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What would the mileage difference be between a 351m & 400m with mild upgrades. I am going from a 351m to 400m with all else being the same. what should I expect?
Assuming that the 400 is properly tuned and not overly built, you should not see a difference. This is attributed to the larger displacement engine having to turn fewer rpms to move the same vehicle.
My brother has a stock 351M in a 79 1/2 ton and get around 18-20 mpg and I have a 79 4x4 1/2 ton and I get aound 11-12 mpg. That should give you a good reference.
My brother has a stock 351M in a 79 1/2 ton and get around 18-20 mpg and I have a 79 4x4 1/2 ton and I get aound 11-12 mpg. That should give you a good reference.
Not sure that's a good comparison as there could be things that affect mileage other than the different engines. It would be more accurate if they were both 2WD (or both 4WD), had the same transmissions and the same gear ratios. Before I built my 351m into a 400, I got 12-14 on the highway. After the 400 conversion I got 11-13 on the highway. When towing with the 351m I got 8-9 mpg on the highway. While towing with the 400 I got 9-10 mpg on the highway. Same truck, same tranny, same gear ratio. I do know some however who've converted to a 400 and got worse mileage but this isn't necessarily because they went bigger. Oftentimes, they used too much cam, the wrong intake manifold and too much carb, all of which also affect mileage also.
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