Gas gauge

I repleaced my fuel pump/filter about a month ago, and replaced the ring and pinion. The thing is now getting 325 to a full tank on the highway. That is great but the gas gauge is moving to fast for the amount of fuel I'm using. At 3/4 tank the miles is between 55-85. at a 1/2 the miles are at 125. So at least I follow the trip miles at every fill up. So the question is this, how do I check that the gauge is working right, or is it really the gauge? Thanks. Juice

that seems about what mine does. At 3/4 you are actually using a little more gas than from 3/4 to 1/2. I know when I fill up, my guage goes over the full line and will remain there for about 20-30 miles of driving. I also seem to get worse gas mileage the less gas i have.....kinda wierd, but its no biggie to me.

What you are describing is what engineers call "non-linearity". What that means is that the first quarter of the tank on the guage is equivalent to say, 4 gallons, but the next quarter of a tank is equivalent to 7 gallons.

This is not uncommon at all. Gas guages in cars are NOT laboratory instruments, but give merely approximate readings. Dont' be surprised if, when the guage reads "1/2" on a 20 gallon tank, that you it takes a bunch more (or less) than 10 gallons to fill it to the top.

Also, don't be surprised if two cars of the same brand, and same year, will show different results on their guages.

If you are really worried about how accurate your guage is, the best thing to do is to run the vehicle until the guage reaches a chosen point (say, 3/4 for example), and then fill the tank to the top. Record the number of gallons filled. Repeat this for other points on the guage (1/4, 1/2, etc.) This will give you an idea of how linear your guage really is.

I remember a Ford I owned in the 1970's had a guage where the movement from "F" to "3/4" was only THREE gallons. Of course, the movement from "1/2" to "1/4", on that same car, was about SIX gallons.

Ya but it doesn't fill up over the full line anymore after the fuel pump was changed. I can fill it put until it spills out and it still doesn't go past the the white fill line. It would before the pump change.
The guy who worked on it is going to get a yelling at...oh ya that was me.
Any ideas would be great. Thanks

When you removed the guage sender/fuel pump assy, did you BANG the thing around?

The sender assembly is somewhat sensitive to handle. You could have disturbed (altered) the float arm and wiper system used in the sender when replacing the pump.

When my "Low fuel" alarm sounds (guage is on E) my tank still
contains 5 gallons of gas - I can bearly get 15 gallons in it!

Aside from the non-linearity, AZ Jeff may have it - I replaced this
assembly on a Ford once, somehow bent the wiper arm (the
metallic arm that wipes across the resistor substrate) and it was
ALL messed up until I fixed it.....

Actually, having the light go on when 5 gallons are left may be within the design parameters for the gas guage system.

Remember, all of these guage systems have manufacturing tolerances. Thus, while your light may go on at 5 gallons, some other XJ might have that same light go on at 3 gallons, or maybe 2 gallons, or whatever.

What I am saying here is a that, although the light goes on at 5 gallons in your Jeep, it may within the upper specification limit for the overall guage system.

I did have to push it in the tank when I put it back. I must have messed it up. I'm just not looking forward to taking it back out this weekend. But oh well. Thanks guys. Juice

AZ Jeff said:

Actually, having the light go on when 5 gallons are left may be within the design parameters for the gas guage system.

Click to expand...

Don't get me wrong - I don't mind at all. It's just that 25% seems
like a BIG tolerance. Anyway, I'll poke around the manuals and
see if I can find any numbers.

I dread even the thought of fixing one of those sending units.....

I get almost 400 highway miles and barely 250 city, I figure (for me at least) for a 13 year old car with 4 wheel drive, that aint too bad. Maybe I should be worried...but I have more important things to worry about than my gas mileage.