boogie86

i need some help on this one. i have only guesses on what to check out on my newer truck that i just purchased a few days ago. the vehicle is a '07 tacoma prerunner with a 2.7 i4 and a five speed. now my old truck was a '00 tacoma prerunner but with an auto. the odd thing is this new prerunner is eating up more gas than the old truck was. i thought sticks were supposed to be superior. i try not to race the engine and generally keep it under 3k during most driving conditions. i drive primarily highway, so it's obvious as i watch my gas gauge drop that something is wrong here.

the new truck has 265/75-16's on it whereas the '00 had 31's. could this be the biggest thing? the '07 has 30k miles on it, so i'm wondering if it's still running the original plugs on it. my father is a firm believer in changing the entire intake system, but i think this is a rather pricey way to go considering my old '00 was putting up consistent 25 mpg with stock everything and 31 in. tires. how can this be??

one other thing on the tires. i looked at the spare to see if i had all matching tires. my spare is a 245! so if i have a blowout, i can't even use my spare. is there anyway to figure out if my new prerunner is geared correctly without taking off the diff cover to count the teeth? i'm starting to think some young kid put big tires on his new truck and then realized he didn't like driving stick. my engine is turning about 2,500 at 70 mph. this actually seems higher than the old '00 prerunner i had. but i'm under the impression that if i put smaller tires on it, the engine will spin faster.

boogie86

i solved this one on my own. it was pretty much what i assumed. someone put jumbo tires on without regearing the rearend. i plan on taking off the tires and wheels and putting on the correct size tires with some cheapo steel wheels. can anyone help me out and tell me what size wheels i should be searching for?

corax

stock tire and rim size should be listed on the driver door jamb sticker

dropzone

iTrader: (6)

you might consider figuring out what the actually mileage was with your current set up using a GPS. it might not be as far off as you think..since the odometer is off from the larger tires.
stock size were probably 245's...my '05 Taco 4x4 came stock with 265's

also no diff cover to remove, it has a third member you would need to pull the axles and than pull the diff to count the teeth you can use this link: http://www.4crawler.com/4x4/ToyVIN.shtml
to determine your axle code.
this link has 2007 gear ratios:
http://www.vibratesoftware.com/html_...0Gear%20Ratios
according to that your truck had 265's...

boogie86

thanks for the replies, guys. much appreciated.

William

From what I have found, the PreRunner comes with 265s just like my 4wd did. Not unless the 4 cylinder PreRunner comes with smaller tires.

Not to start a post war, but you were smart in not letting your father talk you into changing the intake system.

Like you determined, it costs quite a bit of money for an aftermarket intake. And any gains you MIGHT get would be small enough that it could take a few years to pay for the initial cost.

And notice that I said "any gains you MIGHT get", with emphasis on the word "MIGHT". I'll explain as follows:

Many guys believe that changing the intake to a cold air intake increases fuel efficiency. BUT, the truck ALREADY has a cold air intake. This isn't like it was 30+ years ago when engines drew in warmed air from under the hood.

Second, consider the ENTIRE flow path. The flow path starts where the air is sucked in to the airbox snorkel to where the exhaust flows out of the exhaust tip. Where is the most restriction in the flow path?

At full throttle, the most restrictive part of the flow path is usually somewhere in the heads. At part throttle, the most restriction is the throttle plate (in the throttle body), simply because the throttle plate isn't fully open. And since almost all driving is done at part throttle, the throttle plate is where the most restriction is.

So if the intake is rarely where the most restriction is, and since the truck already has a cold air intake, how is changing the intake going to improve mileage?

Okay, so lets say that the intake IS the most restrictive part of the flow path. And lets say you install a less restrictive intake. And lets say that airflow does increase.

Once airflow increases, regardless of WHY it increased, the MAF (Mass Air Flow) Sensor senses the increased airflow and sends that information to the computer. The computer then increases the amount of fuel that the injectors spray into the intake runners.

The computer is programmed to send a certain amount of fuel to the engine based on airflow, with maybe a few other variables such as coolant temperature and intake tract air temperature, etc. being taken into account. If the engine DIDN'T increase the amount of fuel to match airflow, the engine would run too lean. They already run lean enough from the factory to help meet fuel economy and emissions standards.

So how could increased airflow possibly increase fuel economy?

And not only that, if the throttle plate is the most restrictive part of the flow path during normal driving (being that it is partly closed), how could ANYTHING be increased (whether it be power or fuel economy) during normal driving after installing an aftermarket intake?

So IF the intake were the most restrictive part of the flow path, and IF the intake were made less restrictive, in order there to be more flow than stock, the throttle plate has to be open far enough to take advantage of the increased flow capability of the intake. This would be at or near full throttle.

Tell me, what kind of fuel mileage would your truck have at or near full throttle?

The engineers that designed the truck designed the entire flow path to be as efficient as it can be with the technology available at that time. And they designed the entire flow path so that the engine gives ideal power and torque characteristics for the vehicle in which it is installed, taking into account the uses that type of vehicle will likely encounter. Changing ANYTHING in the flow path could change the flow characteristics in such a way as to negatively impact the power and torque characteristics of the engine. And it could possible negatively affect MPG.

I said all this to make an obvious point: It's highly unlikely that any modifications you do to the engine will improve fuel efficiency.

Change the plugs. If it's the same as my V6, the plug change interval is 30K miles. So it is due for a plug change.

If the tires are not the stock size, go back to the stock size. If they are the stock diameter, maybe you can look into a tire of the same diameter that is narrower. This will decrease rolling friction between the tires and road surface. It will make the tires lighter and a little more aerodynamic.

One thing that may help is increasing tire pressures. I'm not advocating a major increase to 50 pounds or whatever. I'm saying go maybe 3 pounds more than stock, and no more than 5 pounds over stock. That minor increase should have no adverse effect, but it COULD be a cheap way to increase MPG by 1/2 to 1 MPG. I know it wouldn't be a big increase, but it's worth the cost.

You should also check for things like a dirty air filter and check the MAF sensor for cleanliness (since a dirty MAF sensor can give the computer incorrect readings). The MAF sensor can be cleaned with spray carburetor cleaner. Just make sure that if the MAF sensor is plastic, that you find a plastic safe cleaner.

It may be possible that there could be a problem with the engine that has triggered a trouble code in the computer, yet it hasn't triggered the check engine light. Retrieving any stored codes could point you to a problem that may be easy to correct.

You should also measure your MPG. Use the same method, same station if possible, and same pump if possible each time you get gas. It may be possible that the whole MPG issue is simply your perception and not reality.

But measure MPG for a few tanks before and after you take action. Then let us know the result.

boogie86

wow, i'm blown away by your detailed thoroughness and appreciate it greatly. i agree that leaving the engine stock is going to yield the best efficiency numbers in this case. i'm 99% positive that my truck needs smaller tires just due to the fact the spare is a 245 grandtrek, which i'm under the impression is the OE tire with these trucks. i have changed the air filter and spark plugs have been on my mind as the next improvement. i have pumped up my tires to 35 psi as i usually run my tires at 30 and have not had a problem yet. i do intend to buy a set of hankook dynapro atm's, but i was viewing some specs and something put the brakes on my decision. i have motometal 16x8 wheels. i was reading a review by fourwheeler on the tires and down at the bottom (i'll include the link) they listed approved rim width. if the approved rim width is 6.5"-7.5", does that mean i'm SOL since my wheels are 8" wide? or is there some leniency on this? i have 265/75r16's on there now, so the more i think about it, this may not be an issue. but maybe.

http://www.fourwheeler.com/techartic...est/index.html

William

Like someone said, the stock tire size will be on the door sticker.

My only 2 concerns with a stock size tire on the wider rim would be having enough tire sidewall sticking out beyond the rim to help protect it from curbs, etc., and having a rim so much wider than the tire sidewall that the tire bead could become unseated from the rim.

It appears neither would be a problem since the section width (from sidewall bulge to sidewall bulge) is 9.8". That means you will have some sidewall sticking out past the edge of the 8" wide rim. And the rim is only 1/2" wider than the maximum 7.5" recommended rim width.

So although I would prefer the factory width rim for the factory size tire, I wouldn't think your 8" rims will be a problem. Just be careful with curbs and such, because the sidewall can still flex sideways enough to damage the rim.