bigger tires = speedo error

[johnceggleston]

now i wonder if a speed sensor from an outback should take care of that...

 

not the speed sensor, it's dumb, the TCU. the speed sensor rotates with the wheels the TCU does the math. neither has to consider the final drive ratio since the sensor is after the final drive.

 

borrow a tcu from an outback.

 

 

Really?

 

I thought for sure the final drive gearing played a big part in the speedo.

 

When I went with taller tires my speedo and odo went off (with OBs as 4.11 and running taller tires than my stock 3.90 and 185x70x14s)...

 

There was a tire size calculator on BFG (tires) website (many others available on the net) that tell you gearing ratios and how RPMs and speed changed withh tire size. I thought I could regain my lost power from taller tires by changing final drive tranny gearing, which would also recompensate my speedo.

 

So I too could get an OB TCU to recorrect my speedo and odo?

 

That'd be nice

 

(but tranny gearing, would still give better power and performance by recompensating for tires right?)

 

 

 

yes, using a tcu from an outback should help correct your speedo / odo.

 

if my math is correct, the difference between the legacy auto 4.11 and the outback auto 4.44 final drive ratio is about the same as the difference between the tire circumferrenc. in other words, when they made the rear diff 'more powerful' with lower gearing, they put on larger tires and offset the gearing reduction.

 

now the GT has the legacy tires and the outback final drive ratio, making it more powerful and quicker.

 

by putting outback size/diameter tires on a legacy you will throw off the speedo, the outback tcu will help correct it.

 

the drive gear in the front diff that drives the speed sensor turns with the ring gear/axles not the pinion. the gear reduction has already done it's thing. the TCU just does the math, X wheel revs per mile; wheel rev per minute = mph. larger tire size increases the distance traveled for each revolution and reduces the number of wheel revolutions in a mile.

 

you're right about swapping the final drive gearing, and it re-compensating, power-wise, for the larger tires. you would come closer to the original power curve but still limited by the 2.2L engine. is there a significant difference in the power curves for these 2 engines??

 

want your outback to drive like a GT, put on GT / legacy size tires and swap in a GT tcu. want your GT to drive like an outback swap the tcu and tires.

 

as stated in numerous other threads, there is not a dramatic difference when you swap larger tires on a legacy but there is the speedo/odo difference.

 

just my opinion. any thoughts?

 

ps: this is for late 90s cars.

Edited March 17, 2009 by johnceggleston