Why your trip computer isn't accurate

[timf]

Now that the weather has started to warm up, I've been able to better observe the under calculation of fuel used by my MKZ Hybrid. I haven't used remote start in over a month. For the first tank of gas while it was still relatively cold, the car was about 1 gallon lower than I added at the pump. The most recent tank has seen a greater number of 50+ degree days, and the car was only about half a gallon off from what I added at the pump. At least I finally got a tank of over 40 MPG on Fuelly, while the car has been estimating around 44 MPG.

[acdii]

Over time I noticed the difference gets smaller, though it will never be accurate.

[Fastronaut]

On a recent round trip to Florida from Maryland, I noticed that several times the Trip Odometer Fuel Used calculation used was up to 1.5 gallons less than I actually pumped into the tank. No real pushing to multiple clicks.

[acdii]

Last night, first click off, 10.88, trip read 10.83, .05 difference. I never top off either so thats where it was.

 

However, as I drove off, I looked at the gas gauge and it was 1/8th below full. Confusing for certain, clicks off full, yet isn't according to the gauge.

[jsolan]

Last tank was the only time I've ever had the trip computer be higher than what I actually filled up with. The trip computer said I used 11.05 and after 2 clicks I only put in 10.9. Normally I put in close to a gallon extra without any extra clicks. Different pumps I guess...

[acdii]

I filled at a different station last night too, and now that I think about it, these pumps fill faster than the one I usually fill at and completely forgot about that, so I did not get a full tank.

[Fastronaut]

Last night, first click off, 10.88, trip read 10.83, .05 difference. I never top off either so thats where it was.

 

However, as I drove off, I looked at the gas gauge and it was 1/8th below full. Confusing for certain, clicks off full, yet isn't according to the gauge.

I've noticed that the innacuracy grows as you maintain higher highway speeds.

[ptjones]

There seems to be a correlation between getting better gas mileage and trip gauge and actual gallonage being closer to the same. I just got my first 700 mile tank this year and it was only .2th's off. All of my high mileage runs also within about .2th's. It is important that the car is level when fueling. :)

 

Paul

[acdii]

Well I am wondering if the switch to summer blend happened, this last tank is at 45 and I am starting to see 47 MPG where previously I was lucky to get 42. This morning was my best commute ever at 47.5. Supposed to hit 80 today so I pulled my lower stuffing out of the grill. Keeping the main cover on though since it is only going to be warm today and drop back to lower temps next week. I need to get MyGirl cleaned up she needs it badly, and looks like I just might be able to do it this weekend.

[corncobs]

I think you are right about the summer blend I also see higher numbers consistently. No commute for me today but I had a 50.5 result yesterday.

 

I agree on the cleaning up there have been a few light rain showers and she looks really dirty now.

[acdii]

48.8 on my way home, Really liking it

[hybridbear]

I only buy gas once a month so I'm hoping that when I fill up in a couple days that I'll get summer blend to improve fuel economy.

[murphy]

I last bought gas before Christmas, 265 miles from home. I still have a quarter tank.

;)

[tr7driver]

I last bought gas before Christmas, 265 miles from home. I still have a quarter tank.

;)

One of the issues of driving the hybrid is the dirty windshield. I have stopped at gas stations to wash it without buying gas. I'm sure it drives the folks inside crazy as they watch to make sure I'm not doing a pump and run.

[murphy]

One of the issues of driving the hybrid is the dirty windshield. I have stopped at gas stations to wash it without buying gas. I'm sure it drives the folks inside crazy as they watch to make sure I'm not doing a pump and run.

You do realize that if you pull the windshield wiper stalk toward you the car will wash the windshield for you.

:happy feet:

[tr7driver]

You do realize that if you pull the windshield wiper stalk toward you the car will wash the windshield for you.

:happy feet:

Wow! who would have thunk it?.

 

Now if only the wipers could get the bug carcasses off as well as the nylon sponge squeegees do.

[acdii]

That just bugs me.

[Texasota]

We just got back from a 1320 mile trip to the Michigan Upper Peninsula and our new FFH had ~250 miles on it when we began the trip. The trip computer reported 48.4 MPG for the entire trip and the hand calculation for the entire trip resulted in 45.9 MPG (2.5 MPG difference) which is approximately 5.4% inflation by the trip computer.

 

Will the MPG reported by the trip computer become more accurate over time?

Edited September 7, 2014 by Texasota

[GrySql]

We just got back from a 1320 mile trip to the Michigan Upper Peninsula and our new FFH had ~250 miles on it when we began the trip. The trip computer reported 48.4 MPG for the entire trip and the hand calculation for the entire trip resulted in 45.9 MPH (2.5 MPG difference) which is approximately 5.4% inflation by the trip computer.

 

Will the MPG reported by the trip computer become more accurate over time?

It might, but will never be as accurate as doing the math yourself. Almost every post regarding the TC shows it as optimistic, and YMMV.

 

How was the ride? Better than the Focus I hope.

[Texasota]

It might, but will never be as accurate as doing the math yourself. Almost every post regarding the TC shows it as optimistic, and YMMV.

 

How was the ride? Better than the Focus I hope.

 

I am very happy with it so far. A much nicer ride, much quieter, more room, and significantly better MPG than the Focus. So far I have not found anything that requires a visit to the service department. I think it might be a keeper.

[hybridbear]

We just got back from a 1320 mile trip to the Michigan Upper Peninsula and our new FFH had ~250 miles on it when we began the trip. The trip computer reported 48.4 MPG for the entire trip and the hand calculation for the entire trip resulted in 45.9 MPG (2.5 MPG difference) which is approximately 5.4% inflation by the trip computer.

 

Will the MPG reported by the trip computer become more accurate over time?

One thing we've found is that the odometer doesn't count enough miles. The odometer seems to be off by 2%. If you add 2 miles per hundred driven and then compare it to the actual gas used the results are very close. It seems that the car does slightly underreport gas used, but that seems to be very common. Most Prius owners find that their car is about 8-10% optimistic. That's what we found in our time with the Prius. That's much worse than in the FFH.

[Bob Cooke]

Optimistically confused. Just did my first fill up, so this would be the 2nd tank stealer filled first one. Now mind you it's been very cold here in Chicago.

I did remote start the car both morning and evening. So I know this is some of the culprit.

My numbers as recorded on the trip 344.7 158.8EV 42.6MPG low fuel light on @50. I put 10.907 Gals in. Now for the NEW MATH. If I assume correctly

344.7/10.907=31.609 this is nowhere near 42.6. 344.7-158.8EV miles =185.9 Actual gas miles driven which even worse 185.9/10.907=17.047MPG.

So can someone with deep knowledge of new math point me in the right direction? I traded in a 04 Buick Lesabre that was getting low 20's even with

remote starting every day so this is not exactly blowing smoke up my skirt. Nearly every trip I took showed nothing lower then 38 and no higher the 54.

Am I missing something? I love my new car but she needs to do better.

[corncobs]

I would try without remote start and check with next tank.

 

Over the last three month my tank avg. (calculated) has been 38.64 MPG. Considering how cold it has been not unhappy with that number.

[Cobra348]

In theory? or do you have documentation that says that. I ask because I have been unable to find absolute documentation of that or verification of that from the dealership. I think this is a huge issue and I would love to find some definitive info ..... :)

Logic dictates it ... if you remote start any vehicle, it is not moving at the time ... thus no TRIP computer registration. However because the engine is running, MPG of any computed type is likely to drop by some portion the longer it sits idling.

[ptjones]

Bottom Line Remote Start and Heater Kill MPG's, in a month or two you will be surprised how much our MPG's improve. :)

 

Paul