brian2sun

Any of you 3rd gen'rs running 265/75R16s w/ 4.88 gears and auto trans? I plan on running 285 MTs for winter tires, and I was thinking about buying some cheap 265 ATs for the spring/summer/fall.

I like running MTs in the winter because they have always done better for me in the snow and of course they're better for wheelin', but ATs are better for daily driving. I just think it would be nice to have the choice.

I like the idea of 265s because they are cheaper than 285s and I should get a little more power too with a smaller tire, but my concern is how much my rpm will go up at freeway speeds. Right now with the weight of tools and other crap in my truck, I do about 2800 rpms @ 70 mph w/ 305s (BYW, I don't like my 305s, they rub in the rear and are even more $$ than 285s, so it's either 285s or 265s for me).

How much do you guys think my rpms will go up going from 305s to 265s?

mt_goat

No but I'm running 5.29s with 33s which is even geared slightly lower than that setup.

Your setup 265/75/16 with 4.88s (auto): 2943 RPMS at 80 miles per hour

My setup 33s 5.29s (auto) 3059 RPMS at 80 miles per hour

Numbers from this gear ratio calculator: http://www.grimmjeeper.com/gears.html

mt_goat

What gearing is that with?

brian2sun

Thanks for the reply, that's with 4.88s and 33s but I'm also carrying 4 or 500 lbs of tools and gear in the truck. IIRC, when I'm not loaded down, I do about 2500 rpm @ 70 mph. Knowing that you're running 33s and 5.29s w/ those rpms, I'm thinking I'll be ok if I run 265s with my 4.88s. Thanks for the info.

Salue

I'm a little confused; are you saying that your rpms are different depending on how much weight you're carrying? If so, shouldn't your rpm @ whatever speed should stay the same no matter how much/less weight you have in the rig? Since your tire size and gearing haven't changed, it should stay constant, no?

Salue

brian2sun

Now you got me thinking, you're right. It shouldn't matter how much weight you have once it's up to speed right? Maybe I just need to pay closer attention to my tach and speed and test it again.

hellfire

Running 4.88s with 275s... My revs are way high, but I have the 22r.

mt_goat

You have a 5 speed, the autos have a higher OD gear (.71).

James Dean

Weight can effect rpm slightly with an automatic. There is about 200-300 rpm loss in the tourque convertor between the motor and tranny. The more weight the more loss. The less weight the easier the fins transfer the power.