I have a 1991 Suburban. What engine and/or transmission do you suggest to change to get a better MPG?

Question

I have owned this suburban for four years and I have made these improvements to spend less money in fuel.
It has a LPG equipment, so it can use LPG gas and gasoline.
It has two mufflers, one for four cilinders and another for the other four.
I bought also a K&N filter for my truck.
I even bought a magnetic fuel economizer.

Whith all these things I get only 8 MPG (city) and 13 MPG (freeway) driving with LPG.My truck is in good condition and I use it on weekends and to travel with my familly.

I have read that I can do these things:

Change the 3 speed automatic transmission for an overdrive automatic transmision.

Change the eight cilinder 350 cu in carburated engine for a six cilinder fuel injected 292 cu in called vortec engine.

If you have a better suggestion than these or than changing my truck, thanks in advanced.

Answer 1

If you do anything, I would go automatic overdrive. Unfortunately, you may find you need to change out the rear-end gears (differential) because the older vehicles had very tall (lower numerically) gears in order to get better gas mileage. Once you install an overdrive with these tall gears, you may find you are not using the overdrive until you are going in excess of 50 - 60MPH, because the engine does not have enough power to keep the OD engaged. I would shy away from any engine swap, as that has the possibility of making WORSE gas mileage because you will be pushing the pedal to the floor more often just to get out into traffic or climb a mild hill. I would stick with the 350, get it tuned up properly with the timing set for best efficiency. Unfortunately, with an LPG and gasoline conversion, or "dual fuel" conversion, the timing is set for a best-of-both-worlds compromise, and does not allow for the best efficiency. If you use the LPG often, and have good access to LPG refilling stations, then I would get the truck converted to LPG only, and if you do not use LPG all that often, then ditch the LPG setup and tune it for gasoline only. LPG will typically not get as good of gas mileage as gasoline, but it does burn cleaner making it very easy to pass emissions tests.

Another helpful solution is to get a crossover pipe installed in your exhaust, this will help not only in MPG, but in power, as it helps low end torque, which is critical in a vehicle as large as a Suburban.

Another way to go would get a better induction setup on your truck. Many of those older GMs came with TBI (throttle body injection) setups which were not the best idea for economy. Check into getting a TPI (tuned port injection) for it. It will make it not only more efficient, but also more peppy. For this idea I would direct you to summitracing.com since their techs are very knowledgeable, and talking to someone on the phone will give them a better idea of what you are trying to do. This is provided you have a general understanding of your vehicle and automobiles in general.

Good luck to you and I hope I was able to help you somewhat.

Answer 2

LPG has a lower energy content per unit than gasoline, therefore your mileage is actually LOWER, on the order of about 25% running your truck on LPG rather than gasoline.

It's just not economically feasable to try to swap engines to improve mileage.

However, typically diesel engines get better mileage, all other factors being equal...I'd recommend if you're determined to stay in a large truck, sell your Suburban, and find a Suburban with the diesel engine.

Answer 3

I have a 99 Suburban with a 350 vortec engine and it gets 15mpg city and 17.5 mpg highway. If you don't have overdrive I would bet that is your issue. I wouldn't downgrade to a six cylinder at all.

Answer 4

Changing the motor/ or transmission isnt cost effective.. just keep it tuned up, check your air pressure in your tires, or go buy a newer vehicle, thats your best bet.

Answer 5

change the engine to a 305 because the motor mounts are the same.