Hey fellas i just recently did a major tune up to my baby. Before the tune up i was getting 15mpg and after the tune up im still getting the same mpg! I thought it would of improved but to no avail i still get **** gas mileage. My friend has a 400 and he gets around 17 mpg! Anyone know why my baby is still getting ****ty gas mileage? Thanks.

 

It would help if you listed what major tune up was done on the car. There was a thread on here that discuss what people have done and recommendations.

 

I replaced my water pump+gaskets and o-rings, timing belt+idler and tensioner bearings, radiator flush, sparkplugs, sparkplug wires, valve cover gaskets, serpentine belt, drain and refill on tranny fluid.

 

It's all going to be how you drive it and your city vs. highway ratio. I have a 99 GS400 which I struggle to get over 16mpg, but it is 99% city driving with the AC on. The factory rated it at 16mpg city so I'm spot on. The GS300 is rated at 18mpg city, so you aren't far off, just depends on how you drive it. Also, do you have any wheel/tire mods?

 

some other stuff you can do is the air intake and the fuel pump. Check your tire pressure as well. If you are using after market wheels they can impact your reading. Bullitt180 is correct, it also depends on your driving.

 

I have some stock is350 rims wig low profile tires. I think that's why it's effing me up. And I drive like a grandma honestly lol. Half highway and half city. Should of got a 400 or 43o

 

Your wheels should be fine as long as the overall diameter is roughly the same. OEM wheels aren't going to be "much" heavier than other OEM wheels. On another note, I just filled up this morning, and got 13.5 mpg on my last tank in my 99 gs400.....down from 16.5 on the previous tank....but I drove it like I stole it most of the time.