most of my roads traveled are 55 or 65 mph speed limits. i check my speed all the time with those roadside speed checkers the county or local police put up, and at 65mph on the speedo the checker thing shows 66, and 75 shows 77. so the tires didnt throw off the speedo to bad,that was my first concern. calculating my milage i top off the tank and divide gallons into miles driven. the reason i wait till the light comes on is to run a full tank of the same fuel.(thats my OCD i think,lol) at idle i can almost stop the truck from running every time the bass hits if i crank it up, the rpm's go from 500 down to 250 then stumble back up to 500. it think its time for a new battery and start using a 1farad cap, i got 2 of them laying around, im just lazy i guess.

 

Rule of thumb is 1 farad per thousand. We have a cap too, from the system we had in camaro.

That's a pretty big amp-draw effect.

 

I mean, it seems like you did your calcs correctly. I think your tires threw it off just a tad, but still those are really good numbers for an AWD QC w/ 193k miles on it. Honestly I would have guessed you were getting 14 or so with that setup.

 

i will agree the tires added some mpg, when winter comes i usally got 14 to mid 15's. i let my truck idle alot during the day while im working, so i will prob drop to 10. oh well, it is what it is

 

It gets a little chilly up there in WI doesn't it?

 

not as cold as some places, but damn, it gets cold enough. teens and 20's with snow. sometimes below zero. i do alot of work in parking lots of dealerships, so i let my truck run so i can hop in and warm my a** up.

 

I did a highway trip today. Don't know exact gallons yet (I'll get that tomorrow morning), but...

Overhead readout said 222.7 miles traveled. I have about a 300 mile range per tank (around 16.2 general average).

Per "My Tracks":
91.20 miles (starting at 10:25am) (max speed: 81.65 mph) (travel time: 1h28m)
42.25 miles (starting at 1:17pm) (max speed: 69.35 mph) (travel time: 58m)
42.28 milles (starting at 2:16pm) (max speed: 69.35 mph) (travel time: 57m)
102.41 miles (starting at 7:12pm) (max speed: 88.92 mph) (travel time: 1h33m)
--------
278.14 miles.

278.14 - 222.7 = 55.44 mile difference between GPS and onboard computer.

55.44/278.14 = 19.93% error

265/70-16 = 96.15" per rotation

.1993 * 96.15" = 19.16" difference of circumference = 6 inch difference in height.

damn, my onboard computer is all kinds of screwed up. Because i dont think the GPS could be all that screwed up..

 

Really? I thought it was colder than that in WI. I gets below zero quite often out here.

 

So I filled up first thing this morning (3 blocks away). 13 gallons on the dot. By the GPS, I got 280 miles divided by 13 gallons, 21.54 miles per gallon. By on board data, I got 224.3 miles over 13 gallons, 17.23 MPG.

The window sheet for my truck shows 16 MPG city and 20 highway. Also shows a 136 amp alternator and 255/65-16 tires (Sport Plus Group). So ASSUMING the speedometer is calibrated to that size tire (29.05" vs my 30.60"), I should only be off by 6%. I went by a police trap, and it showed me as going 55 when speedometer said 50, which is right around 6%. So I know that's the only error I have in the speedometer. I assume the odometer would have that same 6% error, which leaves me looking for the other 14% of loss....

 

I did a mixed MPG run using the trip odometer this week. I got 204 miles out of 14.3 gallons of gas, for a mixed MPG of 14.26 MPG. The tank lasted from 9-13 to 9-22, I filled up when the fuel light came on. That was with OD in normal mode.