You can't over gear a lawn mower with a 3.9X gear. Ever. A /6 wouldn't be over geared with a 3.9X gear.
The problem is now, and always has been, that guys don't use ENOUGH gear and ENOUGH converter. It's just dumb really. It's simple math. RPM, MPH and tire diameter. How so many guy miss I can't say, but it's epidemic at the very least.
Had a customer with a Dart and a 383 Chevy in it. Built the engine for him and told him what gear to run. He says ok.
Car goes out and it's a pig. I mean a real porker. And he tells me it's turning 8200 at the lights.
After I fall on the floor, laugh my *** off with tears running down my face I say I told you what gear to run (it was 5.38) and he says yeah, I told everybody what you said, and I put the 4.30's in there so it wouldn't rev it's guts out and now it's still wound up too tight.
So I tell this mental pigmy there ain't now way on this big blue marble your silly *** in crossing at 8200 with a 4.30 gear, a 31.5 inch tall tire and your MPH. Either your tach in junk, the converter is junk, or the clocks are wrong.
The tach goes back to Auto Meter and they say it's perfect (they always are). But now, the car is crossing at way less RPM at the same MPH. Junk tach. But he was impressed when he could tell his drinking buddies his **** was turning 8k through the lights. But it wasn't, and it needed to.
I tell this dolt again, to get some 5.38 cogs in the pig and let it growl. But noooooooo...all the fence leaning gurus told him 4.88 max.
Fortunately he compromised and used a 5.13 and on a good day it would cross at 8000.
Just another example of guys listening to guys they should ignore.