A friend/former boss just bought an 05 Silverado Z71 4 door 5.3 with a conversion package that includes a ~6" lift along with 35" tires. His truck is a turd though and gets worse mileage on the interstate then in the city because it's always having to downshift and lug. It's gotten as high as 14 in the city with a fairly light foot, but it goes down to around 11 on the interstate where 70-80mph cruising speeds are common.

My question to you all is between 4.10's and 4.56's which would be the best choice? 4.10's would put him about near stock rpm cruising rpm's, but I think 4.56's would be the best all around choice for power and mileage with the added mass of taller tires and more ride height. The truck sees about half city and half highway driving and it won't be used for towing, at least in the near future. Also, since this will be a 1 time thing, what programming tool would be best to calibrate the speedometer? thanks in advance

Jason

 

Sorry, I posted this in the wrong section. Could a moderator please move this to "GM Drivetrain and Suspension"?

Jason