I seriously considered putting a spring under the accelerator of an Isuzu van I had in New Zealand. after 80% throttle it would start belching black smoke and burn oil.
It was a bit anemic on the power side with a 1.6 L naturally aspired diesel engine pushing a 5.5 meter (18 foot) square nosed van. It took a bit of finesse keeping the speed up without rolling coal and a spring that would increase resistance at that threshold, but would also allow WOT when necessary would have been nice.
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