MPG Computer Not Accurate?

[Old 5 Oh]

I have had several cars with MPG computers, and I have to say none has matched the results of my hand calculations, and all have been a little optimistic. Of course. But the computer in my Mustang is worse that the rest. It consistently shows 1.5 mpg more than it's really getting, sometimes 1.6 or 1.7 mpg. That seems like quite a lot and I am just curious if that matches anybody else's results.

It IS kinda fun, now that it's broken in, to see the MPG readout at 31 mpg for a freeway tank, in a 435-hp GT. Heck, my '78 Ford Fiesta wouldn't always get that much mileage on the freeway, especially when the carb was out of adjustment (which was most of the time in the dreary, pre-engine-computer days of the late '70s.)

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[ssyellow]

I was going to comment on this. My MPG calculator is usually correct, but I feel the overall trip/miles left in the tank calculation is off personally. We will see as I put more miles on it.

 

[Qwkynuf]

Mine ranges from a little less than 1mpg off up to maybe 1.5.

I drive mine pretty gentle most of the time, but I am sure not seeing anything like 28 hand-calculated mpg.

I track every fill up with an app on my iPad (for business mileage reimbursement) and am averaging 22.1 over 8400 miles. For 60mph freeway runs, I can get 25 for a tank. At 70 it is more like 22.5-23. I have seen peak instantaneous numbers in the 28 range, but never for a tank.

That being said, I have the 3.55 gears, so that is probably a factor.

 

[P4RKER]

I have had several cars with MPG computers, and I have to say none has matched the results of my hand calculations, and all have been a little optimistic. Of course. But the computer in my Mustang is worse that the rest. It consistently shows 1.5 mpg more than it's really getting, sometimes 1.6 or 1.7 mpg. That seems like quite a lot and I am just curious if that matches anybody else's results.

It IS kinda fun, now that it's broken in, to see the MPG readout at 31 mpg for a freeway tank, in a 435-hp GT. Heck, my '78 Ford Fiesta wouldn't always get that much mileage on the freeway, especially when the carb was out of adjustment (which was most of the time in the dreary, pre-engine-computer days of the late '70s.)

Sounds like there is something up with you computer. Mine is pretty dead on with hand calcs at between 18-19 mpg with 95% highway driving (I have a lot of fun)

 

[Todd15Fastback]

Sounds like there is something up with you computer. Mine is pretty dead on with hand calcs at between 18-19 mpg with 95% highway driving (I have a lot of fun)

Same here. .5MPG over the first 2700 miles. I have calculated it about 5-6 different times. I don't do it any more as it makes no difference for me.

 

[Stangnut]

Mine ranges from a little less than 1mpg off up to maybe 1.5.

I drive mine pretty gentle most of the time, but I am sure not seeing anything like 28 hand-calculated mpg.

I track every fill up with an app on my iPad (for business mileage reimbursement) and am averaging 22.1 over 8400 miles. For 60mph freeway runs, I can get 25 for a tank. At 70 it is more like 22.5-23. I have seen peak instantaneous numbers in the 28 range, but never for a tank.

That being said, I have the 3.55 gears, so that is probably a factor.

That sounds spot on to me. Advertised is 25 for 3.31 gears. You lose about 2 mpg per gear change under normal circumstances.

 

[CooterK]

That sounds spot on to me. Advertised is 25 for 3.31 gears. You lose about 2 mpg per gear change under normal circumstances.

That doesn't sound right at all. I can pull 25 freeway no problem at 70 mph. City I struggle to get 14 though.

 

[P4RKER]

That doesn't sound right at all. I can pull 25 freeway no problem at 70 mph. City I struggle to get 14 though.

You are pulling 25s with the 3.73 at 70?? I have seen times when it hits 25 but that's going down hill with cruise control on And only for a second. I have never seen a consistent 25 in my GTPP except around 55-60.

 

[thePill]

Watch your tire pressure as the temperatures warm up and eventually it gets hot. Improper inflation can occur just by the cooling and heating in the Winter. Most tires are a little low and will affect both real world and MPG on the tach. Even just a few pounds of air can have a noticeable impact on that number.

This can occur even if it wasn't driven much. I would just check on your first Spring detail. It isn't nothing to run out and check right at this hour. Just FYI...

 

[kn7671]

:lol: He said '78 Ford Fiesta... :lol:

These cars can very easily get good mileage when driven nicely, and very low mileage when driven hard.

That aside, to the subject of this thread, the mileage will be off since all manufacturers have to account for the diameter of a brand new tire to a nearly worn out tire. A worn out tire could reduce the overall diameter by as much as 2/3rd's of an inch.

 

[RubyRed15GT]

My 2012 V6 fuel computer was usually .5 mpg under the hand calculated mpg. With 16 fill ups and 3000 miles on my 2015 GT the overall accuracy of the fuel computer is .7 mpg more than the hand calculated. Does anyone know what data the computer uses for fuel consumption? I am assuming the mileage number computer uses is the same as my hand calculation.

 

[GPhilly04]

Mine ranges from a little less than 1mpg off up to maybe 1.5.

I drive mine pretty gentle most of the time, but I am sure not seeing anything like 28 hand-calculated mpg.

I track every fill up with an app on my iPad (for business mileage reimbursement) and am averaging 22.1 over 8400 miles. For 60mph freeway runs, I can get 25 for a tank. At 70 it is more like 22.5-23. I have seen peak instantaneous numbers in the 28 range, but never for a tank.

That being said, I have the 3.55 gears, so that is probably a factor.

What app do you have? I track all mine on a notepad I shove in the glovebox. This app sounds like a lifesaver!

 

[Kiwi]

Why even worry about miles per gallon. Gas is cheap enjoy, drive your Mustang and turn off the miles per gallon read out.

 

[Qwkynuf]

What app do you have? I track all mine on a notepad I shove in the glovebox. This app sounds like a lifesaver!

I am using one called Gas Manager

https://itunes.apple.com/us/app/gas-manager-fuel-consumption/id448662326?mt=8

 

[113]

do 3.73 gears all that dramatically decrease fuel economy? I commute 100 miles/day so even a drop of 2-3mpg compared to 3.31 or 3.55 would really make me reconsider buying the PP.

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