Computer and Fuel Mileage

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shv2sail
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Computer and Fuel Mileage

Postby shv2sail » Tue Mar 08, 2011 1:44 pm

There was some discussion on here regarding the spedo reading "high" by a few mph with stock tires on the truck. I noticed this too as confirmed by my GPS unit. As far as mileage (per the trip computer) I was getting about 20 on the highway on the stock tires. Now, since I've replaced the stock tires with larger 265/75's ATs, I noticed a significant mileage drop (some was to be expected) per the onboard computer.

The interesting thing IMHO is that the spedo now reads EXACTLY correct per the GPS unit. I'm guessing that the fuel mileage, DTE, etc computer is basing it's calculations off of the same raw data being fed to the spedo. So my question is: was the original fuel mileage being calculated by the truck incorrect from the factory since the spedo was incorrect as well? Is the spedo/trip calc/etc setup for larger tires than the stock size, creating an incorrect reading on stock tires?? I've done some paper calculations over the last couple of fuel fill-ups, and it does in fact look like the the compter's current (MUCH lower) calculations are in fact correct.

Anyone else notice this??


serturbo
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Postby serturbo » Tue Mar 08, 2011 5:04 pm

My speedo isn't that far off. It only reads about 1 mph higher than GPS at 60 mph.

Npath
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Postby Npath » Tue Mar 08, 2011 6:14 pm

I have new A/T's mounted back in December and I too have noticed a significant drop in mileage.

Fuel quality (winter fuel) also plays a roll in poor mileage.

In regards to speedo, when I drive side by side with a car, their speedo pretty much matches mine.

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08Datsun
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Postby 08Datsun » Tue Mar 08, 2011 8:08 pm

With the stock 245's, the speedo read about 72 when the GPS read 70. Highway mileage was typically around 21 per the readout.

With 265's, the speed and the GPS are exactly the same, and highway mileage is now consistently about 20.

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shv2sail
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Postby shv2sail » Tue Mar 08, 2011 8:21 pm

Spedo to GPS on the stock 245's was consistently about 3 mph off at highway speeds (70 on spedo was 67 on GPS)...now is dead on with the 265's. Mileage went from 20-21 to 15-16 on the onboard computer.

Sheesh...08Datusn...20mpg highway on the 265's - I'm lucky if I get 16 since the tire change! It looks like we have the exact same truck...how many miles on your 08? I'm at about 86k...

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NVSteve
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Re: Computer and Fuel Mileage

Postby NVSteve » Wed Mar 09, 2011 9:05 am

shv2sail wrote:There was some discussion on here regarding the spedo reading "high" by a few mph with stock tires on the truck. I noticed this too as confirmed by my GPS unit. As far as mileage (per the trip computer) I was getting about 20 on the highway on the stock tires. Now, since I've replaced the stock tires with larger 265/75's ATs, I noticed a significant mileage drop (some was to be expected) per the onboard computer.
I've been pretty vocal on the forum whenever this topic comes up in a thread. Yes, the on board cpu is basing all calculations on a 265/75/16 (for those with 16" wheels) tire. So, if you run a smaller size, everything will be off in the positive direction, meaning that mpg numbers will look better than they are in reality. However, I can attest the winter crap blend of gas has been killing my mpg this winter. I don't know if they use this special blend country wide, or if only select states.
So my question is: was the original fuel mileage being calculated by the truck incorrect from the factory since the spedo was incorrect as well? Is the spedo/trip calc/etc setup for larger tires than the stock size, creating an incorrect reading on stock tires??
Yes & yes. I've asked various technicians more than once if the CPU can be reprogrammed for another tire size (in case I go larger), but the answer has always been a resounding "no."

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08Datsun
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Postby 08Datsun » Wed Mar 09, 2011 12:25 pm

shv2sail wrote:Spedo to GPS on the stock 245's was consistently about 3 mph off at highway speeds (70 on spedo was 67 on GPS)...now is dead on with the 265's. Mileage went from 20-21 to 15-16 on the onboard computer.

Sheesh...08Datusn...20mpg highway on the 265's - I'm lucky if I get 16 since the tire change! It looks like we have the exact same truck...how many miles on your 08? I'm at about 86k...
Mine has 47k. We've been in the 19s on road trips, but no lower. City mileage is 15. Like Steve says, it may be the winter and the different blends of gas. Out here, our temps vary very little.

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shv2sail
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Postby shv2sail » Wed Mar 09, 2011 6:34 pm

Thanks NVSteve...now I know - I suppose I should have just done a search...I've been hanging out here long enough :roll:

...and I agree, I think some of my current horrendous mileage is due to the really bad gas here too. We'll see if this changes any as we move toward summer.

Thanks all for your responses!!

Taavi
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Postby Taavi » Thu Mar 10, 2011 6:51 am

I have noticed that driving with 4wd(auto) or 4wd(Hi) it takes 1-2L/100km more than driving with 2wd. In winter with mostly 4wd(auto) it takes about 15-15,5L/100km and in summer mostly 2wd it takes about 13,5-14L/100km.

Tires: 265/65R17


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