Question om overhead computer.

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Three things happened all at once and I'm tring to sort out what I see.

I installed a trnasfer flow fuel tank. I went from 215 tires to 245's and it got cold in the northeast, 60+ degrees to in the 40's.

My overhead computer says tht my milage went from 18MPG to 14MPG. I am not sure where it gets it's information from. I know the distance to empty will be no good because of the transfer flow. I expected the tires to make milage go up. The cold should make it go down. I didn't think that much. Comsumption seems about the same.

I don't know where that computer gets it's fuel flow info from. Does anyone have an idea? Miles probably comes from the Anti Lock sensors.

 

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I don't know which tire sizes, so I just assumed you went from 215/75R/16 to 245/75R/16. If that is not right, the numbers will change a little, but based on that:
Your speedometer will read "low" on miles travelled; that would take you from 18 to 15. 8 mpg, if you went the same speed.
But your speedometer will also read "low" on speed. If your speedo says 70, you are now really going 77 mph. That will drop your mpg from 15. 8 down to 14+ in a heartbeat.
The trip computer only estimates mpg based on throttle demand, but it's accuracy is very poor after any change to the stock set-up. Most of your difference probably comes from travelling further, + driving faster, without even realizing you are doing it. Slow down so you don't get a ticket, use the mile-markers, figure exactly how much to add to your mileage in %, then do your mpg calculations manually with this correction factor. You'll probably be happy again and like your bigger tires to boot.

 

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Did you ever check mileage the old fashioned way, i. e. miles traveled / gallons used before the changes? The overhead computer is a "nice toy" kind of gives you a rough idea of whats happening, but is no way accurate enough, (IMHO) to be used to figure mileage, distance traveled etc.
. If you had figured your mileage before, you could do it now and see what really changes, of course with the new tires you need to make an adjustment, (old tire hieght vs new tire hieght), I think I remember somewhere on old posts either a chart, or a calculation you can use to make the adjustment.

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1999 3500 quad cab, auto aero fendor trim and running boards,
bugflector II, K&N filter, Geno's monster tip