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Hard to say if it would help or hurt. I will say this. I also have a 86 4Runner with a 22re (2.4l like the previa). It is on 33's spinning 4.88 gears has a lumpy engine builder "RV" cam. It gets better mpg then the SC Previa.
As the displacement grows the "sweet spot" comes at a lower and lower RPM. I know my 4Runner likes to spin between 2800-3300 RPM. Thats where I can squeeze 24mpg out of it despite the 33's. Not only does it get better MPG it is faster 0-40.
The N/A Previa was geared to spin about the same RPM at the same speeds. (think they came with 4.10 gears maybe a 4.30) and people all over the place claim 24-26mpg and up to 28mpg with the manual. The complaint was on the hwy they topped out about 70mph and felt very labored. The supercharger let them run the 3.73 and improve the top end. at 70 my SC still feels peppy. The bottom end and take off was improved with the clutched super charger giving about 6psi of boost but it gets significantly worse MPG.
I wonder how it would do to split the difference and run say a 3.90. That is a common gear to the Supras and I think they run a 7.5 ring like the Previa so it should be a usable R&P. I wonder if the 2wd 4cyl trucks ran a 3.90. That might be a plug and play pig.
Somebody on Toyota nation or Toyota van people site stuck a NA diff in a SC to improve towing. Said it took the labored feeling out of it. He had to swap out the tone ring to get the ECM and speedo happy. He ran a bit taller tire on the rear as well to slow it down some on the hwy.
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