Mileage gauge?

Is there such a thing as an accurate, real-time mileage gauge that can be easily installed on my car?

You can’t save what you can’t measure.

Yes and know. I know of some OEM gauges that do well, but I don’t know of an after market that you can add later. Unless one came with your car or at least with a different trim level of the same model, I doubt if you are going to find anything. Even the good OEM’s are not 100% accurate. They are good at telling you are doing better or worse than a few minutes ago.

I would suggest that if you remember to take it easy on the peddle and anticipate traffic conditions so you don’t need to use the brake etc. you are going to do noticeably better. The additional you might do with a meter is going to be very small.

Yes, the popular one is ScanGauge.
See here:

Years ago I installed a similar device in an '84 Buick. I bought it from JC Whitney. It worked great and lasted for the rest of the car’s natural life.

I believe Steve is correct. I did not think it worked on most current models, but it appears I was wrong. However I do stand by the fact that they are not 100% accurate and that you really don’t need one to figure out how to drive for economy. $150 will buy a lot of fuel even at today’s rate. I doubt if it will pay for itself. What it will do is be able to tell you if what you are doing is helping or hurting, and that should not hurt.

Probably one of the best guides to getting good gas mileage is a manifold vacuum gauge. A regular parts store vacuum gauge plumbed to a manifold vacuum source will indicate how much effort the engine is producing. At idle it will run about 25 inches; at full throttle it will show <2 inches; and what you want for economy is between 10 and 18 inches. If the reading stays below 5 inches for a long time, down shift and get the vacuum up. I remember one of my vacuum gauges had the scale marked as idle, economy, power, and overload while going from 29 to 18 to 10 to 5 inches.

A vacuum gauge is actually a better instrument for managing an engine than a tachometer. In fact I have wondered about putting a vacuum gauge on the inner circle of a tach gauge so both instruments are readable at one glance. I remember that the '68 Dodge charge had the clock in this postion on its tach.

Hope that helps.

“I do stand by the fact that they are not 100% accurate…”

The DIY gauges can be made extremely accurate because you calibrate them yourself. Use interstate mile markers to adjust the distance measurement, and several fill-ups to adjust fuel usage. Tweak any time as necessary. They can aproach 100% accuracy; I’ve had experience.

just noticed the OP wanted real-time.For non real-time just the value on the pump(quanity you put in) and your odometer (how far did you go) then the simple math you guys implied I couldnt do (Im getting over it)Probably best (maybe required) same pump,same temperature. Accurate to 1 mpg, agree?

The DIY gauges can be made extremely accurate because you calibrate them yourself. Use interstate mile markers to adjust the distance measurement, and several fill-ups to adjust fuel usage.

I agree as far as that goes, but it will likely be off at any given time. For example it may underestimate fuel usage when accelerating and over estimate very slightly when cruising at 70 mph and slightly under at 55 mph.

That does not mean it is worthless, but I would not rely on it too closely.

No, like Steve F said, you first have to calibrate your gauges. If your speedo is off, your odometer is likely hooked up to it in some way & will be off too. Temp is important, I don’t think the pump is, but pump rate I think is - the faster you pump, the more the valve (or whatever) interacts that keeps the pump from exceeding the capacity of the hose to deliver, and recycles the overage back to the station tank, with increased vaporization. So if you pump slowly, the more accurate the delivery.

I have one of the current scangauges. It was a simple plug in install. It will not work on older cars, but it works on most 2000 and later models. It uses the car’s sensors. For accuracy, you need to calibrate it. That includes a few adjustments at each of the first few fill ups and adjusting your car’s speedometer using the highway mileage markers or a GPS unit. The more accurate you adjust it, the mofore accurate it will be.

It appears to sample and refresh the display about every 2 - 3 seconds. 

I have not finished the calibration procedure, but it appears to be doing well.  

That said, I doubt if it is going to be able to give accurate figures across the board.  That is it may be right on for a typical tank full of your usual driving.  However going on a trip with a lot of highway driving, unlike you local city driving and I would expect it to be less accurate.  Even then though it should be able to tell you that driving at 70 mph is using about 7% more fuel than at 60 mph, even though both numbers are about equally too high or too low.  

I repeat, my comments are based on my car, my scangauge, my experience and expectations.  Your results may vary. I believe they are only producing one model currently, but I could be wrong about that as well.