I've got a 89' with 22re....31's and 4.11's. Keeping 70-75 on the highway can be a challenge, and acceleration is pretty poor. I was thinking of going to 4.56's...any idea's on what that will do to my mileage? I get about 18-19 now. Thanks

 

I'm not positive but wouldn't it give you better mileage in the city and worse on the highway??

 

i think that's a bit up in the air. if you can get crusing alright with the 4.10s, you could theoretically save gas. however, you'll get the power back to stock with 4.56 gears, and your speedometer should be close to correct

 

This is something I've wondered about, what RPM's are the most gas efficient? When I'm driving, say 40mph 3rd gear in my stock 90 with 5sp and 22re the RPM's are 3k or a little higher, 4th drops it under 2k (this is from memory) so third seems too high, 4th seems a little low...

are lower RPM's always more efficient or is there an ideal RPM range?

 

I'm no expert, so take this for what it's worth...

My wife's BMW has a dial that shows estimated gas mileage in real time (i.e. you're estimated mpg at any given moment). From what I've learned watching this dial, fuel economy is more about strain on the engine and less about rpms. I've noticed that if I stay in gear longer and push the RMPs higher, but keep acceleration slow and gradual, my gas mileage is better than if I shift (thus lowering the RPMs), but accelerate faster. So it seems to me that gas mileage is all about how hard you are pushing the engine, not RPMs.

I'm interested to see how people in the know respond to this.

 

I'm running a v6 automatic with 33's and stock gears were 4.56. I just put in 5.29's I should have gone lower. My gas mialage was around 18 it is now around 14. It did drop but with the lower gears I am always hitting the throttle more just to feel the power or if you want to call it lock of power. I am building this to go off road so gas mialage is not so important to me. My suggestion is to do it. You will only see about 2 miles per galon differance. The gear change will help you pull out from the stop[ signs faster. My budy has the same set up as you and he is running 5.29s. He is very happy with the stick shift. So consider the 5.29's.

 

Yeah, we have a 97 BMW 328i with the same fuel economy gauge. What I have noticed is the fuel economy is directly related to how far down you push the accelerator pedal. Think of the accelerator pedal has a valve that dumps your fuel, the heavier your foot is the more fuel you dump out of the gas tank. So basically if you can select a lower gear and ease off the pedal a little bit you will save fuel. For example if you are climbing a slight hill in 5th gear and it is taking full throttle to climb the hill, but if you drop down into 4th gear you can climb the hill at 3/4 throttle, you will save fuel even though your rpm's are higher. That's my theory anyway.

 

So what of the effect of the OD? For instance: if you're on a street that's 45mph should you go to OD or stay in your next lowest gear for the best MPG?

 

The diff gears will have an affect on mileage. To tall of a diff gear and the engine will be struggling all the time on the highway and you will loose MPG. To short of a gear and it will be wound out all the time loosing MPG. It is a function of load and that can be reported as manifold pressure. There is a manifold pressure, RPM relationship that will provide the best drivability and economy. It will be different for each engine, but,,

Generally when you put bigger tires on your engine will be working harder all the time and your MPG will suffer even when you factor in the odometer error correction.

What I am planning to do is to install 4.56 gears in my 96 auto that came 4.10s, 31s and I am now running 33s on it. That will put my highway cruising about 100 RPM more then it was stock. That should give back most of my lost MPG without making the engine wound out on the highway and get back some of that lost spunk.

Remember you can gear the hell out of it, but if you go to tall or to short MPG will suffer. You need to find that sweet spot.

Gadget

 

On my set-up 33's w/5.29 gears and a 3.0L V6 the OD isn't much use under 55 mph unless going downhill. Going up slight hills 70mph is a good shift point for going to OD for me.

Just start getting a feel for your accelerator pedal, if you can lighten up on it by going to a different gear then you are saving fuel. IMHO

 

yeah, this is a good way to think about it, but you need to factor in one more thing: the engine speed.

here's how i think about gas milage: as the pistons are going down, they're tyring to suck in as much air as possible. pushing the long skinny pedal opens up the throttle and allows more air to get in the cylindar. the o2 sensor watches how much o2 is coming out of the exhaust and tells the injectors how much fuel to spray in to as closely keep about a 15:1 air:fuel ratio. so the idea is if you keep your foot off the accelerator, the throttle doesn't allow much air into the cylindar causing you to burn less fuel. however, if the rpms are really high, then the pistons can suck the air in quicker with the same throttle position and you'll burn more fuel. so to save fuel you want to keep the throttle closed and rpms low. opening the throttle and increasing rpms will burn more fuel.

so yes you are correct that you want to keep your foot off the long skinny pedal as much as possible, but you need to also watch your rpms.

 

Any "formulas", rules of thumb, or general theories that might give us some indication where that sweet spot is for our rigs? My 96 came with 3.90s and 29" tires. I'm probably going to go to 33s this spring, and I would also like to get back my lost power, fuel economy and correct my spedo/odo.

 

Here's a great place to start. I think you will find that it agrees with 3.90's with 29's, so for 33's on the same rig, it's going to want 4.27's or a bit to either side...

http://www.4lo.com/calc/geartable.htm

I found the chart to be right on as I increased tire size and regeared my rig.

 

my guess is that the toyota engineers knew what they were doing. so if you're going from 29 to 33 tires.

33/29=1.137 increase in distance per rotation, so you'll need an equal change in gearing

3.90*1.137=4.44 gears. i don't know what the closest gears for the toyota are to 4.44, but i'd try to get as close to that as possible - maybe slightly shorter because the 33's will have more rolling resistance than the 29's.

 

I have better mileage with 33's and 5.29's than I did with 31's and 4.56's.

I do not recall mileage with the 28's.

Throw those calculators out the window. They do not consider the added weight and resistance of the new rubber.

New trucks get 4.88's for 33's.

Old junk should look hard at 5.29's. Assuming in both cases they are runners.

You can get by with less, but I do not think it will be grossly out of whack with what I am suggesting.

 

I hope you are right Adrian. I just ordered my new diffs with PG 4.88s and ARBs.

This chart doesn't show it but all of the others I've seen say for vehicles with Automatic Overdrive, the RPMs will be 15%-30% lower than the RPMs shown. The fine print above this one just says the RPMs will be lower.

 

Where did you snag them from?

Steve posted one the other day that had a list and said I was way undergeared with 5.29's and the auto.

Looks like you can hit the big stuff this fall then, eh?

 

I am getting them through Inchworm Gear. So far, it's been great dealing with the owner.

I still plan on being there, wherever it's at. I just need you guys there to keep me from getting too stupid with my new toys.

 

If it revs too high get some 315's. After a taste of the 315, 285's are just way too small for me.

Huge difference.

:telephone:

 

:telephone:

Yeah, I'm a knucklehead.