3.55 gears running 235/85-16 tires.

My math says I should be 2000-2200 rpm but mt tach is showing 3000.

I am trying to figure if my tach us wrong of if something in tranny is wrong?

I just put this engine in, and haven't really drove it for a year prior, not sure what it ran rpm before.

Nothing should have changed.

It seems to run strong, I even in hooked kickdown to see if it was binding.

I am going to try to locate a different tach to compare readings.

It almost looks by the numbers that it isn't going to third. But I think I feel 2 shifts from a stop.

 

I'd say your math is about right. Off hand I don't know my gearing or tire size but for me, on the straight and level;

2nd gear 45 mph is somewhere around 3,000 rpm
3rd gear 63 mph is about 2,600 rpm
3rd gear at 55 mph is around 2,200 - 2,300 rpm.

How's the fluid level in the trans? If it's low it might be slipping? Might also be good to pull the trans pan and check the screen/filter.

 

My truck with 2.75 gears turns about 3000 at 60 in 2nd gear. Your RPM would be quite a bit higher with 3.55's in 2nd.

I think someone may have swapped out your axle for 4.10's.

 

I am hoping that I have either a bad tach, or low fluid level ( I need to find someone to Comeover and hold it in drive), or misaligned linkage.

I don't notice any abnormal movements in the tach as if it were slipping.

The Rpms weren't this high before engine r&r.

 

Do you have a factory tach? If so, it may be in 6 cylinder mode. Have you swapped in the wiring harness to the ignition or swapped from a six to an eight? There is a Bl/Lg wire at the connectors near the ignition module that must be grounded to tell the tach it is an 8.

 

Yes it's factory. Original wiring, perhaps something got disconnected or broke on engine removal.

 

Mine used to turn around 2200 with C6 and 215/85R-16 tires (remember I have 6 of them).

 

Mine is drw also.

 

How did you get 235s to fit without rubbing? Every tire place I looked tells me you can't go that wide or the rear sidewalls will rub and overheat.

 

Bill, there is ton of room between tires. I can email you a pic if you like?

I have even hauled 10000 lbs if trailer/cattle over the summit us rt 40. With no issues on the tires. That was the size on it when I bought it.

Tire guy never second guessed me when I got 4 new ones.

 

Update:

I borrowed a friends snap on timing light with digital tach, it was reading way different than my gauge.

Timing light readings were 2200 rpm when 3000 showed on my gauge.
670 at idle compared to 950-1000 on my tach.

I think I might be missing that ground that makes it think its a 6 cyl.

Can someone tell me what and where theses grounds are?

 

I found it. It now reads within 100-200 rpm of snap on timing light.

For anyone following this, there is a two wire connector coming out if plug that goes to module. This is on the truck side of connection with module.
There is grn/yellow and blk/grn that leads toward cab. Blk and grn leads toward coil/distrib.

My connection was bad at that plug.

It must have read as a six cyl.

 

It would, about 1 1/3 times as high.

 

Yep, that's what that bk/gn wire is for. Glad you found it.