Gear Ratio??? Got bigger tires losing MPG, HP

Ok i have a 08 reg. cab 2.7l tacoma. I recently put on 29" tires on my stock 15" rims i think it made it look like a truck. Now it seems the truck has slowed down, and lost considerable gas mileage down to like 18 MPG!!!! So i checked out gear ratios cannot get a definitive answer, toyota says it could be 3.15, 3.33, and 4.10 i checked on my door it is an A05A, which i believe is 3.15 but i dont know> What type of gear would give me back good mileage, increase the HP. I think 4.1 gears maybe a little overkill but i really dont know? I do like 85% highway driving so i like my MPG, but i can sacrifice like 5 MPG for better HP. Can someone throw me in the right direction also how about brands of gear too? im kind of learning as i go.
Its also making a vibration while i drive on the highway i think it may be the U-Joint
P.S. the truck looks sick with the BFG 235 75 R 15

 

You can have the power back or the MPG, but not both. Pick.

 

From my SG II, in-town I am getting 21.2 MPG with a lift and 31" tires.

How many miles do you have on your truck? In the manual, it says there is 1k break in period for the engine. But it may vary, sometimes it could take about 1.5k miles for the engine to break in. Another factor could be a winter blend of gas.

 

Also, did you factor in the difference from going from stock tires to the new bigger tires?

 

I have like 42K for mileage it was great i was getting around 27-30 depending how i drove. The only thing now with the bigger tires which i really wouldnt think would do this much damage, on the highway 5th is stuck at 70 before i could cruise to any speed not an issue. I have a CAI but thats it. Could this be the case of a failing U-Joint? I understand you cant get both the MPG and HP but i will sacrifice some MPG for some HP was what i was saying

 

neslerrah, do you have a stock 2.7 or is it a prerunner? Because i heard there gears are a bit higher like 3.73 or something like that. I see your running larger tires did you run into an issue?

 

I have a 5 lugger, not a prerunner. I had to lift my truck in order to fit some 31x10.5 BFG All Terrain TA KOs on my truck. No issues with the tires I am running.

 

According to this:

http://www.customtacos.com/tech/files/specs/08/08_specs.pdf

Your truck has a 3.307 axle, 3.31 for short. This is what was specified for the '09s when I was shopping.

The automatic has a 3.583, or 3.58 for short.

The calculator here can accept all combos of tire/gear/axle you can think of.

http://www.novak-adapt.com/knowledge/gearing.htm

Stock at 70 mph, 27 inch tires, (205/70x15, right?) with your .81 OD gear = 2335 rpm. A bit low, but it probably pulled it ok.

Add the 29 inch tires, and now the revs drop to 2174. Your into the throttle more, and downshift more, correct?

Here's where the wheels fall off for me, as I can check out a Ford 9" all day, but have not found the same interchange for the Toyota axle. IF the carrier and axles are the same spline and type for the automatic truck, you can swap in a 3.58 axle. IF it's interchangable, it's a 2 hour job. (20 min after you practice a lot)

Then, with your 29s, your revs at 70 are 2351, right back where you started. Due to the increased weight, your mpgs won't come all the way back, but you'll be OK.

3.73s would be better, but that would mean buying gears and having the axle reworked. You should be able to buy the whole center section from a scrappy and just plug and play. SHOULD: you need to confirm axle splines and driveshaft yoke.

Good luck.

FYI, manual trans Prerunners have 4.10 axles, but are six luggers. Not sure the splines match the 5 lug axle. They also come with a 30" tire, and are just starting to show the shortness of the gear at 65-70mph.

 

Hey thanks for the post yeah thats where I'm at exactly what you stated, I'm curious if i got some 3.73's if that would put my truck back closer to where it was with MPG and add a little HP too. I mean i lost about 10 MPG im a little P.O'd, could the U-joint be causing any of this do you know.

 

A bad u-joint won't help mpg, and should be replaced asap. If it lets go, you will not like the result.

You are correcting your miles travelled when calculating your mpg, correct?

Note the 4.0 Prerunners and 4x4's have 3.73s, but again, are 6 lug axles and I do not know if the splines or even the housings are the same--

 

I may be jumping the gun. i should really do about3 or 4 tanks to get the real results. They dont make a 3.73 for 5 lugs?
Yeah i think the U-joint went when i got stuck in the mud the rock and roll failed i can post pics when i get em.. Thought the truck was out of balance for the vibration on the hghwy but then the new tires went on and the vibe was still there.

 

Good info.
The correct size for stock tires on 2nd generation 5 lug is 215/70R15.

 

5 lug tacomas with the 2.7 have a 7.5in rear end. You can't swap 3rd members with any other tacoma.

6lug tacomas and the xrunner(5lug) have a 8.4in rear end, and also a 8in but that only comes with the locker on the TRD offroad.

 

Cappy, answer that question because you haven't yet. You HAVE to adjust for the difference or your figures will be off.

 

OK, great. So the OP should be able to swap the entire 3.58 third member out of a 2.7, 5 lugger, automatic, right?

Also, thanks for the correction on the tire size. The calculator on the Novak link still lists them at 27 inches, so the numbers above still hold. (approximate though they are)

As for the gas mileage, yes, run a couple more tanks, and be mindful of your fill up method and calculations, as well as the odometer correction.

 

Bigger tires and wheels are heavier they take more HP to turn them.
It is a large difference plus the higher gear ratio caused by taller tires will take away low end power.

 

Just to clarify... anything with the 4.0 has the 3.73. The 2.7 Prerunner and 4x4 has the 4.10 rear end.

You may want to do a search in the 4cyl section, there were some discussions awhile back about the rear ends in the Tacos. My memory was that they were all interchangeable, but my memory has failed me before!

 

I really think he isn't adjusting for the difference in tire size, and that's why the MPG is down that far.