New Shoes, Crappy Mileage

I too have Iridium spark plugs, and I went with the Cooper Discoverer S/T at 255/85/16, load range D.
Let me save you a little frustration. First, if you have the original tires and rims, take a measurement, and you may see that your original tires are a little bit smaller in diameter than the calculator predicted. Second, if you have the original tires and a GPS, you may see a little difference in the odometer miles and the GPS distance, where you actually traveled less miles than the odometer thinks you did.
Coopers run a little bigger than the calculator (at least mine did), and for me, going from worn BFG Rugged Trails at 265/70 to oversized 255/85 yielded about a 17% change in circumference. Now the odometer reads less than the GPS distance traveled.
After a few months at roughly 50 miles a day (mostly highway), I still fill up every six days, just like I did before, except the odometer reads high 200's instead of low 300's.
Oh, and I run at 50psi front & rear empty, 60 psi front & rear loaded, 65 psi max load, but the edges of the tires don't touch the ground at 65 psi even when fully loaded. I used the Firestone Ride Rites to level.

 

Did he say anything about the new tires being balanced? What about making sure alignment (toe/camber) are correct after the install? My wife's neon MPG took a dive when her alignment/balance was off after new tires...

 

Tires were balanced when I installed them and alignment was checked immediately afterwards.

 

I kept thinking about this on my drive home. The change in tire size just doesn't account for all the mileage loss. If the tire pressure is low it will feel like brakes when you let off the gas. Mine actually picks up speed going down slight hills.
I think I would pull the spark plugs and make sure that none are fouled. I'm guessing that it idles ok?
A bad oxygen sensor would give a low idle when at temp.
A dirty maf usually gives a high idle.
Bad bearings and dead gear oil get hot after driving, though the rear diff runs hotter than most.

 

Well, I figured it out, bad tank of gas. I filled it up with Shell 91 and a bottle of B12 Chemtool and headed home. I got 20.4 mpg driving up into the foothills. Guess it was a coincidence, I would have never guessed it was bad gas...funny thing is it ran fine. No rough idle, no misfires. Oh well, at least she's better now. Thanks for all the feedback, everyone.

 

Got a 4% difference in the tire diameter, so even if you still use exactly the same amount of fuel to get from point-a to point-b, your calculated MPG will drop by 4% due to the odometer error.

Add to that, the taller tires are hurting mileage during start and stop driving... they'll help on the highway if you keep to conservative speeds.

 

No adverse effect, but no benefit other than a longer change interval.

That is assuming the replacement plugs are the proper heat range. That can make a big difference if they are incorrect.
Seriously... in the long run, the plug cost is the same. You're going to spend $4/plug every 30k, or $16/plug every 100k.

 

I no longer have a heavy foot thankfully. I'm still finding 22 mpg in urban driving hard to believe.

 

Wow! Bet ya feel better now? I knew something was off.

 

wut?

All this over one bad fill-up?

 

Toyota says not to put that stuff in.

 

I also saw a drop in mileage going from P245/75-16 Dunlops to same size Michelin ltx ms2's. Ltx's are not only heavier but .4 inch wider tread. Dunlops at 35k miles had lost a full inch in diameter. Recalibrated ScanGauge by mile marker on a 100 mile trip, plan on recalibrating every 10k miles.

AFAIK, no vehicle has any programming (or way to) adjust for tire wear, so odometer is changing in accuracy as tires wear. Don't know if manufacturers set it to be on the money for new tires, or 1/2 worn tires or whatever.