13 or 13.5 gallons. Do not let it run down and leave you stranded before you calculate MPG. Go fill up the car (dont top it off, just let the pump shot off normally). Drive it around until you get to the 1/2 tank marker if not 1/3 or so, and then fill it again.

However much you put in (7 gallons or whatever), versus how many miles you went is how you calculate. 7 gallons and 140 miles would be 20MPG.

I have noticed that on my 73, the gas gauge does not read linearly, many cars are like this. You will have to get used to what the gauge means, but generally once you calculate your average MPG you should know by # of miles how much gas is gone and how much you have left about. When I hit the 1/2 way point on my gauge, it starts going down fast, so it is obviously not reading linearly.

No matter how you do it, you need lots of data. I have a log book and enter into it religiously. MPG varies from 23 ~ 10. Average 15 MPG - But I have a very heavy foot.

Fuel tank capacity for a 240 is 15.9 gallons.

280's had a slightly larger tank at 16.3 gallons if I remember right.

Have you checked your vent hoses on the tank to see if any are blocked off or crimped? You may have trapped air in the tank while filling it, and therefore not getting the tank full.

ok, i jus filled up today, put in about 12 gallons, according to my guage, it is almost full. which means i have somewhere around a 13-14 gallon tank, which according to 2ManyZs is incorrect and that i have a 16.3 gallon tank, i dunnoe, n e way. i will keep track or my miles per fillup and report bak.

I sure thought it was a 13-13.5 but I guess I am incorrect(?)

Just keep in mind gas gauges are almost never accurate, usually this is because of the float device and the shape of the gas tank, modern cars are often the same way depending. My 94 Camaro for instance, same thing, 1/2 tank on the gauge is really 1/3 tank. I have to burn 5 gallons after filling it (not topping it off, just full) for it to say 4/4 FULL.

For my 73 240Z it is probably similar, when it gets down to 1/2 tank the tank empties FAST thereafter. So I assume it is around 1/3 or so. Either way, to get an accurate number you should fill it, so that you can fill it the second time around, but you should be pretty close so long as you dont get sloppy with the math, break out a calculator if needed

Ok. Now im pretty sure that im getting around 10 miles a gallon or so.

So what is there i can do to fix it. Again, I have a 78 280z fule injected. Seems like i have a vacuum leack, can that b a problem. but i cant seem to find the leak.

My timing might b off, seems like i have no more room to turn the distributor to get the timming right. it say timing mark 10 under the hood"s lable. Help me out guys.!

I'm getting great mileage with my 73 240. The trick is to put in a 3.9 rear end and not change the speedo gear. I seem to be getting around 30 mpg. Not bad for a six with triple Webers. Dream on.

I have a 78 FI 280z with the stock r200 3.545 rear, with the stock 4 speed. Only thing i'v done is put on an k&n cone filter.

Im getting about 10 miles per gallon. What could be the cause?

My injectors dont seem to b leaking. but like i said earlier, i have a vacuum leak, but i cant seem to find it, so not sure if it is a vacuum leak. And my timing might be about 4-5 degree advance.

the timing mark under the hood shows 10 its at about 4 or 5 but i have no more room to turn the distributor to get to 10i can only turn it the other way.

ANY ONE?