New Guy / Guess my MPG ('05 GMC Canyon Crew Cab 4x4 Z71)

[evil03mustang]

The laziest cruise I ever used was on my old '83 Cougar.

I set it at 65 and it pretty much did whatever it wanted. I hit [whatever all the way to the right was] several times coasting downhill, and it was so slow it couldn't do more than about 50 uphill anyway.

The car was a 90-hp 3.8L 2-barrel carbed 3-speed auto no overdrive SLUG with airplane gears out back and got 34mpg from PGH to Cleveland and back...

Pulling out up a hill was nearly suicidal, however.

[KY_Canyon]

LOL. The worst I've had was in a Toy Tacoma manual trans and oversized tires. Uphill with cruise just wasn't going to happen. It would loose mph so quick that it would just finally kick off completely well before i needed to down shift. Not a good feeling to be running 65 up a hill and your cruise kicks off. I just got in the habit of holding the pedal going up hills.

[JRMichler]

I drove a 4X4 Colorado AT once. I did one coast in neutral with the engine running, and it coasted about as well as my truck.

I believe that well over 25 MPG is practical. Possibly close to 30 MPG with a grille block, better tires, at least a tonneau cover, and slowing down to 55 MPH.

[KY_Canyon]

Originally Posted by JRMichler

I drove a 4X4 Colorado AT once. I did one coast in neutral with the engine running, and it coasted about as well as my truck.

I believe that well over 25 MPG is practical. Possibly close to 30 MPG with a grille block, better tires, at least a tonneau cover, and slowing down to 55 MPH.

I'm sure the 4x4 hampers a bit but think my tires prohibit the coasting more than anything. I have an 05 Mustang V6 that seems to coast forever, so that may be part of the feeling I get when in the truck. I've actually been reading your threads and saw where you did your coasting test.....quite thorough. I was extremely surprised to see just how long you were able to go.

I read where you are running 235/75-15 (if i recall correctly); not sure what your differential gearing is, but given that OD in the auto and 5th in the manual are the same ratio I was curious what RPM you are running at 55 MPH?

[JRMichler]

The 235/75R15 tires are 2% oversize, so about 1800 RPM at 55 MPH. My truck has the 3:73 rear end.

I did an informal coasting test after installing the current tires. After increasing the pressure from 33 or 35 PSI to max sidewall of 47 PSI, it coasted much better. They have been at 47 to 52 PSI ever since.

[KY_Canyon]

Originally Posted by JRMichler

The 235/75R15 tires are 2% oversize, so about 1800 RPM at 55 MPH. My truck has the 3:73 rear end.

I did an informal coasting test after installing the current tires. After increasing the pressure from 33 or 35 PSI to max sidewall of 47 PSI, it coasted much better. They have been at 47 to 52 PSI ever since.

Interesting, the 2005 Z71 came standard with 265/75R15 which is ~ 6% oversize (diameter) from where you are now and with the same 3:73 ratio. My 265/65R17 tires are just a slight bit smaller than stock for the Z71 and I'm turning about 1825 RPM @ 65 MPH. I'm wondering how my oversize tires really impact MPGs. It seems as though the Colorados & Canyons with the best mileage tend to have the smaller diameter tires, even though this creates a need for higher RPMs at a given speed. Perhaps the increased torque puts the engine at a more efficient load rating?

I was planning on replacing with 245/70R17 once these are done. The tires I'm looking at would reduce my tread width from 9.75" to 8" and would reduce weight by 4 lbs. Diameter change would be negligible. Perhaps a bit of testing is in order.

[JRMichler]

Originally Posted by KY_Canyon

I was planning on replacing with 245/70R17 once these are done. The tires I'm looking at would reduce my tread width from 9.75" to 8" and would reduce weight by 4 lbs. Diameter change would be negligible. Perhaps a bit of testing is in order.

Please do that and post the results. Tire width has been a continuing topic of interest.