Gear up for fuel mileage ?
 

I know You can gear down for big tires, etc.. Does anyone know where I can get parts to gear up ? I'm installing a buick 3800 in a 90' pickup and would like the motor to run about 2200 rpm @ 65 mph with 245/15 tires. If You could do this I think You could get around 26-29 mpg and have better power. Thanks Kiwi

ive never heard of any toyota gears higher than the stock 4.10's unless the first gen trucks had higher gears, other wise you could get different transmision gears put in.

There were some 2WD trucks (so a smaller rear ring gear, I think 7.5", but I could be wrong) that came with 3.90 ratios. There might even be some 4WD models that had that, not sure though.

Just put on taller tires, same effect.

Hey tc, You are correct, but increasing the unsprung weight and rolling resistance is bad for fuel mileage. Dont people with monster tires just change tranfer case drive gears in gear driven 4x4's. My 90' 4x4 is gear driven. Thanks Kiwi

usually people will change differential gears before they go for tcase, 4.10 is the tallest that i know of for our toyota trucks. the only way i could think of getting taller gears would be to swap out axels for ones that can get taller gears. or try and get the toyota auto tranny to work with your 3.8, with its huge overdrive that will bring you down to where you want.

Hey Kiwi,

My feeling is that tire size affects mileage exponentially with size. That is, if you went from a 29" to a 31" tire but geared it so your RPMs were exactly the same, you would barely loose any fuel economy. The jump to 33" would cause you to lose a little more (I am running 33's, 4.88 gears, 3000rpm at 70mph and I get 20mpg). Then the jump to 35" would start showing bigger signs of economy loss.

My off-the-cuff calculations tell me that with a 33" tire, stock 5-speed case, and 4.10 gears you would be running 2500rpm at 65mph. That might not present much difference over 2200 rpm either... a higher rpm does mean more friction (a little) but also higher engine vacuum, so in a sense your engine is not working as hard.

I've experimented with that a little with a vacuum gauge on my 4Runner. When I'm going 70mph at 3000rpm and hit a steep hill, I can either pull it at full throttle (0 psi vacuum) or, in 4th gear, I will be at 3400 or so but be at 8 psi or so, which I believe is directly related to how much fuel the computer is telling the injectors to deliver. For reference idle is about 30 psi or so.

Hey Rust,
If 60 mph means 1 mile per minute over land, and My rpm's increase by 100 revolutions per minute, drawing 3.8 liters of atmospher per revolution, at a syncometric ratio of 14.7 to 1 , what would be the 100 rpm increase in fuel consumtion be. Answer in liters would be ok, I'll convert. Touchee!! luv ya. Thanks Kiwi. p.s. Yes I am drinking

Clearly.

NOTE: throttle position has MUCH more of an effect on gas mileage than RPM.

For example, the difference between 3000 RPM and 2700 RPM is only 10%, but if you're having to floor the throttle to hold the gear/speed, that difference can be over 100% ...

Hey tc,
Thank You for the quote spanking, My only regret (after sobering up) is not thanking the RustMan for the thought and time He put into His answer to My question. Kiwi Quote "If this cant be fun, I ain't playin".
My question still stands sir, throttle blade angle has little to do with fuel consumption, as anything above 14.7 parts air to 1 part fuel, is inefficient and will piss Your 02 off. Thank You Kiwi. p.s. I look foward to bantering with You in the future.

Ahhh, yes, but on a FI engine running on the highway, the computer will always keep you at 14.7:1. More throttle = more air = more fuel.

Your engine does not take in 3.8L of air @ STP for each revolution for all throttle positions... search Wikipedia for "Otto Cycle" and let's see what we learn... I could use a refresher :hillbill:

Ummmm - yeah it does take in the displacement each revolution ... otherwise there would be a vacuum in the cylinders, and that would suck :D

Good Morning tc,
No matter where the throttle blade is, if the rpm's do not increase the enginein can not draw in any more air. No Air No Fuel. You said Yourself 14.7:1. Thanks Kiwi

Oh just to clarify, the word you wanted was stoichiometric, not "syncometric." Love ya, Kiwi!

Not exactly true Kiwi - You think RPM's just increase magically? Where does the energy for increasing the RPM's come from?

The throttle blade opens first, allowing more air in, which allows more fuel, which generates more RPM's.

More RPM's at the same throttle position burns more fuel, but only by the difference in RPM (each squirt is the same - just more squirts at higher RPM). By comparison, the difference in the size of the squirt between idle and WOT is MUCH greater.

Well tc... think about it this way: At low throttle position, the intake is effectively a restrictive port. A cylinder is fully exhausted, the intake port opens, and then the piston comes down. At wide open throttle, when the piston has reached bottom dead center (BDC) the cylinder is completely full of air at standard temperature and pressure (so just like the atmosphere). But at a low throttle position, only so much air can flow into the cylinder, so it might be at 1/2 atmospheric pressure when the piston is at BDC. So at lower throttle position the engine is actually taking in less air than at a higher TP for the same RPM. Then the FI system adjusts for how much air is in there by delivering the right amount of fuel (to create the 14.7:1 ratio of air 02 molecules to gasoline molecules). Does this make any sense?

Hooray,
We found the missing displacement, induction vacum. Thank You Men. Thanks Kiwi,Whats next?

Yeah - that kinda makes sense - it's a timing thing too. The system isn't airtight, so if given enough time, it would all balance out at STP, but with the time between the intake cycle and the compression cycle that won't happen...

Oh just to clarify, the word You where looking for was expotentially, not exponentially. I was saving that. luv ya, Thanks Kiwi