I put 265's on my truck when new so I didn't note the engine rpm's with the stock 245 all seasons that came stock.


Can anyone with Duramax/Allison CCSB combo tell me what the rpms are at 60 mph, 65 mph, 70 mph and 75 mph to see how far off my speedo is? I'm going to get it flashed for the 265's at the oil change but I was curious how far off it is. I'm guessing about 5% or so?


Thanks!Edited by: Raven

I switched mine very early as well, but going from memory, cruising speed was off by about 200rpm and 3 mph.

Here is what I had sent out previously to another memeber, Hope it helps!


I Just got Back from work, and here it is.


1500 RPM = 50 MPH


1625 RPM = 55 MPH


1850 RPM = 60 MPH


1975 RPM = 65 MPH


2200 RPM = 70 MPH


This is the best judgement I could make as it's rather hard to tell exactly where the RPM'S are when there in between the marks, but if you do the math it makes sense. Note: the tach at 65 MPH is not fully centered on 2000 RPM'S so this is how I determined the RPM'S. I haven't done 75 MPH yet, but will try to remember and do it today and edit the post tonight. And yes it's an 04 Dmax Ally cc/sb 4x4, but the rear end ratio is the same on all these, just have to make sure it's another automatic of the same year to be safe (I don't know if they ever changed any of the gear ratios of previous year transmissions).


Hope this helps, Fran

Thanks Fran - that is precisely what I was looking for.http://www.dieselplace.com/forum/smileys/Clap.gif

I went to 285/75-16 . The speedo is now exactly 3 mph slower (it reads 60 actually 63). How do I know exactly, because I used the radar unit in my patrol car. Although I have corrected that now with the power pup downloader. So you should only be about 1.5 - 2 mph off.





04 Dmax LB7
Edited by: NightStalker

What is; "the power pup downloader"? Does it correct the speedo? If so, will it work on the LLY motor?

The Power Pup plugs into your data port and you can add a 50HP, 70HP or 95HP tune to it, change the tire size to correct speedo, remove the speed limiter, remove the rpm limiter and you can use it to diagnose codes. But no it wont work on the LLY yet, they are in the process now of making one for the LLY.

Here is a great website that allows you to enter stock tire size vs new tire size and any mph, and it will tell you what speed you are actually going.........


http://www.paspeedo.com/calculator.htm


Enjoy!