6 speeds, can we drop out interstate rpm's with differential gearing ???

Rolling resistance cannot be discounted for hwy milage, its a factor at all times.

And then your theory regarding mass falls flat on its face at the first hill - mass DOES have a large effect. Not on a pancake flat prairie, true, but not everybody is a flatlander.

 

It simply doesn't and isn't. Go find the Mythbusters episode that proved it out, if you need any clarification. Or speak to folks that do this sort of stuff for a living.

 

THE EARTH IS ROUND, THE EARTH IS ROUND!

 

Several owners have stated that taller and lighter wheel tire combos with no re-gearing has made a difference beyond doubt in fuel mileage and cruising rmp's

 

Of course. The reduction came from the fact that the wheels were taller. It had nothing to do with them being lighter. The mileage dropped because the RPMs dropped, not because the mass dropped.

On city streets: it makes a difference. Not on the highway. Not unless you're grinding through a traffic jam on a highway; but then you're not really getting highway mileage that way.

 

On 35s, my milage went up a bit when I regeared from 3:73 to 4:88. Brought my rpms back up to where they were from stock. I think about 2200. With 3:73 I was at about 1700 iirc.. Had no power. Engine was trying harder and down shifting often to keep me going. Too low of rpm can hurt as well as too high.

 

I would love to see more arguing, but does anyone have any expertise on rebuilding the RA60 transmission ? http://www.customtacos.com/tech.old...6toyrm/06toypdf/06rmsrc/rm2006ta/00303010.pdf

some good points here

 

Tire and Gear Math

I’m posting this as a point of reference, so that everyone can see the MECHANICAL (not cost) difference between changing transmissions, differential gears and tires. I hope that this may help somebody who is considering modifying their Tacoma for high-speed highway driving. There are many places in the USA where traffic flow is at 75+ MPH during rush hour, and there are many other areas that are so vast (Texas, Montana, etc.) that slowing down is beyond frustrating.

IF it is possible to swap transmissions, or to swap gear sets within transmissions, one obvious approach is to use another transmission from the AY-6 family, of which the Tacoma’s RA60F is a member. I see two possible paths: First, the RA61F transmission from the Toyota FJ Cruiser, and second, the GM variant from a Chevy Camaro.

FOR HIGHWAY DRIVING IN 6TH GEAR:

Using a Camaro transmission is equivalent to
- increasing tire size to 315/75R16
OR
- changing differential gears to from 3:73:1 to 3.291:1

Using a FJ Cruiser transmission is equivalent to
- increasing tire size to 275/70R17
OR
- changing differential gears to 3.510:1

THESE NUMBERS HAVE BEEN ROUNDED-OFF SEVERAL TIMES IN THE PROCESS, AND I USED BFG AT T/A KO TIRES AS LISTED ON TIRERACK.COM

If you see an error, please say so and I will correct it.

———————————————————————-
MATH

Tacoma final drive = 3:73:1

First gear ratios:
RA60F = 4.17:1
RA61F = 4.17:1
Camaro = 4.48:1

6th gear ratios:
RA60F = 0.85:1
RA61F = 0.80:1
Camaro = 0.75:1

The stock Tacoma tires are 683 revolutions per mile
————————————————————————

AT THE “CRAWL” END

Overall gearing in 1st gear:

RA60F/RA61F = 3.73 x 4.17 = 15.5541
Camaro = 3.73 x 4.48 = 14.2016

Using the Camaro transmission gearing:
To get the same overall gearing with tires, the tires must be
14.2016 / 15.5541 = 0.913
683 x 0.913 = 624 revs/mile to match 1st gear
Tire needed is approximately 295/75R16

Using differential gears
15.5541 / 4.48 = 3.472:1

—————————————————————————
AT THE HIGHWAY END

RA60F 6th gear 0.85:1
RA61F 6th gear 0.8:1
Camaro 6th 0.75:1

3.73 x 0.85 = 3.1705
3.73 x 0.8 = 2.984
3.73 x 0.75 = 2.7975

RA61F tire size correction factor = 683 x 2.984 / 3.1705 = 643 revs/mile (275/70R17)
Camaro 683 x 2.7975 / 3.1705 = 603 revs/mile (315/75R16)

Using differential gears
RA61F = 2.984 / 0.85 = 3.510
Camaro = 2.7975 / 0.85 = 3.291

 

I have experienced this myself on past trucks... but i have no need for a lift or 35's.. I wonder what the max tire height would be that we could use without suffering from too much RPM drop (but ofcourse lighter wheels/tires plays into this)

 

Mike, that is extremely useful I think It answers some questions for quite a few of us.

Yut!

 

I have been trying to find if the transmissions are a direct bolt in fit from the FJ to the Taco and am finding the trans mentioned in articles, but i cant seem to find any good pics. But I do see that the camaro folks and x runner folks complain about the shift from first to second and the Taco guys complain about the 3rd gear grind and 6th being too short.

here is something interesting from an fj onto a taco http://www.tacomaworld.com/forum/ga...-case-install-2007-tacoma-6-speed-manual.html

 

Changing the tire size and rear diff gear ratio will affect you speedo. Changing the 6th gear ratio will not affect the speedo. As for the overdrive gear box, it depends on where it connects I to the system. If it is before the Tcase it won't affect the speedo. If it is after the Tcase it will.

For autos, only tire size changes will affect the speedo.

Manuals read speed at the Tcase and the autos read it at the wheels.

 

Good to know. thanks

 

After everything in this thread I thing your best approach is to get some 235/85R16's. It's a fairly common size, is taller to get the rpm down and skinnier so can be lighter and will be more aerodynamic at speed.

Plus if it doesn't workout for you, you just get a new tire down the road. Whereas if you change the diffs or 6th gear and it doesn't work out you are out a lot of money...

I still think that when you change that ratio to drop the rpm you are just going to find yourself downshifting more often which will negate the gains of the lower rpm.

 

With any gearing or wheel size change you will need to get the speedometer recalibrated.

Install smaller - shorter wheels and don't recalibrate the speedometer.
This way you can think you're going 70 but are travelling at 58 mph.

 

I DON'T THINK SO

1. AFAIK, 235/85R16 is only available in Load Range E tires. A single example, the BFG ATKO weighs 49 pounds. The OE Dunlops weigh 32 pounds. A 53% increase in tire weight will be immediately noticeable in throttle response, braking, fuel economy, and all will be negative effects. It will NOT save fuel.

2. There is plenty of solid science available that shows that wider tires do NOT negatively affect fuel economy. However counterintuitive that sounds, it is real.

 

I'm glad that it helps.

It got me to thinking. I understand that this is purely hypothetical. The FJ tranny should be a relatively simple swap. The Camaro has a lower first gear that most people would like off-road. We don’t know if it is possible at all to put the Camaro gearing into a Tacoma transmission case. The numbers, for me, make it worth exploring. If you did only the transmission swap, and did not modify the differential gearing or change tire size, you get the following engine RPMs at cruising speeds:

Engine RPM

Tacoma RA60F
683 x 3.1705 =
2165 @ 60 MPH
2346 @ 65
2526 @70
2707 @75
2887 @80

FJ Cruiser RA61F
683 x 2.984 =
2038 @ 60 MPH
2208 @ 65
2378 @ 70
2548 @ 75
2717 @ 80

Camaro Trans
683 x 2.7975 =
1911 @ 60 MPH
2070 @ 65
2229 @ 70
2388 @ 75
2548 @ 80

BUT, AT WHAT COST?

From a quick look on car-part.com, it seems like buying two transmissions and combining parts to make one (again, if possible) would likely be in the $5,000 range with professional labor.

Used FJ manuals are in the $800-1300 range. I don’t know what other parts you would need, but I’ll bet somebody on TW has already done this.

We had commentary on the absurdly high cost of the Gear Vendors overdrive. As far as I can tell, they are a Big-Three-only company and don’t make anything for Toyotas.

FWIW, I called Marlin Crawler about an overdrive version of their $1700 Crawl Box. It is a no-go.

 

Thanks, Well my 3rd gear syncro wont last forever... so at some point I will have to do something. A bit of an experiment couldnt hurt.

But it does seem like the most practical thing to do is Lighter aftermarket wheels and tires...

But if you got 2wd, just get a ring for the rear dif of a Base model taco.

 

I see, I didn't realize the 235's only came in E-rated.



So out of curiosity, what kind of mileage are you getting OP and what kind would you like to get with this change?