Manual Previa Gearing Issues

[batook]

I love my 5-speed MT Previa, but there's one problem: the 5th gear O/D is only .85, as opposed to the 4th-speed O/D gear in the automatic at .75. With the 4.30 Final Drive Ratio on the naturally aspirated Previas, that means I'm running around 3900 rpms at 70, and over 4000 anytime I'm going 75-80. This is a problem for me since 90% of my driving is on 75mph freeways, and running the engine for long periods at 4,000 rpms can't be good for it...plus it's REALLY loud. I have an aftermarket tach installed in my Auto Previa, and it runs way lower: around 3,000 rpms at 70-75, and around 3500 at 80-85. Much better.

Ideally, this van needs a 6-speed gearbox. That's probably a pipe dream though. Anyone know if Toyota made any 6-speed transmissions in the early 90s that might work with the Previa? I'm not opposed to having a custom adapter fabbed if necessary in order to bolt it up, but I'm not sure if a 6Spd even exists.

Another option would be trying to fit a 5-speed from another Toyota with a taller top gear, something like .75 or less. Not sure if that exists either, and again, probably a lot involved in getting it to bolt up, but I'm up for it if I can find something workable.

I'm not too familar with manual gearbox internals as I've never had to crack one open, but I do have a spare 5-speed that I could experiment with. I wonder if it would be possible to swap out the .85 5th gear in the constant mesh gears for a .75 or lower gear. If I can't buy or source a pre-fabricated gear, I could maybe have it machined out of brass too if it would be possible, but I'm not sure if that could even be done.

One solution that would definitely be do-able would be to pull the diffs from an S/C Previa for installing on my MT, which have 3.73 FDR instead of 4.30. This would probably put the revs more in line with the automatic Previa when doing 80-85, but at the cost of more sluggish performance in all gears and worse hill climbing abilities given the less aggressive gearing. I'd rather not go this route due to that compromise and due to the cost: because it's an AllTrac, I'd need both Diffs, and I'd also want to switch out the speedometer driven gear in the transmission to get the speedometer to read accurately, and I'm not sure if the speedo-gear from an S/C automatic transmission would even fit/work with my manual transmission.

Just brainstorming here and trying to figure out what could work. I don't think Toyota anticipated modern 75mph speed limits when they designed the van in 1990...they certainly don't drive that fast in Japan (at least not 20 years ago!). Thanks, Eric.

[timsrv]

Yeah, good question. You could go with the SC diff's & a 2 speed transfer case from an 87-89 MT van, then you'd have 10 different speeds to choose from......... Internal transmission gearing and possible gear ratio mods is one of my weak spots, so I couldn't help with that question. Tim

[batook]

Well, I know the Previa's transfer is 1:1, so I assume that 2H and 4H in the TV transfer is also 1:1, right? I'll have to look into what it would take to change out the gears in that transmission...someone at the Yahoo group pointed me to the Marlin Crawler forums...apparently they mess with gearing a lot there, so maybe they can provide some guidance, or at least let me know if what I want to do is even feasible. Eric

[maxemum610]

I have read in other forums of putting the diff from the Supercharged version in, as it has a different ratio.
They say it stops the over revving when in top gear.

They say the Manual Previa is great for around town and the auto for highway.