The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

**hatzie** · 11-22-2021, 10:22 AM

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.

11-22-2021, 10:22 AM	#8
hatzie Moderator Join Date: Oct 2008 Location: Wentworth, NH Posts: 4,920	Re: No start??? 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. __________________ 1959 M35A2 LDT465-1D SOLD 1967 Dodge W200 B383, NP420/NP201 SOLD 1969 Dodge Polara 500 B383, A833 SOLD 1972 Ford F250 FE390, NP435/NP205 SOLD 1976 Chevy K20, 6.5L, NV4500/NP208 SOLD 1986 M1008 CUCV SOLD 2000 GMC C2500, TD6.5L, NV4500 2005 Chevy Silverado LS 2500HD 6.0L 4L80E/NP263 2009 Impala SS LS4 V8 RTFM... GM Parts Books, GM Schematics, GM service manuals, and GM training materials... 1973-1991 GM Light Truck Service, Unit/Overhaul, Wiring Manuals, Parts Books and Some 1968-1999 GM Service Training textbooks in PDF 1988-1998 GMT400 CK service manual project 1988-1993 GMT400 CK parts books. 1967-1972 GM Light, Medium, & Heavy Truck Service Manuals in PDF Please include at least the year and model in your threads. It'll be easier to answer your questions. And please let us know if and how your repairs were successful. Last edited by hatzie; 11-22-2021 at 10:43 AM.