Right now I have 285/70/17s on my 03 hemi. I am about to get 315/70/17s. How much of a difference will I see in mpg and loss of power?

 

Quite a bit.

 

pretty sure 315/70/17 is close to a 35" tire your gonna average about 12 mpg with 3.92 gears.
I didn't notice a big power loss

 

Only downhill with a tail wind.

 

what are you talking about, i get 12mpg with my truck daily and i drive the **** out of it with 35's

 

Help! I have an '05 Ram 1500, and currently have 285/70/17's. Purchasing some 315/70/17's......will they fit with a 2" levelling kit, or will there be some serious rubbing? I have yet to figure out the science behind measuring tires to ENSURE they will fit in my wheel wells, but currently with the 285's and no levelling kit there is no rubbing. Does anyone else drive a 1500 with 315's? Did you lift it? Crank the torsion bars? What do I need to do to make these tires fit?! Thanks guys!

 

Well, being from the US, I don't think in metric, so let's do some convertin'...

The 285/70/17 is essentially a 32.7" x 11.2" tire and the 315/70/17 is a 34.4" x 12.4" tire.

You have more to be concerned with more than just the diameter (height) of the tire, you also have to consider the width. Are you planning to use the stock wheels (where the offset/backspacing is NOT very friendly to a wide tire) or do you have/plan to get aftermarket wheels with a much better offset/backspacing?

The other thing I need to know, before being able to give a definitive answer is if this truck is a 4x4 or 4x2, because torsion bar fronts offer a little more room for wider tires than coil over fronts (and obviously a T-bar crank is NOT an option on a 4x2)...



The amount of power and mpg loss with going up about 2" in diameter and 1" in width would be greatly effected by what gears are in your trucks differentials. If you have 3.92 gearing, then the effective gearing after the tire change becomes about 3.69, not great but not catastrophic. You'd feel a slight power loss and probably could expect to lose 2-3 mpg, especially around town. With 3.55 gears, the effect would be a little more dramatic, with an effective ratio of about 3.3, you'd not be happy at all with performance, will tend to overwork your transmission and gas mileage will suffer probably in the 5-6 mpg range.

Ideally, you want your Effective Gear Ratio (the combination of the physical gears in relation to tire size) to be in the 3.92-4.10 range for optimum power and fuel economy. So if you changed to 4.10 gearing with the tire change, your effective gear ratio would be 3.86 which ain't bad. A swap to 4.56 gears would have you at 4.29, not bad either but would lean toward more performance/less mpg at highway speeds...

 

I plan to keep the factory 17" rims, and yes its a 4x4. Please advise. Thanks!!!

 

Crank the keys 2", I wouldn't go over that. Some like 2 1/2" but it puts considerably more strain on the CV axles which could result in a failure, also premature wear of tie rod ends, ball joints and the rack & pinion come into play here as well.

With 2" of leveling up front, you'll clear those 34.5" tires, be a little full in the well, but they'll go. Might scrub a bit if you are into off-roading, but normal driving you'd be fine.

As for the width, they'll clear. Might be a hint of scrub at full lock (100% wheel turn), but if so, just a smidge. You're kinda pushing the envelope a bit on width as the stock 17s are 8" wide and the rule of thumb is the tire should be 3" wider than the wheel with a -/+ 1" considered within spec. At almost 12 1/2" wide, you are just outside spec, but unless you air down for off-roading and cut the wheels hard at speed, you shouldn't have a problem losing the bead. Again, general everyday driving you should be fine.
Only other consideration is that the base 17" all steel wheel is a 17x7 and I wouldn't go 12 1/2" wide on the tire, as a matter of fact most tire shops will refuse to mount it. But the base, all steel 17s I believe only came on the RCSB 4x2 trucks...

 

Thank you so much! Since my truck will NEVER see mud, I should be fine with regular daily driving? My other concern was the tranny pushing this huge tires - any thoughts on that being an issue?

In your professional opinion, would putting these 34.5" tires on my truck with a 2" leveling kit be a good idea? In terms of wear & tear on the vehicle, and appearance?

I have such a hard time visualizing what my truck will look like & if things will 'work' - I know what I want in the end, but can't decide how to get there! Indecisive girl I suppose....

Thanks again