I'd have to go with that.... when our friend got the tires for us they were 8
ply because we tow the trailer... they said "max pressure 65 psi" so I ran them
When we took it back in for rotation, he freaked and lowered it to 35 front and
40 rear... said to go to 45/55 when towing..
In every single case I have ever put larger tires on the result was
lower mpg. While the effective ratio should improve it, the engine has
to work harder to turn the larger tires. I've yet to see a case where
it improved mpg.
Miles makes an excellent case here. Some years ago we had a great e-mail
list, it was called the RTML, or Ram truck mailing list. Some current
newsgroup members were on it. One of the members ran an experiment with his
overdrive. He ran back to back tanks of gas, one using overdrive as normal,
one with it locked out. He got one MPG less with it locked out. Overdrive
on a Dodge is .69, about a 30% reduction in RPM for given road speed and
MUCH more than you'll get with a larger tire.
One of the other caveats is rolling resistance, one of the major
considerations of the factory selection is rolling resistance, and most
aftermarket tires don't give this much thought. I've seen people lose two
MPG with aggressive off road tread tires. If you use a larger diameter and
usually wider tire, you may gain some rolling resistance.
Lastly I've seen discussions of axle ratios and fuel economy, too low a
ratio and fuel economy suffers, too high a ratio and it suffers again. The
discussion I'm referencing took place before automatic overdrives were
offered. The synopsis at that time was about a 2.9 final drive was about
optimal for fuel economy. This obviously was also well before fuel
injection was commonly available. But consider this, most half ton trucks
today come with 3.42 to 3.55 ratios standard and automatic overdrives offer
a 30% reduction of RPM's and now we're down into the 2.5 range. IMHO, this
is mostly for emission control to reduce amounts of pollutants per mile, NOT
to increase fuel economy. Maybe with the better atomization of fuel with
injection this is the new optimal ratio, it's hard to say, I could be wrong.
But if we're already this high in axle ratios, how much could a little
taller tire help?
Now if you want taller tires because they look better, I'm with you there.
Just don't want you to do something expecting one thing and not getting it.
Ken.. before or after speedometer recalibration?
I would guess at about 7 or 8% error with that tire change...
We went up 1 size on the Ram and it reads 60 when you're doing 63... yours
should have been maybe double that error with 2 sizes...
What about going to a "slightly" taller tire that is lighter and more
narrow like 235/85-16s (I checked 225/85-16s but no one makes them). My
current 245/75-16s weigh 53 pounds (Michelin), the 235/85-16s weight 45
If not, would at least this help us; go "skinnier" - what about going
with 215/85-16s. It's just about the same height, less friction with
the road and the tires weight about 18 pounds less than the stock
tires. That should help with MPG, right?
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