OK, Dig this.

Put 33" Interco Super Swamper TrXus MT's on the stock 17"s (along with the KORE leveling kit) and had the dealer recal. the speedo (since the BDTD cant).

They set it for the radious of a 305. Checked the speed over measured distance at dif speeds and turned out the speedo was off (fast) by 5 MPH.

So drove around for about a 3 days until I could get back to the dealer. In the mean time got used to adjusting for the 5 MPH. Had them adust it to a smaller tire radius (asked for 295 but the ECM only had the option for 285). Checked the speed over distance and its about spot on (+/- about 1-2 MPH).

Now comes the crazy part. While the ECM was set for the 305 radius I noted that at 78 MPH (aka 73) the RPM's were coming in at 1700. Now with the ECM set for the 285 and at 76 (aka 73-74) the RPM's come in at 19-2000 which is what it was at stock.

Now I know the MPH is pretty close +/- (used the tach on my watch) so why the big dif. in RPM's?

And maybe the bigger Q here is could I reset to 305 and expect to get better MPG as at the same speed the RPM's were lower?

Any thoughts?

Could they have set the ECM and chose the wrong rear end gear ratio causing this anomally?

Truck drove fine.

Hmmmm....??? [sm=confused06.gif]

 

mmmmmmmmmmm weird the only thing i know is im looking at doin 33s with level kit and thought this, slightly larger tire less rpm at freeway speed eg; 2000 rpm stock doin 75mph, or 1800 rpm with 33s doin 75 dont think they can reprogram this because bigger tires are physicaly changing the gear ratio with the larger diam tires . i think they can reprogram the spedo but rpm are what you got

my oppinion could be wrong if anyone knows better way please elaborate so i dont screw truck up , THANKS

 

sounds like your in tow haul mode or your torque converter is not locking now.

 

I didnt notice any change while going through the gears (like it slipping) and I didnt mess with the tow/haul. Also the exceleration on the top end was just like normal. I put my foot in it and the rpm's and speedo would advance (fast ) together so good torque / power there.

I threw the rear end gears thing out there couse I went to a dif. dealer to reset it to the 285's and in gen conv. about the process / truck he said this that and the other and then mentioned he set the rear end gears in the process. I had no idea this was needed and he said yep its all done together.

I mean the truck is the same as it was before the tires, after the first speedo cal, and after the sec. speedo cal. The only thing I noted was the change in speedo and lower RPM's when set at the 305's.

At highway speed at the lower RPM's the motor did sound a little dif (quieter) but again lower RPM's.