MPG loss

[arrowheadracing]

Well I finally installed some parts. One part at a time , though I havent installed the most important parts yet ( EGT and Boost gauges ). Chip, polished fsd cooler with #9 resistor, boost controller set at minimum, Removed air box silencer, installed new fuel filter and air filter ( both stock items ), ported and polished the upper plenum ( removed around .200 of material , about 5 inches into the intake ) and polished the plenum area itself. Pictures available of all the porting all polishing. Will be offering polished, chromed and powder coated parts for 6.5's soon. Special discount to members ( had to get a plug in there ). My question is my loss of mpg. I was averaging about 17-18 mpg and now dropped down to 13-14. I did add 285 75 16 tires and noticed a drop in mpg from those, but figured my upgrades would bring me back to the same area hopefully. I havent checked the injectors and I get a loud knock noise ( least I think its loud ). Everything runs great, no problems, just lackluster mpg.

Any thoughts?

Thanks
Todd

PS picturs avaiable of the parts, if I figure out how to put them in the message ( hint if anyone wants to give a quick tutorial on how to do it ) I ll post them up.

[arrowheadracing]

Down a few threads about the ac compressor/defrost surgine, I live in Michigan. So I constantly have the defrost on. Not so in the summer, as I dont really like ac, rather have the wind in my hair ( must be the motorcycle thing ). Could this be another reason I see a drop in my mpg. They said it could cost 2-3 mpg on the highway. I run alot of 50-60 mph speeds with the defrost on, and I have noticed the surging will at idle waiting for my son to get out of school.

Hmmm ..... ideas, I love this page.

Thanks
Todd

[slagona]

A couple of things come to mind:

First, winter blend fuel. I lose 1-2 mpg during the winter blend fuel months. I typically get 17-18 mpg in my everyday summer driving - then 15-16 mpg in my winter driving.

Second: Did you adjust your VSSB for your larger sized tires? Larger tires mean the appearance of slower speeds and lower miles than actually traveled - thus another margin of error in your MPG calculation - probably in the area of 3-5% or 1 mpg.

--Scott

[arrowheadracing]

I havent adjusted the vssb yet. I did a quick calculation and figured my speedo to be off about 4-5 mph. Running on the highway one day I looked over and saw a caddy with digital speedo. So I got up next to him and stayed even, his speedo was 76 mph , mine was 71. With errors on his or mine, I figured it was pretty close to my math figure of 4-5. I just keep in mind that my speedo is off, so I usually stay around 55-60 mph ( a true 60-65 mph ). Normally I just try to keep the rpms in the 1800-2200 range. Seems I get my best mpg there.

Thanks
Todd

[rjschoolcraft]

Keep in mind that your speedo error with larger tires is a slope error, not an offset error. In other words, your 4-5 mph error is that at a particular speed...at a higher speed, the error is more, at a lower speed the error is less.

[moondoggie]

Good Day!

rpm = (mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (tire dia in inches x π) X (driveshaft rev/tire rev [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio])

@ 60 mph w/ standard tires (assumes 245/75R16), 4L80E AT, 4.10 gears:

rpm @ 60mph = (60 mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (30.47 inches x π) X (4.10 driveshaft rev/tire rev [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio])
rpm @ 60mph ≈ 2035

@ 60 mph w/ 285/75R16 tires, 4L80E AT, 4.10 gears:

rpm @ 60mph = (60 mph) X (1 hour/60 minutes) x (63,360 inches/mile) / (32.83 inches x π) X (4.10 driveshaft rev/tire rev [axle ratio]) X (0.75 engine rev/driveshaft rev [AT OD ratio])
rpm @ 60mph ≈ 1889

Because you haven

[DennisG01]

Hey arrowhead,

Just a heads up - don't know if it's a problem on my end or not - but I clicked on your website link in your post and I got a "website not found" message.

ON EDIT: *** Nevermind - just saw your answer in another one of your posts ***