Help with mystery

[Jonathan V]

total newb here, so please forgive me if this post is in the wrong spot.

I have a new to me 2005 Tahoe 5.3 4x4

At 65 mph (as measured on my gps- my speedo says 70mph, but appears to be off as it is doesn’t match gps or radar) my engine is turning 2000 rpm, I have 3.42 gears (checked by turning the back wheels and counting driveshaft turns). I have 265 75 r16 wheels/tires (31.6” tall)


It seems to be going into 4th gear and locking up.


2000 rpm seems about 400 too high at 65.

What am I missing? Any help would be appreciated

 

[Scottydoggs]

pull the shiftier into 3 when at hwy speed, rpm should go up, if they stay the same you lost 4 th gear, not sure if thats a common thing with our tranny, but it is with others.

 

[Jonathan V]

pull the shiftier into 3 when at hwy speed, rpm should go up, if they stay the same you lost 4 th gear, not sure if thats a common thing with our tranny, but it is with others.

Appears to have 4th (rpm definitively different in "3rd" vs "D")

Any other possibilities? or is my car just odd?

thanks for the help!

 

[Scottydoggs]

you have the stock dash cluster still. or is it upgraded with a face plate?

its possible that the servos are out of whack if some one pulled em off and didnt get em back on right. normally they would need to be re calibrated

 

[Jonathan V]

you have the stock dash cluster still. or is it upgraded with a face plate?

its possible that the servos are out of whack if some one pulled em off and didnt get em back on right. normally they would need to be re calibrated

firstly: Thanks for your help!

It appears to have a stock dash (but not sure how to tell)... the Tach looks right, car idles at about 700-800. But could the tach be mis-calibrated? Goes to 0 when off.

How would I tell the actual RPM, with a ODBII scanner?

 

[Scottydoggs]

yeah, the torque app will do that for ya. every thing read off the obd port is a better reading, the cluster can be off for sure.

 

[UrbanSuburban]

Your right 2000 rpm is rather high for 3.42 gears, but it is about right for 4.10 or 3.73, in your glove box check for a GT rpo code. GT4 is 3.73. If you had Transmission trouble it would light the CEL as it effects emissions. Your idle speed is also a little high, my 04 suburban with the same motor would only idile at 550 or so. Something is off.

 

[Fless]

Your factory tires were probably 265/70R16s, and you're running 75s; look at the sizes listed inside the driver's door. Technically that ought to make your speedometer read slower than what it was on the original tire size. Based on the difference -- the other way -- between your GPS and your speedometer, that's a mystery in and of itself.

 

[swathdiver]

Based on your tire size, gearing and transmission, 2000 rpms in 4th gear would be 79 mph. If you had 3.73 gears it would be 73 mph. 66 mph for 4.11s.

265-75-16 tires at full tread depth are 31.65" tall.

 

[Jonathan V]

Based on your tire size, gearing and transmission, 2000 rpms in 4th gear would be 79 mph. If you had 3.73 gears it would be 73 mph. 66 mph for 4.11s.

265-75-16 tires at full tread depth are 31.65" tall.

I know. I just measured again today... 2000 rpm was 66mph (on my gps, which I measured as accurate against my other car)

That implies 4.11 gears, but I jacked the back of the car up, chalked one wheel, turned the other twice and counted 3.5 turns of the driveshaft...


So I am really confused!

 

[swathdiver]

I know. I just measured again today... 2000 rpm was 66mph (on my gps, which I measured as accurate against my other car)

That implies 4.11 gears, but I jacked the back of the car up, chalked one wheel, turned the other twice and counted 3.5 turns of the driveshaft...


So I am really confused!

Did you check your RPO codes? Can you tell via scan tool if your torque converter is locking up and how much slippage there might be? I don't remember that 3.5 turns stuff, just use my homemade Excel calculators.

 

[John Ley]

GT5 4:10 GT4 3:73 GU6 3:42 GU5 3:23 GU4 3:08 GU2 2:73 GU1 2:41

 

[Jonathan V]

GT5 4:10 GT4 3:73 GU6 3:42 GU5 3:23 GU4 3:08 GU2 2:73 GU1 2:41

RPO: GU6, so 3.42 was stock, the 3.5 turns actually measured how many turns the driveshaft (therefore pinion) has to make to turn the wheels 1x (the ring gear). Mine is approximately 3.5 driveshaft turns (therefor 3.42, as .08 of a turn is too small for me to see by hand)

 

[swathdiver]

RPO: GU6, so 3.42 was stock, the 3.5 turns actually measured how many turns the driveshaft (therefore pinion) has to make to turn the wheels 1x (the ring gear). Mine is approximately 3.5 driveshaft turns (therefor 3.42, as .08 of a turn is too small for me to see by hand)

Your tach may be off some and your converter might not be locking up. Are you able to detect whether it is locking up or not? It would seem like another shift and drop the rpms a bit.

 

[Jonathan V]

Your tach may be off some and your converter might not be locking up. Are you able to detect whether it is locking up or not? It would seem like another shift and drop the rpms a bit.

Sorry for the late response, I was out of town. It does seem to lock up, I see a small rpm drop distinct from 4th

But perhaps my tach is wrong, but I don’t have a scanner to see the real RPM

 

[Walkercraig01]

u can grab a elm327 on amazon pretty cheap and get torque or dashcommand for your phone. torque is free tho. can you post a video ?