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MPG and Manifold Vacuum
My T5 OD transmission gear is 0.76 with a 3.73 rear axle but I have not seen any improvement in MPG compared to the factory 3 speed with 1:1 final drive.
Others have suggested that I'm increasing throttle when in OD and that the Rochester B power valve kicks in - thus wasting fuel. I disagree since I do not lug the engine when driving, and I usually don't shift into OD unless I'm on the highway. Back road cruising in OD is tempting but manifold vacuum drops a lot, so I don't do that.
I hooked up a vacuum gauge to the manifold to check what happens when driving. If I'm on level highway and keep the throttle steady, manifold vacuum drops a little when I shift into OD. I think a slight drop in vacuum should be expected since the engine strokes that produce the vacuum are fewer than when in 1:1 and it's not a perfectly sealed system.
I hope this will remain an academic discussion. I know I can spend a lot of cash on EFI or just swap to LS, but my questions are more related to MPG and Overdrive transmissions.
Here are some details.
1963 230ci with a Rochester B carb. Engine is completely stock.
Oil pressure is 25 PSI at idle and near 40 when driving.
I have 135 PSI in all six cylinders and absolutely no smoke.
1963 C10 Fleet short bed.
Rochester B carb
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