Here’s the latest reader question, along with my reply!
Nicholas asks: I read with interest your articles about the mileage of the (current) Chevy Tahoe. I’m a big fan of Chevy generally. I just bought a 96 Tahoe with 75,000 miles, almost 100 percent compression on all cylinders. She runs super clean and I just cruise gently on the gas pedal. Do you know of a way to improve the fuel efficiency of these vehicles? I’m only averaging 12.5 mpg, highway driving and here in NZ gas is $2.29 per litre and rising. Any help would be very much appreciated!
My reply: If you’re only averaging 12.5 MPG on the highway, I suspect your Tahoe isn’t running optimally. It may need a tuneup. It might have a restriction in the exhaust (it is almost 20 years old) or the driveline.
So, the very first thing I would do is go over . . . everything. Compression is one thing. How are the plugs and plug wires? Is the rest of the ignition system working properly? Is the fuel system clean and working properly? Have you cleaned the throttle body (IIRC, your Tahoe has TBI rather than PFI) and checked the fuel filter?
I would check – and probably replace – all fluids. Not just engine oil. Also the gear lube in the rear axle, the transmission/transfer case fluid, too. Use premium synthetics, which flow better, reducing friction. Check all wheel bearings.
What kind of tires are you running? If your Tahoe is shoed with knobby, M/S-type tires that would account for the horrendous mileage. Switch to street radials.
Is it jacked up? If it is, un-jack it.
I’d also check the rear axle ratio. If it’s higher than about 3.08 I’d consider changing out the ring and pinion to something around 3.08 or even 2.73 or so. The truck won’t pull as hard from a standstill, but the V8 has plenty of power and torque to move it adequately and once rolling, the lower gear ratio should noticeably reduce your over the road fuel consumption.
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I run a 96 K1500 pickup. Your 96 tahoe should have the same 350 (5.7) Vortec. My 4WD K1500 has 3.73 gears, and I get around 14.5 MPG overall and around 17.5 on the highway.
One thing on that engine you should look closely at is the fuel injection- it is an inside-the-intake-manifold central port inhection- a “spider” harness where one or 2 injectors batch fire a bank of nozzles. These routinely fail and run rich and get awful mileage. Usually one or more of the nozzles sticks and drizzles fuel in. I was getting your 12mpg and replaced mine with an upgraded MPFI “spider” off Ebay, cost a couple hundred bucks and got the Vortec’s mileage back up where it ought to be.
After changing to Amsoil it used a quart in 4000 miles . I changed the oil and filter before it could use another quart. The second round ran leak free and used no oil in the next 7000 miles or so. It no longer has the weep I could never find and uses no oil.
I’ll be changing transmission fluid again this year. I replaced the tailshaft seal on the transfer case and replaced the fluid with Amsoil. It now has no leaks there. The stuff is magic in a bottle.
Don’t mean to be a broken record but switching to Amsoil on several vehicles that used some oil and a couple that even leaked a bit has cured both problems.
I never had the problem solved immediately with an engine but have seen the problem cured in just the second round.
My Z71 used a quart ln 30p
In 3000 miles with conventional oil. After g