Horrible gas mileage. Need help
 

I drive a 1992 gmc Sierra k2500 4x4. Has a tbi 350. I drive it like a grandma barely on the gas rarely ever seeing anything over 2500rpm. I filled it up and estimated my gas mileage. I'm only getting 7mpg. I don't understand what's going on. I put a new filter on it, motor runs good and it doesn't leak any gas. Modifications are a 3" body lift, 33" tires, 15" American racing rims, true dual exhaust with glass packs.

Any help would be appreciated.

33 inch tires!!!! That's definitely a chunk of your problem if not all of it. With that large of a tire you have effectively increased what is known as your 'rolling weight' meaning how heavy the stuff that turns is, ie your wheels, hub, discs and so forth. This is the worst place to add weight to a vehicle for mileage purposes. Not only that but since your tires have such a large circumference you have effectively put more weight further on the outside of the wheel which is worse yet.

Oh, one more thing, your mileage may not be as bad as indicated if you are going by the trucks dash read out or even figuring by hand. The tire size will throw the mileage indicator off until you have the trucks computer reprogrammed. You can actually figure this all out by driving 50 mph and using a gps to see what your real speed is. If your speedo says 50 and the gps says 55mph then you are racking up mileage 10% faster than you should and need to subtract 10% of you mileage readout from your calculations.

To basilkies point, not only will your speed be off but so will your odometer. If you buy something like a Hypertech module you can reprogram your computer for the larger tires and all will be well again as far as speed/odometer accuracy.

I have 33" all terrians and I get worse gas mileage. Look good but I think it threw off my odometer also. So I just use a gps now

To be exact there 33" Dayton timberline MT tires. I thought the same thing about the tire size and speedo being off. I used a gps and it's dead on the spot. If I'm doing 30mph on the dash, the gps is saying 30mph. If I'm doing 70mph, gps says I'm doing 70mph. I realize that the gas gauge is horribly off anything under 1/2 a tank. But I ran it empty on accident and it came out to 180miles on a full tank.

i have a 1997 4x4 1500. i get 15mpg. and also a 97 4x4 2500 suburban. gets about 9 mpg. its is supercharged, and both trucks have 33" mud tires.
the reason the suburban is getting bad mileage is because the catalytic converters are cloged, therefor giving bad readings to the sensors and not allowing proper flow.
the 1500 could also be getting better gas mileage if the exhaust leak was fixed. (also giving sensors a bad reading )
so you might want to check your cats
hope this helps

i have a 94 silverado(K1500) with 265/75/R16's and im getting around 3mpg around town 7 on the highway. The truck has a 350 5.7L thats been bored out 3x's over and magnaflow exhaust, stock air filter, stock everything else...no leaks, doesn't burn oil, and i wld like to know how i can get better mileage too, even if its what y'all are getting!

94 is carbed right.?
fuel injection would help. kinda an expensive project though. but i bet it would deffinately help your 3mpg. and it has alot to do with driving style also.

94 is TBI, Throttle Body Injection. Its an early fuel injector system, more like a cross between a carb and fuel injection. One fuel injector which mixes the fuel with the air within the throttle body.