I've seen the mileage my truck can get when I go the speed limit of it's era (55) and I like it. The problem is all the freeway driving I do anymore is at least 70, sometimes more. Does Marlin sell a taller 5th gear or does anyone know of where I could get one? All the other ratios are fine, but when I do the 4th-5th shift, RPMs barely drop. The truck has plenty of grunt to maintain speed, I just need to get the RPMs down another 250-500 RPM.

that would be the answer to all my problems aslo, i wish one of the fabricators could make out it should be that tough

Taller tires are cheaper and easier. At 75/80 mph you want to be doing about 3K rpm.

I did - - - by switching to 33's without changing gears.
It cruises beautifully at 125-135km/h at around 3000rpm (with maybe 1/4 throttle needed)
Try some 33x9.5 bfg a/t's or m/t's with some aftermarket aluminum rims. My mickey thompsons were WAY lighter than the stock SR5's....Your rpm's should drop a good 500RPM, making cruising at 60mph good in 4th, and 75 in 5th...

I'd go for bigger tires, but I have absolutely NO need for them as the truck is a DD and the wear and tear and loss of acceleration in the other gears isn't worth it to me. A taller 5th is really all I need. Mostly I was hoping there was another gear from another transmission or something that could be used.

a new ring & pinion ratio.

Well if he doesn't want to change his ratio by going to taller tires, why would he want to spend way more money to change gears to get the same result ? <img src="/forums/images/graemlins/rolleyes.gif" alt="" /> <img src="/forums/images/graemlins/rolleyes.gif" alt="" />
The answer is there is no way to change just the 5th gear ratio, other than modifying the tranny.

You can't change just one gear because the cluster gear is one-piece. An aftermarket "kit" would require both a custom cluster and 5th gear ($$$), plus the labor to install it and then you might as well do a rebuild while its apart...

If you have an R150 trans (V6), you can swap in an R154 (Supra) gear set. That is the only thing available that I know of and even that hardly seems worth it. And it changes more than just 5th gear too.

A few observations that I have seen on my truck is that I got better MPG when I ran 33s with 5.29 gears than I do now with the same size tires and 4.88s. Higher RPMs worked out to higher MPG in my case. I do find the truck more driveable at the lower RPMs, less noise and vibration make long drives much more tolerable. I also find that on my truck, at 55, I get better MPG that I do at faster speeds. Air resistance is likely the main factor there.

I highly doubt you'll see any significant change in mileage at 70 mph, even with a higher first gear.

The problem between 55-70 is a more than gearing. You're dealing with a significant increase in wind resistance and that's probably where most of the mileage loss comes from. With higher gearing, the engine is at lower rpm, yet it has to overcome much higher wind resistence. The lower the rpm, the harder the engine has to work to achieve the same output and that mean less gas mileage.

To see what I'm talking about, trying climbing a slight grade at speed in 5th gear and then 4th gear. Both gears can maintain the same speed, but 5th gear will cause the engine to work harder and use more gas than 4th gear even though 4th gear is lower.

The only Toyota transmission that has higher gearing than yours is an automatic. Instead of a .85 overdrive, they have a .70 overdrive. But since I've never heard of an automatic owner bragging of excessively good gas mileage, I suspect that wouldn't be worth it either.