If the gears were set up right they should be Quiet take it back who installed them.
I have ran different gears over the years the only more noisy ones were Richmans because they are a harder metal but by more noise there were very little noise.
Ive had the 4:10s installed now for about 4 months. I dont drive it on the interstate so thats not an issue but around town I noticed that I get BETTER mileage. I get about 18-19mpg. Granted where I live it mostly hills and 40-50mph. With the gears, and servo, It drives so much better. I just wish the cheap gears werent so LOUD at times. Around 45mph, it is sort of annoying unless I turn the radio up or get into the throttle and drown it out.
---------- Post added at 09:30 AM ---------- Previous post was at 09:20 AM ----------
I have Factory 4:10 in my 2001 Yukon XL 4X4 I get on a good Day 15.3 to 15.7 MPG.
I running 265/75/16 AT tires these tires are 1 inch taller than stock.
With the Stock 265/70/16 I was getting about the same MPG.
I also running a Hypertech programmer to adjust for the tire size not so much for the performance I do have set on Performance and 50% firmer shifting.
What I can tell the Performance mode more of less reprograms the timing curve and the injector timing nothing to write home about.
But I also have a Series 70 big block Flow master muffler I have noticed it dose not down shift as much climbing hills staying up on HWY speeds get up to speed easier but It not no race car by far.