I had AAMCO do the work on the truck. They rebuilt that 5sp manual transmission around 176,000 miles ago, and figured I'd let them have a go at it again. They found a used transmission with around100,000 miles on it for $1100. I said go for it. Well, I got the truck back on Wednesday. It seemed to drive well when I drove it home. I took it beaver'n on Thursday and noticed it used a lot more gas than normal. So today, I figured the mileage. Before the transmission repair, I averaged 17mpg. After the transmission repair, I averaged 12mpg. It was a puzzlement to me, so I called my retired Toyota mechanic friend and asked him why it would do that. He figures they put in a transmission from a 4 cylinder, which has different gears than the transmission for a 6 cylinder. I'll make a trip back to AAMCO to get this straightened out.

It'd have to be way off to make that big of a difference I'd think.

Has me puzzled too, maybe they damaged an oxygen sensor or something. Strange one.

Wow 17mpg prior to the repairs?? Seems steep for a newish Toyota. I’ve had several older Toyotas, granted non with a v6 but 17mpg?? One 85 I had with a 22r 5.29s and 49 inch tires, ran out at 20 mpg...I could also cruise at 70 on the hwy. Wife has an 05 Chevy 1500 4x4 4.8 v8 and she gets 20-22mpg on the hwy.
I have always been under the impression that newer vehicles were to be better economically?

Not a new Toyota - lol - 20 years old. This is a 2000 model with 677,000 miles on it.

Maybe your catch was weighing your truck down.

The 4 cylinder has a different model number transmission than the 6 cylinder. Supposedly the gears are different sizes in the two different transmissions.

4 cylinder - 330303D410
6 cylinder - 15398914

Did they try remapping the transmission ECU ?

Ok ok, I’ll give it that...I will also admit that 85 I had had been gone through almost completely. All but the body...it was built to be crunched on the trails. Engine and drive train was all rebuilt or new.
I am thoroughly impressed by your mileage at any rate.

Do they do that for a manual transmission in a 20 year old truck? I have no idea what remapping is - lol.

I don't doubt they're different ratios, but i think you would know if they were far enough apart to make that big of a difference in fuel mileage. Like engine sounding like you need to shift again, ect.

Try marlin crawler, advance adapters and trailgear for info on gear ratios. Seems like I’m forgetting a few others to check. But I know there’s a website that has all this info for parts swaps and such for the Toyota nuts. For example if your gonna swap up to a better trans for say crawling your gonna wanna know which one to pick by number. In my case the output shaft told me a lot..

Have fun ogling all the fancy off-road parts while your searching for ratio info;)

No, clutch is operating normally. I've put 612 miles on it the past two days and there's no problem, except gas mileage.

I am going to bet that the speedo drive is different in the two tranny's. Does the speed seem to be close to what the speedometer says?

Yes

Good call bear guy. I believe there are a few different gears, that said it wouldn’t effect the mileage, just the speedo.

Well, I guess it could effect your calculations but I think you’d notice a 5mpg difference without math but buy the frequency you’d have to hit the pump.

X2

1st, 2nd, and 3rd might be different between a 4 cylinder and 5 cylinder truck. 4th gear's gotta be direct drive 1:1 for both, and 5th gear is an overdrive gear, but the ratio's gotta be pretty close between the two transmissions. Is your cruising RPM pretty close to where it used to be, or is your RPM way higher than before? If your RPM is pretty close, I would suspect the gear ratios are pretty close. The speedometer issue is another possibility. It doesn't have to be too far off to make your trip meter/odometer way off over time, which would give you a false calculation of MPG.
There's likely no reprogramming needed for a manual trans in a vehicle that old.