can’t deliver economy or performance until engine gets back up into its designed power curve. This is a reason for not seeing any significant economy increase’s by adding an overdrive to such vehicles.

An overdrive installation would work a lot better mated with an engine having a torque build sporting a vacuum secondary carburetor where torque comes on just off idle, and carburetor always sees a good vacuum signal at idle.

We have become spoiled with modern automotive drive trains delivering the best of both worlds; efficient fuel injected, variable cam timing, HO engines, and eight speed transmissions utilizing mutable overdrive ratios in two plus ton 400 hp monsters capable of great ¼ mile times, and mpg in the high 20’s all day long.