Cruising RPM

[oldbrown]

Good morning. I am curious what RPMS you guys are seeing with vortec 454, and automatic with OD in crew cab dually. Based on GM code, I have 4.10 gears. I have original tire size. My speedo is definitely off. At 2500 RPM in OD it says 70mph but GPS says 63. The 4 variables are gear ratio, tire size, RPM and mph. I have done the math and something does not equate to what I am seeing. I have taken into account the .75:1 ratio of OD in trans in my math but still off. Thanks.

 

[Supercharged111]

In the dually I get 2000 RPM at 57mph on the cluster which is within 1mph of GPS. I have 225/75/16 tires, 454, 4L80, and 4.10 gears. I forget where I see 2500. In the 1500 with 285/70/17, 4.10, and 4L80 I see 2000 RPM at 62mph.

 

[RichLo]

I'm usually puttin along at 150mph @ 5000rpm in OD with 4.10, 31" tires. Hope this helps

 

[Tavi]

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My guess is your gear ratio was altered at some point in its life. Based on the numbers given by each responder and myself in this spreadsheet. 62-63mph puts you at 4.56 gears.

 

[oldbrown]

Thanks guys. The 4.56 ratio did come into my mind as well. I think I will physically check the rear end to confirm.

 

[Sparkysikes]

I just switched gears 4.56 to 3.73.

AT 65 mph I was arou d 2200
At 75mph I was around 2600.

Now 65 is 1800
And 75 is 2150

My tires are 215/85/16 rear tires and 265/70/16 Front. Same height but wider

 

[Pastor T]

I just switched gears 4.56 to 3.73.

AT 65 mph I was arou d 2200
At 75mph I was around 2600.

Now 65 is 1800
And 75 is 2150

My tires are 215/85/16 rear tires and 265/70/16 Front. Same height but wider

How is it taking off when towing?

 

[Lanny]

If it doesnt end up being your rear gears, it could be your tach reading slightly off.

 

[Supercharged111]

I just recently flashed an LQ9 swapped RCSB GMT800 and its tach was waaayyy off. I was really surprised, like it was a 6 cylinder tach being fed an 8 cylinder signal. I don't see how that's possible though. Yes the truck was originally a V6 truck but it had an Escalade cluster and PCM.

 

[L31MaxExpress]

I just recently flashed an LQ9 swapped RCSB GMT800 and its tach was waaayyy off. I was really surprised, like it was a 6 cylinder tach being fed an 8 cylinder signal. I don't see how that's possible though. Yes the truck was originally a V6 truck but it had an Escalade cluster and PCM.

The LS PCMs have a programmable tach output. In stock form the tach output is equivalent to a 4 cylinder output. My buddies 1997 stock tach is fairly accurate with his LQ4 0411 after I played with the output settings.