LT Tires: Wider or taller - best bet for mpg

[steak2k1]

OK:

First post. I live in Southern Alberta. 4 months summer & 8 months poor ski-doing. Well, winter seems long that is for sure.! Anyway i digress...

2006 GMC2500HD D,Max/Ally. I currently have a set of OEM 245/75R16's AT type. They look pretty small on this truck actually, but that is not really a concern to me.

At some point in time I wish to have 2 separate sets: 1 for winter with a more aggressive tire likely being an AT. And a set for summer with an HT type tread.

My question to this forum is: what is preffered for best possible mpg..? summer and winter.

265/75R16's or
235/85R16's or

Driving is pretty much all hwy (80%) and the rest city. No off roading. I do not load heavy very often if at all. Summer trailering is with a 4500lb / 22' HTT.

This is my Daily driver - 55,000 km annually. Truck has had some mods: exhaust, intake, trans upgrade & I run a programmer in it's middle range.

Thank you,

Stk

 

[cibachrome]

Presume these are LT sizes you mention (LT245/xxR16). Your best rolling size at your load at a reasonable pressure (56 psi) is the OEM size (General best brand, Bridgestone a bit higher but took higher pressure to get it).

The LT265 could do it but it would take 80 psi to be equivalenty. Aren't LT235/85s trailer tires?? They can do the good rolling but the cornering stiffnesses are too weak to give good handling on your truck for the pressure it would take.

Stick with the LT245s. There are some very good reasons those are the tires on your vehicle. For best fuel, keep up the pressures and keep your wheel alignments good at the load you generally run. Pressure always gets you fuel economy but ride and handling slide back. Steering could get a bit light and cross-wind sensitivity will increase.

 

[steak2k1]

Actually 235/85R16's are LT Tires with an E load rating. I have seen them on many vehicles. Toyo makes em. My thinking in these is that they are narrower and as such would present a smaller profile to the pavement reducing resistance somewhat. Combined with the slight increase in diameter over OEM size, would give a slightly better mpg rating givent hat the increase is similar to changing the gear ratio in the rear diff. if ever so slightly.

the 265/75R's 16 are only a D rated tire...however as noted in original post, that should not be an issue even when towing/light hauling. But definitely wider and taller.

I guess the other option is going with a 17" rim (H2's) or something like that and then going with 265/75 or 70R'17's.

Rgds,

stk

 

[cibachrome]

Data doesn't support the notion that tire rolling resistance of the 17" tires you mentioned is better. The OE tires are running about .0065 load normalized rolling force at 56 psi. An LT265/70R17 takes 85 psi to do the same job but the handling properties deteriorate. Plus, the 17" tires have about a 30mm taller loaded radius. This jacks the truck up for worse overall aero section, AND your speedometer will be wrong. So if you do the typical miles-per-gallon estimate, which miles are you going to use: the odometer or the actual distance traveled?

Besides, Smokey the Bear won't buy your explanation about the speedo error, either, in spite of the cost of 5 new 17" wheels (you'll need a new spare, too, or do you accept the posibility of ruining the rear diff if you get a rear flat (the most likely nail scenario)? Finally, aero drag is less than about 18% of a tires rolling resistance. Most is hysteretic losses. Air pressure helps this but drives the handling properties down, not to mention the brickyard ride at 80 psi. Might want to buy some spare shocks and brackets while you are at it...

 

[steak2k1]

Well for Smokey/speedo, I'll use the PDA on the Banks system as it can be calibrated for diff tire sizes. As for actual mpg..I calculate. Once tires are diff from OEM, (If I go that route), there really is no other choice but to calculate.

17's are on the outside of my possibilities. Cost is a factor and 5 new rims would likely be well over 11-1200CAD.

So I am back to either 16" rims I agree, I do not wish to take the truck up higher than "normal". So I shall wait for a while and check up on other sources as well before making up my nind as to which way to go.

Thank you,

Stk