Make it make sense...

[clthorn]

I have a 1998 C3500 with a 4l80e, 4.10 and 245/75/16 tires. Now with all that when I'm cruising at 3000 rpms I should be going about 88mph right?!
No I'm going 70mph. I have verified that I am in 4th gear and the torque converter is in lockup. I'm 100% positive I have 4.10 gears. My speedometer reads incorrect I have verified this with gps. Below is a photo of me driving 70mph but if you input all the information into Spicers ring and pinion calculator I should be going 88mph at 3000rpm just like the speedometer says but I'm only going 70mph at 3000rpm for some odd reason please help

 

[Moleman]

Yeah something ain't right there. Now I only have 3.42s but I do about 1900 at 70 in lock up

 

[Hipster]

I would verify the rear gear. DS should turn 4.10 revolutions for one wheel rotation... Old truck , 3500, maybe been swapped to 4.56's or something for towing use. You say 100% positive 4.10's but..... didn't say how verified. 4 distinct gear changes, then a 5th drop of a few hundred when the tc locks,

 

[scott2093]

Can you get a hold of a scanner and see what the computer is showing everything is at?

 

[CumminsFever]

It would make sense... if you have 4.56 gears.
Or, if you aren't getting high gear, being 4.10 in 3rd gear.
My vote is on 4.56 gears that were swapped into a truck that originally had 4.10, and speedometer was never corrected.

 

[yevgenievich]

Check with scanner. I seen too many clusters that will show rpm and speed incorrectly. Both rpm and speed will show hight. And verify gears as well. Either different gears or cluster it self

 

[clthorn]

So I had a friend come over with a scanner and I never in a million years would have thought both my speedo and tach were wrong. Going 70mph using the scanner my speedo reads 88mph. Turning 3000rpm using my tach, the scanner reads 2450 rpm.

 

[skylark]

So I had a friend come over with a scanner and I never in a million years would have thought both my speedo and tach were wrong. Going 70mph using the scanner my speedo reads 88mph. Turning 3000rpm using my tach, the scanner reads 2450 rpm.

Chance are that someone has had the needles off of the cluster. You can reset them.

 

[GoToGuy]

Before changing anything. Are you using the tire size your truck was built with. Or at least the same diameter as OE equipt. Your tach is off. Is it a percentage ? Or needle position? If your axle gearing is stock OE you can go into the eng computer and change to current tire size, it will adjust the math, for speedo.

 

[yevgenievich]

Motors in the cluster can act up and be off track