Anybody try this?

[cyclone03]

I've be playing around with gear ratio's vs tire size and found that if you swapped the tires front to back at the track the over all ratio would be the same as a set of 3.89's in the rearend.Anybody try this yet?
With the 4.10's everybody seems to be going to does it spin the tires off the line?How hard?
The front tires are 26" tall vs the rears 28".
I wonder if it would hook with the shorter tires on the back?Hum..... :rolleyes:

[Logan]

Swap your tires around and your shift points and speedo will be screwed. The wheel speed sensors assume the tire sizes are what they are from the factory.

With 4.10's, you still won't be spinning your tires anyway, at least not without a 3000rpm stall convertor. The 4.6 DOHC simply doesn't make it's torque down low, it needs rev's. You can be at a dead stop and simply mash the pedal and all that will happen is you'll get up and go. By the time the 4.10's get the motor into the power band, you're already at speed.

[cyclone03]

With 4.10's you still have no wheel spin off the line?
Hum.............
4.30:1's
4.56:1's...................... ....

I've noticed from other post that some say gears through off the speedo others say no.
If the speedo is read off the ABS wheel speed sensors ONLY and the rear sensor is not on the ring gear like the trucks and there is nothing else the CPU is trying to calculate for then no calibration is neccessary.

But if the CPU is trying to calculate wheel speed to engine RPM,or something else..................
So do you NEED a chip to make the speedo read correctly after a gear change or not???

[Logan]

In the 03's, they eliminated the speed sensor in the tranny, and I've personally seen a 3.73 equipped marauder where the speedo wasn't off at all, without a chip. The factory says that the PCM calculates speed based on the average wheel speed from the wheel speed sensors at all 4 corners.

That being said, MY speedo >IS< out as a result of the gears, but I corrected it with a chip, BUT the dealership is insisting it shouldn't be out and is going to trouble shoot it the next time the vehicle is in for service.

Also, gear ratio's have nothing to do with wheel spin. That is purely a result of torque. I could put 5.13's in a chevette and it'd never spin the tires either. That's why I said you'd need a much higher stall convertor to have any issues with traction, unless you brake stand the car off the line.

[03 Merc]

It makes sense a 3.73 change wouldn't impact the speedometer as that is only a 5% change from the 3.55s. A change to 4.10 is a 15% change and should be more noticable to the system.

I would like to go with gears, without a chip for the time being, so I hope they get this straightened out soon on the computer / sensor /calculations...

[Logan]

03Merc, When I mean it didn't affect it, I mean it. We took two cars out and paced it, and then hooked up to the PCM reader to check the gear ratio. The PCM had dynamically changed the field to 3.73 instead of 3.55...

I should be in next week getting some trim work done, they'll be looking at it then.

FYI, Dennis can burn you a stock chip with a gear ratio override if desired. You don't have to modify the performance.

[03 Merc]

Logan,

That is weird...so if I understand this right your car did not automatically correct to the new gear ratio, 4.10, in the PCM. Another car, with 3.73 installed, did correct itself in the PCM... Either it can only correct itself within a certain tolerance range, or your car is different... Did you check the PCM code on the other car to see if it is the same as yours? A long shot but it might be different...

I just checked my PCM code and it is BMD0, is that the same as yours?

[Logan]

You understand the dilemma perfectly now. It should work, but it doesn't. My PCM is BMD0 as well.

I >HAVE< seen other marauders with different PCM's though. FDH0 or some such.

Anyway, doesn't much matter. Once the dealership figures it out, I'll advise...