Regearing transmission 5th gear?

[spookybathtub]

I'm trying to maximize fuel economy in my Yaris. I have several aerodynamic mods planned, but one thing that always bugs me is the high RPMs on the highway. I think that 5th gear on this car is way too close to 4th. Has anyone tried regearing to make 5th gear bigger?

[Betrivent]

People have completely swapped out the transmission for a 6 speed. It requires some cutting of the frame and some pretty advanced mechanical know-how.

[fnkngrv]

changing out that 5th gear most likely would take a long time to reap a benefit if any honestly unless gas goes up over 5 bucks a gallon. there is a reason for the gearing design and that is to help an already anemic mill NA stay in its power band at hwy speeds otherwise you would very well get worse mpg.

[DFWxB]

Echo trans...

[RagnaCaT]

True what fnkngrv is saying... with luck and used you can get from 1zz corolla Trans the 5th gear set .725 it will be around $160 new expect to pat over $280 then to take down the trans maybe $500 and to do the swap maybe another $150....
.725 is the only possible swap but if it falls below the powerband which it DOES your car will have a harder time trying to push the car which will translate to lower MPG so a $800 dollar investment is about 32 tankfulls...
For economy it won't payoff but for the hobby and joy I would do it anyway if its valuable... I actually swapped a longer 1st gear and a shorter 4th to my trans

[CrankyOldMan]

The .725 5th gear set is really only needed with a more aggressive final drive (4.3 from the xB) to reduce highway RPMs. Toyota has done a really good job picking the gear ratios to maximize fuel economy at common speed limits.

I agree with fnkngrv about the reduced economy--overdrive gears reduce torque in exchange for speed, but at freeway speeds you still need a significant amount of torque to overcome wind resistance and hills. The stock final drive with the super overdrive would mean you'd have your pedal to the floor more often, which means open loop fuel maps, which means pouring stupid amounts of fuel into the engine.

RagnaCaT and I have been working on custom gearboxes for a while now, and can both attest to the difficulty of pulling the gearbox yourself. If you have the tools, mechanical skills, and funds to get the parts, it may be a worthwhile mod. If you're going to pay someone else to do it, I don't think you'll ever recover the cost in fuel economy.

Perhaps a better investment would be some low rolling resistance tires?

[spookybathtub]

Thanks for everyone's input. I guess a 6 speed transmission swap would be better, to keep the powerful 5th gear, and then have 6th when cruising on flat highway. I think I could get a used 6-speed for under $200. I could do a lot of the work myself, but I think I'd need a little expert help. If that sounds appealing to anyone in SoCal, let me know.

[IllusionX]

Originally Posted by spookybathtub

Thanks for everyone's input. I guess a 6 speed transmission swap would be better, to keep the powerful 5th gear, and then have 6th when cruising on flat highway. I think I could get a used 6-speed for under $200. I could do a lot of the work myself, but I think I'd need a little expert help. If that sounds appealing to anyone in SoCal, let me know.

You would be cruising at about 3500rpm at 62mph with a 6 speed, so you won't get what you wanted to achieve.

The echo has the longest final gear, I wonder if you could swap that in. That would bring you down to 2500rpm at 62mph. But let's not forget the Yaris is a heavier car, and doing this is still counter productive I think.

[echo-francis]

no offense but you guys are wrong with the ''you will burn more gas if you lower your gear ratio because your engine will work harder''

lets take a look a some trans really built for fuel economy.

92-95 Civic CX/VX 1) 3.250 2) 1.761 3) 1.066 4) 0.853) 5) 0.702 final) 3.250

00-06 honda insight 1) 3.461 2) 1.750 3) 1.096 4) 0.857 5) 0.710 final) 3.208

those cars have less power then ours and there are more exemples of cars like that

and if you look at bsfc maps you will understand that a engine working harder is more efficient

[CrankyOldMan]

Originally Posted by spookybathtub

Thanks for everyone's input. I guess a 6 speed transmission swap would be better, to keep the powerful 5th gear, and then have 6th when cruising on flat highway. I think I could get a used 6-speed for under $200. I could do a lot of the work myself, but I think I'd need a little expert help. If that sounds appealing to anyone in SoCal, let me know.

That's unlikely, as the C160 is much harder to find, will be more expensive, has a much more aggressive final drive (4.5 to the Yaris' 3.7) and requires notching the frame rails on the driver's side. The C60 is more common, but is too wide to fit between the frame rails. If you've got access to a used one in good shape for $200, more power to you!

[CrankyOldMan]

Originally Posted by echo-francis

no offense but you guys are wrong with the ''you will burn more gas if you lower your gear ratio because your engine will work harder''

lets take a look a some trans really built for fuel economy.

92-95 Civic CX/VX 1) 3.250 2) 1.761 3) 1.066 4) 0.853) 5) 0.702 final) 3.250

00-06 honda insight 1) 3.461 2) 1.750 3) 1.096 4) 0.857 5) 0.710 final) 3.208

those cars have less power then ours and there are more exemples of cars like that

and if you look at bsfc maps you will understand that a engine working harder is more efficient

I'll keep an eye on my Scangauge the next time I'm on the freeway, but I'm pretty sure that fuel economy starts to decrease after about 3500 rpm. I've got the 4.3 final drive and .725 5th ratios, specifically to keep the freeway rpms in a more reasonable range than the .815 stock. There is an optimum point for efficiency, and it's not a linear relationship to rpm and speed. Wind resistance plays a huge role in efficiency.

[Bluevitz-rs]

The 6 speed runs 2950RPM at 60mph. The only advantage is that all the gears are very closely spaced so you can shift at really low rpms and still accelerate without bogging the engine. My MPG increased slightly in the city driving and stayed relatively the same at highway speeds.

[spookybathtub]

Looks like the 6 speed has a larger final drive, which combined with the smaller 6th makes a larger ratio than the 5 speed in 5th. So what about taking a C160 and using the final drive gear from the C50? That should achieve much lower RPMs on the highway, but maybe not enough torque in 1st.

On the other hand, Toyota's stock gearing is designed to pair with the stock aerodynamics. With reduced drag, maybe it makes sense to just swap the 5th gear for a .725.

[CrankyOldMan]

Originally Posted by spookybathtub

So what about taking a C160 and using the final drive gear from the C50?

I think the two gearboxes are mechanically incompatible, specifically the differential. The C50 uses an 8-bolt ring gear, the C160 uses a 6-bolt. BlueVitz can confirm since he's done the swap. You would also have to find a pinion shaft for the 6-speed that came with the 3.722 final drive, and I don't think that exists.

[IllusionX]

Originally Posted by CrankyOldMan

I think the two gearboxes are mechanically incompatible, specifically the differential. The C50 uses an 8-bolt ring gear, the C160 uses a 6-bolt. BlueVitz can confirm since he's done the swap. You would also have to find a pinion shaft for the 6-speed that came with the 3.722 final drive, and I don't think that exists.

You have to Frankenstein the 2 gearbox together to make a 6spd. It won't fit bolt on for sure.
And the Yaris uses the c150

[bronsin]

WHat you want is a 3 speed trans with the 3rd speed the same ratio was 5th is now. The more speeds the more wasted gas as you row thru the gearbox.

[fnkngrv]

Originally Posted by echo-francis

no offense but you guys are wrong with the ''you will burn more gas if you lower your gear ratio because your engine will work harder''

lets take a look a some trans really built for fuel economy.

92-95 Civic CX/VX 1) 3.250 2) 1.761 3) 1.066 4) 0.853) 5) 0.702 final) 3.250

00-06 honda insight 1) 3.461 2) 1.750 3) 1.096 4) 0.857 5) 0.710 final) 3.208

those cars have less power then ours and there are more exemples of cars like that

and if you look at bsfc maps you will understand that a engine working harder is more efficient

You are trying to compare apples to oranges here. regardless of what the numbers sheets say the platform makes a dramatic difference in real world experience. This is just as you really can't compare the LB and the sedan when making adjustments to the trans and anything having to do with speed. power, and economy as they have different drag coefficients as well as aerodynamics in general to include center of balance/stance. I have been doing a lot of research for a converse reason which is to enable me to obtain a higher top speed. Man it really is looking seriously like I would have to pour thousands into a totally custom gearbox with custom fabbed gears as well.

[RagnaCaT]

Originally Posted by IllusionX

You have to Frankenstein the 2 gearbox together to make a 6spd. It won't fit bolt on for sure.
And the Yaris uses the c150

IllusionX you've been around long enough to know that the Yaris USES a C50... And Cranky is almost a master on this subject.
And talking about Frankenstein my trans gets installed today with 1st 3.166 2nd 1.904 3rd 1.310 4th 1.031 5th .815 and a 3.94 final with helical LSD special thanks to Cranky for info and help...



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