Individual Gear Ratios When Swapping Transmissions

[the_bard]

I convinced myself to start looking around at parts yards instead of "U Pull It" yards for the replacement transmission for my 2006 Baja. Granted, it's more expensive, but at least I'm not going to spend half a day laying on my back on top of packed dirt.

 

During my searching, I'm finding a few 2000-2001 Forester manual transmissions that reportedly bolt up without difficulty. There's a bit about the Forester transmission using a couple of pins where bolts are used on the Baja transmission, but that's about it. Both have male stubs on the tranny side, so there shouldn't be a problem with the axles.

 

The gear ratios are different, though:

                                           1        2         3        4       5       R       FD

US Baja MY06                 3.454 2.062 1.448 1.088 0.871 3.333 4.111

US Forester MY00-01     3.545 2.111 1.448 1.088 0.780 3.333 4.111

 

I've already heard that taller fifth gear in the Forester transmission will drop a few RPM on the highway, improving mileage but hurting the ability to power up those hills and pass a bit, depending on my driving habits. I'd call myself mild on the highway, so I doubt it's going to bother me much.

 

Will I notice the shorter gears in first and second? I don't spend much time in 'em, cruising back and forth to work, usually in third or fourth in town. I want to do some light off-roading at some point, with emphasis on light... it will be my daily driver, once my company enforces their "we're parking the company vehicle at the office" policy this fall.

 

Any thoughts?

Edited May 31, 2013 by the_bard

[presslab]

5th gear is 10% different so a decent change.  But 1st and 2nd are only ~3%, I doubt you will notice much.

 

Do those two vehicles have the same diameter tires?

[the_bard]

Not exactly. Looks like:

Baja = 225/60R16 = Diameter of 26.63" = Circumference of 83.66"

Forester L = 205/70R15 = Diameter of 26.30" = Circumference of 82.62"

Forester S = 215/60R16 = Diameter of 26.16" = Circumference of 82.18"

(according to http://robrobinette.com/tire_diameter.htm )

 

So the Baja's stride is about an inch, inch and a half longer per tire revolution than the Forester, depending on the model and assuming brand new OEM tires. It's too late for my head to comprehend more math than that. 

[johnceggleston]

the tire size difference is less than 2%, not much.

that difference is so small i doubt the speedo gears are different.

and you can change the tire size on your car if you want, when you need tires.

 

 

the difference in gearing probably has more to do with EPA mpg rather than  model / weight / performance.

well, the gearing on 1st and 2nd may be performance driven, is the baja lighter ??

 

are other 06 5 speeds geared the same as your baja?

[the_bard]

Looks like the Baja should be about 450 lbs heavier than the Forester.

 

The '06 US Outback has the same gearing; I believe that's true back to '00. '99 and earlier had the same shorter 1st and 2nd gears (3.545 2.111).

 

The Legacys generally have a different final drive, as well as the Imprezas, so I'm not considering them.

[Fairtax4me]

You'll notice about a 300 rpm decrease in 5th gear at 70mph.

1st you will notice an increase in off the line torque, 2nd you might notice a bit of change but not much.

 

Speed sensor gearing is dependent on final drive gearing (4.11), which is the same on both, so your speedometer will read the same.

[the_bard]

Thanks! Nice to know about the VSS. The change in gear ratio sounds fine.

 

Gotta make a couple of phone calls to confirm details on a transmission... hoping to have it in by the end of next week.

[ivans imports]

doubt you will notice anything

[johnceggleston]

 

Speed sensor gearing is dependent on final drive gearing (4.11)

actually the speed sensors are all the same and they are on the axle side of the diff carrier.

they measure wheel rotations regardless of the final drive ratio.

different size tires will affect the accuracy of the speedometer.

a trans with speedo gears for one tire will not be accurate for another size tire.

but the size difference is so small, i doubt they have different  speedo gears.

but since one is an 06 which i have no experience with they could in fact have different speedo rears.

 

a great way to test your speedo is with a GPS.

on a long trip, select your destination, note the miles, and reset your trip counter.

then drive 100 miles. does the trip counter match the GPS?

if there is a 1.5 mile difference, then your speedo has a 1.5% error, etc.

this error should be consistent with the speed displayed as well as the distance traveled.

 

you can run the test on shorter distances but 100 miles works well.

Edited May 31, 2013 by johnceggleston

[Fairtax4me]

The sensor itself is the same but the number of teeth on the sensor driven gear in the trans is different.

[the_bard]

Huh. So the number of teeth on the gear could differ, and the speedometer/ECM (or is it ECU?) is configured to match. Theoretically, at least. Seems to me that if the final drive is the same and the size of the OEM tires is just about the same, there shouldn't be much (if any) difference in that sensor driven gear. Can't see why someone would go changing it between transmissions.

 

Wouldn't be the first time that reality fails to meet my expectations, though. I'll check it against a GPS once I've got the new transmission in.

Edited May 31, 2013 by the_bard

[Fairtax4me]

Sensor, ECU configuration and speedometer calibration are all the same. The only difference is the number of teeth on the drive gears.

It's different depending on final drive ratio and tire size of the vehicle the transmission is going in.

An Outback with 4.11 FDR will have a different number of teeth on the speedometer drive gears than a Legacy L with the 3.90 FDR. I know there is a difference between those two because I ran outback size tires on my L with the 3.90 FDR. The speedometer was reading approximately 5 mph too high at highway speed. I did a trans swap to an outback trans with 4.11 FDR, same size tires, now the speedometer is 3mph slow, which is the same way it was with the stock L size tires and 3.90 FDR.

[johnceggleston]

a more correct way to say it is that the speedo drive gears, match the stock tire size.

if you install an outback trans in a lego L and put outback tires on it, I THINK the speedo will read true.

 

but we are splitting hairs here.

and it only impacts the OP's situation if the stock tires are a different size than the new trans' stock tire size.

and if the difference is small enough, it mat not matter at all.

 

do the swap and test drive the car with a GPS. if the speedo is off , be careful when you drive.

 

the only reason to use another trans is if you can find one cheaper.

 

www.car-part.com

Edited June 3, 2013 by johnceggleston