Ok, here's my observations and data so far from owning a 91 Pickup (3VZ-E engine found in 2nd Gen 4Runners) and a 98 4Runner (5VZ-FE).
- Larger Cat-back exhaust and straight-through muffler: gained 1.5 MPG on the Pickup. No change on the 4Runner.
- Upgraded injectors to 12-hole fine spray: gained 0.5 MPG on the Pickup. Haven't done it on the 4Runner.
- Synthetic oil: No change on the Pickup. No change on the 4Runner.
- Synthetic transmission fluid: Manual Pickup. No change on the 4Runner but does shift smoother.
- Replacing front oxygen sensor: Helped regain lost mileage on the Pickup. Helped gain lost mileage on the 4Runner.
- High flow air filter: No change that I have recorded on the Pickup. Gained a small amount <0.5 MPG on the 4Runner.
- Larger intake: No change on the Pickup. Lost a small amount <0.5 on the 4Runner (doh).
The 3VZ-E had a lot of inefficiencies on the exhaust and fuel delivery that could be improved. The 5VZ-FE on my 4Runner does not and as a result most changes made little impact. However, all that being said I have a custom tuned 4Runner that makes over 300HP to the wheels and I still get a combined 16 MPG and up to 21 MPG on pure highway over 60+ MPH. That's nothing to cry about.
A few other notes:
- Not all injector cleaners are made the same. I've tried pretty much all of them, Gumout, Lucas, Royal Purple, Chevron, and AMSOIL. The cheap ones I noticed no change. Royal Purple smooths my idle out. AMSOIL did actually make a different in my Nissan Altima. It was at 34 MPG for years and I put one in earlier this year and it's now at 37.3 MPH. That's a 9.7% increase in fuel economy for a $8 bottle of cleaner. Not bad at all. Have not measured the 4Runner yet as it's been garaged more due to gas prices.
- About when I bought my 98 4Runner CA added 10% ethanol to all fuel. That really killed MPG's of these older vehicles as they don't run well on ethanol blends and was an instant ~1 MPG loss on my 4Runner. You can see the whole chart shift lower between 2016 and 2018 when ethanol was added: https://www.fuelly.com/car/toyota/4runner/1998/gamefreakgc/330715/fuelchart
- I don't really notice a huge difference on whether I'm driving quickly or slowly in a pure gas vehicle. Driving style is more of a detriment than a way to add MPG if that makes sense. Driving like an old lady on the road isn't going to net you 30 MPG in a 3rd gen 4Runner. I drive my 4Runner hard at times but not for miles and miles on end. Plus it's not any fun to drive slow.
- Considering a 4-cylinder commuter car really is the best solution. You can pick one up for cheap under $5000 and can get 30-40 MPG. Hybrids are even better. Mine is rated 33 city/33 highway but can get more if you drive slow since it's a hybrid. Here's an example of the low end of pricing:2009 Nissan Altima Hybrid - cars & trucks - by owner - vehicle.... Owning a hybrid as a 2nd car allows me to use the 4Runner for short drives and snow conditions and a hybrid for the daily errands and commutes. Really makes a big difference.