Quote:
Originally Posted by hatzie
The engine size and vehicle weight drastically affects the fuel mileage. Not to mention the availability of 6 speed automatics on the 1/2 ton trucks and SUVs at the tail end of the T800 production.
The 4.8 & 5.3 in a 1/2 ton truck will get significantly better fuel mileage than an LQ4 6.0L in a 9600lb GVW 3/4 ton truck. Add in the extra gears and optimized PCM tables for the 6Lxx transmission and you again bump up the mileage noticeably.
If you lifted and installed larger tires the speed and odometer are affected quite a lot. Both read lower than actual speed and miles traveled. 3.45 gears with 245/75R16 tires (29.47") changed to 33" tires drops your original 3.45 final ratio down to something roughly similar to 3.08 gears. If I did that with the NV4500 in my GMC the RPMS would drop from 1,103 to go 5mph in 1st gear to 985.
You need to change the final drive ratio correction factor in the PCM and adjust the tire size in the ABS computer to compensate for the larger tires. Otherwise the PCM will be using the wrong portions of the fueling and transmission torque reduction tables. Not to mention getting tickets because the speedometer is reading several MPH too low.
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I agree with pretty much all of the above but most of it doesn’t apply to my situation. Stock tires and no lift. However it is a used truck with 222k on the clock so I’m not expecting like new performance or gas mileage. 4L80E in it and stock gears as far as I know. Could a previous owner “tweaked” the tune, sure. Not sure I mentioned it but it’s a non HD 3/4 ton.