Computer not computing?

[Moe_Fugga]

Less than 1000 miles on my JT and I’ve noticed a couple issues.

1. Computer showed 19.3 average MPG on last tank at fill up. I calculated it by hand at 18.4. Never had a vehicle be that far off. Not a huge issue, but an issue nonetheless.

2. I never though anything of it until I calculated the MPG, but my TPMS shows 40 PSI in each tire when two separate handheld gauges show 36 PSI in each tire. It has since I brought the JT home. Again, not a huge issue, but still an issue.

Maybe I have something wrong with my computer? Has me thinking if my speed and everything else are even correct. My JT is bone stock minus a bed cover and small stuff that won’t affect MPG or computer system. Anyone have any thoughts or similar issues? Thanks in advance friends!!

Sponsored

 

[Renegade]

Margin of error. Buy 3 different brand digital tire gauges and they will all differ. I have one which I know reads 2 psi high. Speedometers may be up to 5% off in all vehicles, which is why most of them read a couple of mph high. The only ones which are certified accurate are in police vehicles.

 

[TheITGuy]

Less than 1000 miles on my JT and I’ve noticed a couple issues.

1. Computer showed 19.3 average MPG on last tank at fill up. I calculated it by hand at 18.4. Never had a vehicle be that far off. Not a huge issue, but an issue nonetheless.

2. I never though anything of it until I calculated the MPG, but my TPMS shows 40 PSI in each tire when two separate handheld gauges show 36 PSI in each tire. It has since I brought the JT home. Again, not a huge issue, but still an issue.

Maybe I have something wrong with my computer? Has me thinking if my speed and everything else are even correct. My JT is bone stock minus a bed cover and small stuff that won’t affect MPG or computer system. Anyone have any thoughts or similar issues? Thanks in advance friends!!

I wouldn't put too much stock in the MPG calculated by the on-board computer. Never owned a car with this feature that was close to accurate.

 

[relapsekg]

I have noticed that the more miles you drive, the more accurate it becomes.
I am at just under 10,000 and it is within half a mpg.

 

[jalbrecht55]

I think there’s a few things at play here. Odometers should be +/- 4% by NIST handbook

Handbook 44 ‘Specifications, Tolerances, and other Technical Requirements for Weighing and Measuring Devices' ” (the NIST Handbook).  (NIST Handbook, § 5.53, A.1, T.1, T.2 (2008)

Speedometers seem to have a recommended tol of +/- 4% by SAE J1226 or +10/-0% by UN ECE Regulation 39

But even if the odometer is dead nuts perfect you’ve got the errors associated with the way the ECM records and calculates the mileage. On my own JGC the odometer seems to be right on by the GPS tests I’ve done but the reported mileage ranges from 5-7% high than I hand calculate. Adding to this confusion I notice that it is biased towards the most recent miles driven.

Example on trip B I have 23,000 miles since reset (well 3000 because if only shows 4 digits) and the mileage number will still swing up or down by 2-3mpg per tank if I go from city to highway driving even though I know for fact it’s not changing the 23,000 mile average by that much.

 

[MarineHawk]

My odometer and speedometer both are dead on as compared to highway mile-markers and my Waze navi-computer. This is true after going up to 35s and having the dealer recalibrate; and also was true before I did that. My trip computers usually read about 0.5 mpg higher than reality--compared to when I hand-calculate it myself.

 

[mr_bots]

Are you resetting the trip computer each fill up?

Also, the TPMS sensors aren't live data. They refresh when they sense a pressure change or are spinning and send it every 30 seconds (I believe). If the vehicle is stationary they'll go to sleep.

 

[Moe_Fugga]

Are you resetting the trip computer each fill up?

Also, the TPMS sensors aren't live data. They refresh when they sense a pressure change or are spinning and send it every 30 seconds (I believe). If the vehicle is stationary they'll go to sleep.

I reset trip meter each time. I checked the TPMS on the road right after checking them with handheld and they were 3-4 PSI higher. They didn’t have time to get enough heat in them to raise PSI.

Sponsored