8l90
 

Have an Gen V LT4 8L90 auto in my 66 C10. Have put 75 test miles so far. Noticed when in full automatic at city speeds (30mph) the engine rpm is about 1000 rpm. My rear tires are 31" tall and rear gear is 3:73. The engine does it but rpm seems low?

Do you have it in overdrive?
What gear is it in driving in the city?

Who did the tune? Did you give them the tire diameter and rear gear?

Don

The 8L90 has drive (full auto) and manual (I use paddle shifter). Near as I can tell at 30 mph it is in 5th. I seems to like to be around 1000 rpm at pretty much any speed until your at highway speeds in 8th and rpm goes up with speed. This is a crate package and all "tuning" was done by GM.

Right, but the ECU doesn't know what tire diameter or gear ratio you have. Consequently the shift strategy is based false information. It's close for a lot of applications but with a 31" tire it won't shift the way it should.

I was thinking the same thing. The ZL1 uses 2.85 gears with a 27” tall tire so it’s similar to 3.73 gears with 31” tall tires if I did the math right. Got me wondering what the Connect and Cruise tune assumes.

Don

id have to agree , not that it matters but my 17 silverado with 3:42 gears isnt even that low lol

The install docs for the LT4 8L90 specify 3:08 to 4:11 gear and 26" to 30" tall tire. Yes I went with a 31" tire but assumed the transmission only sees the driveshaft rotation as an end result? Funny my speed to the Dakota speedo was within 5 mph first time out.

Ya I think the rear gear ratio sort of cancels out the tall tires.

Don

Do you have HP Tuners? You can change it that way in the gear/tire wizard in the tune. You are using the Gen V connect and cruise pcm correct?