azdave35

has anyone ever done a test to see if the mpg computer (average) is accurate?

Fred/N0AZZ

It's not the most accurate, I fill my fuel tank to the same point every time and divide by mileage and gal used.

jon-d

it makes a good reference and is also good for direct comparison of any changes meant to improve mileage. for accurate readings miles divided by gallons on fill up is the best way.

dave1123

The overhead display in my WJ is accurate to + or - 1mpg when checked by actual usage. My driving conditions are extreme because I'm either running highway or delivering newspapers. Not much in between. I'm averaging 15.5 to 16 with a 4.0 and 3.55s. Oh..no lockup.

extrashaky

If you don't want to do the math, there are several free smartphone apps out there that will do it for you just by entering your mileage and fuel pumped at each fill. So install and test it yourself.

dave1123

WTF? Can't do simple math? Oh yeah, I forgot about Common Core.

extrashaky

I have explained the math to people enough times to know that it's a waste of time. Some people have a mental block that makes subtraction and division too difficult. Now I just suggest using an app that does it for them. It removes the burden of thinking. I'm pretty sure they can do the math. They just have a mental block that prevents it.

With regard to OP's question, if I wanted to know how accurate the computer's estimate was, I would reset the computer and let it estimate an average through four or five tanks of gas while also manually calculating the mpg at each fill up. Then I would average the results of those calculations and compare it to the computer's result.

However, knowing how often people's eyes glaze over as soon as you use the word calculate, I just suggest using an app. When I was younger, I actually kept a little book in my car I used to calculate and track mileage. Now there are several apps that do the same job and also average the results. Not a whole lot of thinking required. Just let it compute an average based on actual math and compare it to the computer's estimate.

OP actually asked if other people had tested to see if the computer is accurate. That's irrelevant. One car may be accurate, where another isn't. The only way to know is to do the math (or let an app do it) for your own vehicle.

dave1123

Yeah, back in the 50s and 60s, you could buy a little black plastic strip with 4 number wheels you could stick to your dash and set for mileage, then fill up your tank and do the math. Those number wheels were handy when keeping a logbook in a delivery truck. Now everybody has trip mileage settings on their dashes.

Funny story, I had a 61 Chevy Impala with a busted gas gauge. I used a number wheel strip and when it got to around 200 miles, I'd fill it up! I never ran out of gas!

These young cashiers nowadays can't figure change without their cash register.

jon-d

similar story, I had an old ford car that I picked up for $200 while in high school and the gas gauge did not work. I assumed a mileage and went from there. this may be taboo today, but back then when a car broke down you left it where you found it (auto dismantlers would take them) and pick up another $200 car somewhere. today however, the CHP will have it towed, auctioned, and if they don't make enough money off it they will send you a bill for the difference but if they make money of course they keep it. this is in Kalifornia.


what were we talking about??

dave1123

Talking about the accuracy of the overhead mileage display.

azdave35, I would check it by the calculation method and if it's close, just go by that. That's what I've done with mine.

Just for info, I've researched how the PCM figures mpg and it's interesting. It uses injector on-time compared to miles driven. That's a pretty accurate way of measuring fuel flow.

Fred/N0AZZ

In high school all we cared about is 3 of my buddies and I kicked in a dollar and could cruise all night long with fuel left over in my 56 Chevy 265 ci., but reg. was .25-.35 a gal depending on "Gas Wars" at the time lol.

carbuff

Ive wondered about this myself. Right now my 2008 3.7 is getting 13.0 mpg. I do around town driving but I thought Id at least get the 15 that was the low number advertised. I drive it like an old man too. LOL.

dave1123

I AM an old man, but drive "with gusto" in that I accelerate briskly enough from a stop to feel the shifts and like to "feel" curves in my seat. I don't speed except for the +5 everybody does. I tend to gear down on hills saving the brakes and confusing other people, lol! I still get about 16 mpg average with my 198K old 4.0. The 3.7 was designed to replace the 4.0, but it doesn't look good to me from what you're getting.

carbuff

Yeah, the 4.0 in my 2000 xj (204k) is a rocket compared to the 3.7 in the 2008. It seems that the 3.7 relies heavily on the 5 speed trans to keep it in its torque range for power. I guess the 355 gears in the xj help it out a bit too compared to the 307s in the wk.

dave1123

Yeah, I find that interesting. They say they did away with the 4.0 because it makes torque at too low an rpm to keep NOx emissions down and they needed to buzz up a short stoke engine to meet emissions standards, then they saddle it with long-legged gears. It makes no sense.

Did you know Federal emission standards are only in effect for speeds up to 55 mph? Over that, anything goes. That's how Lingenfelter got 625 hp out of a stock Corvette and still met emissions.

Then again they said the production equipment to produce the 4.0 was worn out so they couldn't make them anymore. BS! Chrysler sold the production dies and fixtures for the 426 Hemi to Keith Black and he's still making them. OR was it the 392? Anyway, where there's a market, they can produce!