Gear ratio & tunes?

DZL JIM · #1 Wed, October 14th, 2009, 08:07 AM

I have had a few customers ask if my tunes are set for their specific gear ratio, either 3:55 or 4:10.
I see axle ratio for all of my tunes are set to 64:1.

Is this something that should be altered per each truck?

What does 64:1 mean?

Also tire size seems to be constant, too at 65535 rev/mile. Same deal, leave it alone?

Thanks.

***cleatus12r*** · #2 Wed, October 14th, 2009, 09:31 AM

I have had no issues with leaving it as it is. You can change the values to the correct ones, but I saw no difference between stock and 35" tires on a customer's truck when the proper (or improper) values were used.

You can make up for the taller tires really easily by just shifting the entire shift schedule up by 2-3 MPH in each gear range. These trucks don't use the PCM for speedometer calibration anyway so it's not really that important.

The 99-up Super Duty trucks are more dependent on these two parameters though.

***Jackpine*** · #3 Wed, October 14th, 2009, 12:27 PM

Guys, I'm going to ask a question here. That figure "65535 rev/mile" seems awfully high to me. There are 5280 ft in a mile. If a tire rotates 65535 times in that distance, it only covers about 0.081 feet per revolution! You're running on really tiny tires (diameter would be about 0.31 inches)! Are you sure that rev per mile figure is correct?

- Jack

***cleatus12r*** · #4 Wed, October 14th, 2009, 12:35 PM

Quote:

Originally Posted by JackandJanet

Guys, I'm going to ask a question here. That figure "65535 rev/mile" seems awfully high to me. There are 5280 ft in a mile. If a tire rotates 65535 times in that distance, it only covers about 0.081 feet per revolution! You're running on really tiny tires (diameter would be about 0.31 inches)! Are you sure that rev per mile figure is correct?

- Jack

It's not correct, Jack. With the 95-97 trucks, the data given in the tuning software shows a 64:1 final drive ratio and a 65535 rev/mile. Why? I don't know but it has little to no effect on the operation of the vehicle even when correct numbers are given like 4.10:1 or 635 rev/mile. I don't think that the PCM in those trucks use that particular value when figuring shift points. The automatic-equipped Super Duty trucks are sensitive to those values though.

***Jackpine*** · #5 Wed, October 14th, 2009, 08:57 PM

Quote:

Originally Posted by cleatus12r

It's not correct, Jack. With the 95-97 trucks, the data given in the tuning software shows a 64:1 final drive ratio and a 65535 rev/mile. Why? I don't know but it has little to no effect on the operation of the vehicle even when correct numbers are given like 4.10:1 or 635 rev/mile. I don't think that the PCM in those trucks use that particular value when figuring shift points. The automatic-equipped Super Duty trucks are sensitive to those values though.

OK, I've got it now! It's really just the largest signed integer the processor can store: 2 ^ 16 -1. (I knew the number was familiar to me from somewhere). The processor is a 16-bit machine and for some reason, it's holding a hex value of 7FFF - "maxint", in that position). Doesn't look like it serves any useful purpose at all though. Wonder if the good Doctor knows anything about it?

- Jack

***Power Hungry*** · #6 Thu, October 15th, 2009, 03:16 AM

The tire size and gear ratio values on the 7.3L (particularly the OBS trucks) don't do anything. The reason you are getting 65536 and 64 is because those values are pointing to "dead space" in the calibration.

As Jack indicated, the values are related to hexadecimal numbers, although those particular values are unsigned so the hex would actually be 0xFFFF (65535). However, Jack is correct that for a signed value, 0x7FFF (32767) is the max and 0x8000 (-32768) is the min.

Hole that clears it up.