My MPG gauge is very accurate

Post by **matthew1** » 12 Jul 2009, 22:53

I've done two full tanks comparing mileage from trip odometer/math vs. the MPG computer. It's accurate to within a decimal place.

Post by **VCA** » 13 Jul 2009, 05:33

matthew1 wrote:I've done two full tanks comparing mileage from trip odometer/math vs. the MPG computer. It's accurate to within a decimal place.

...and it will be, unless your fuel level sender goes ape, thinks "empty" occurs when you have 3 gallons left and starts throwing trouble codes

Post by **vegasjetskier** » 13 Jul 2009, 06:14

matthew1 wrote:I've done two full tanks comparing mileage from trip odometer/math vs. the MPG computer. It's accurate to within a decimal place.

So is mine.

Post by **tjts1** » 13 Jul 2009, 10:52

Yup mine is also accurate. I pulled it at the JY from a 97 automatic and installed it in our 96 NA 5 speed. Its always within .2mpg of calculate mileage at the pump. I used to compare the two at every fill up but now I'm confident in the number the trip computer spits out.

VCA wrote:
matthew1 wrote:I've done two full tanks comparing mileage from trip odometer/math vs. the MPG computer. It's accurate to within a decimal place.
...and it will be, unless your fuel level sender goes ape, thinks "empty" occurs when you have 3 gallons left and starts throwing trouble codes

No. The trip computer doesn't use the fuel level sender to calculate MPG. It uses injector pulse width vs speed. The car was designed to show empty at 3 gallons. When the tank is that low, you can starve the fuel pump in sharp turns. Its a safety feature. The reason fuel pumps die is because owners repeatedly keep driving after the low fuel light comes on.

Post by **VCA** » 13 Jul 2009, 19:23

tjts1 wrote:No. The trip computer doesn't use the fuel level sender to calculate MPG. It uses injector pulse width vs speed. The car was designed to show empty at 3 gallons. When the tank is that low, you can starve the fuel pump in sharp turns. Its a safety feature. The reason fuel pumps die is because owners repeatedly keep driving after the low fuel light comes on.

Might be true on the 850s, but on the networked cars (post-99), the long-term fuel economy is reconciled with the fuel level difference over the last 100km. Otherwise, ECM-4050/DIM-0002 gets thrown and the trip computer (I assume) errs on the side of the lower figure. I know this because of the 6 months of throwing that code on my car and being 2+ MPG off (using the same gas pump and computing mileage between tanks). Replacing the sender made the figure agree with the computed economy

. Oddly, the ECM is capable of throwing that code even if you don't have trip computer software installed.

instarx · Post by **instarx** » 14 Jul 2009, 16:15

Very timely post matthew. Just this morning I finished a 1,013 mile round trip. Except for 3 miles at either end it was exclusively interstate. Odometer MPG read 30.2, calculated mileage was 29.8. That's a difference of a little over 0.1%.

Around town however, the odometer usually under-estimates actual mpg by about 5-7%.

Incidently, last year I made the exact same trip but with a small front air dam. Measured mileage was 32.8. Apparently the air dam adds about 3 mpg at 65 mph. That seems high to me, but data are data.

Post by **Ozark Lee** » 14 Jul 2009, 20:27

Apparently the air dam adds about 3 mpg at 65 mph

Funny you should mention that. The best mileage I ever got on my '94 NA was when I bought the car and they had disconnected the rear spoiler and thrown it in the trunk due to a broken mount. Adding the rear spoiler back actually cost me about 1.5 to 2 mpg in terms of mileage with a lot of highway miles. We have a 65 MPH speed limit on the state highways but the highway patrol officers don't even blink until you are 10 over so I run 74 MPH on cruise without a lot of fear except for the speed trap towns that I know about.

I fixed the mounts on the spoiler and - at least for now - I am leaving it because the spoiler along with the Columba wheels makes the car look very sporty for a cruddy old 224,987 mile N/A that runs perfectly.

Do I get a grill badge for 225k?

...Lee

bob850 · Post by **bob850** » 30 Oct 2013, 12:04

tjts1 wrote:Yup mine is also accurate. I pulled it at the JY from a 97 automatic and installed it in our 96 NA 5 speed. Its always within .2mpg of calculate mileage at the pump. I used to compare the two at every fill up but now I'm confident in the number the trip computer spits out.

VCA wrote:
matthew1 wrote:I've done two full tanks comparing mileage from trip odometer/math vs. the MPG computer. It's accurate to within a decimal place.
...and it will be, unless your fuel level sender goes ape, thinks "empty" occurs when you have 3 gallons left and starts throwing trouble codes
No. The trip computer doesn't use the fuel level sender to calculate MPG. It uses injector pulse width vs speed. The car was designed to show empty at 3 gallons. When the tank is that low, you can starve the fuel pump in sharp turns. Its a safety feature. The reason fuel pumps die is because owners repeatedly keep driving after the low fuel light comes on.

A number of years ago, an indy mechanic who did a lot of work on Volvos, and drove an 850 himself, told me that I should fill up my 850 when I get to 1/4 tank, especially in summer. He said the "surrounding" fuel helps keep the pump cool and prolong its life. At the time, he had his car in the bay to replace his own original fuel pump, which had just failed at 190,000 miles. I'm not sure how accurate his advice was, but I tend to fill up shortly after I hit the 1/4 tank mark. Still on original fuel pump at 130K.

Post by **abscate** » 30 Oct 2013, 12:43

3 mpg for an air dam seems really high. Same fuel, temperature. warp core properly aligned?

Ive done some nasty Boston trips in the T5 where Ive ended up with 27 mpg instead of my 29 that I like to see - the good news is the AC hasn't hurt me when needed. Town driving Im doing 24-25 mpg.

I got 13 mpg last trip, but I had 2000# of boat behind me....that gas doesn't count.

Post by **osman** » 30 Oct 2013, 12:57

We have the same car and thats about what I get, I just put on oem roof racks I wonder if that will effect my MPG's noticeably? I also have the rear spoiler on my 99 V70 T5.