tire size , gear size, rpms and power

[shadango]

So right now I have 235 65 15 rear tires and 2.76 gears (open).

I used the cool calculator posted as a sticky here to look at how going to 3.55s would change my RPMs.

I turn about 2500 rpms now at 65 mph....according to the calc, I would turn about 3200 rpms with 3.55s, assuming a 93.5% torque convertor efficiency (I have a 2400 rpm stall convertor).

If I plug in 265 50 15 tires in (I want to fill my wheel wells more) the rpms will be 3k......

The question I have is will I be in effect reducing my 3.55s to a lower numerical gear simply by going to a taller tire?  In effect, negating my rear end upgrade?   

[brads70]

yep, tire size does effect gear ratio!

[72bluNblu]

I wouldn't say negating. But yes, larger tires will reduce the effect of the new gears a little. Might not be a bad thing though, depending on how much freeway driving you do. 3,200 rpm is getting up there for extended cruising. Especially given that almost no one goes 65mph on the freeways (at least around here).

[shadango]

Thats assuuming I have everything plugged into the calculator correctly.....I could have sworn that the last time I tried one of those calculators, the rise in RPMs wasnt as great.....still under 3k......   I am only plugging in 3.55s.....its not like I am looking to go to 4.10s....maybe i plugged something in wrong?

[cwestra]

I run a 3:55 gear with 275-15's and I am over 3,000 RPM at 65 MPH, according to the factory tach.

[shadango]

Ok.....so how bad is that on the highway?? What engine do you have?

[73Chally]

How much highway driving do you plan on doing?  I have 3.91s with a 29" tall tire (255/60-17), and turn about 3,200 at 70 MPH, but I will never drive more than about 50 miles at a time on the highway because I have no need to.

[shadango]

Well, things like trips to carlisle (200 miles or so) etc...most of the time, NOT on the highway for long periods.

I think 3.55s will be "fine"...just wondering if I will be losing much benefot of going from 2.76s to 3.55s if I also increase my tire size?

I guess the key would be knowing where the HP band is in my particular engine setup, which i dont know.

Time to experiment i suppose....LOL

[BB73Challenger]

My Challenger has 3.55 rear, 270 15 inch tires, and mild 440.
I made the drive to Carlisle from Cleveland twice now, I just cruise the freeway at no more than 65 MPH max.
The motor does not seem to mind, and I love the gear for the street.
With that being said, I do want to one day upgrade my 4 speed to a 5, but I don't do enough freeway driving to consider it a "must do" project.
Anything higher would be pushing it.

[cwestra]

Ok.....so how bad is that on the highway?? What engine do you have?

I am running a 340.  I can cruise at 70 but it feels more comfortable at 60 or 65, to be honest.  The only out of town trip I made (which was last weekend) was about 50 miles (each way).  I feel I could run at that speed indefinately.

The 3:55 is a great all around gear.  You almost can't go wrong with it.  And if you don't like it for some reason just sell it.  It will hold its value well.

[Chryco Psycho]

this will help http://vexer.com/automotive-tools/speed-rpm-calculator

[shadango]

I am running a 340.  I can cruise at 70 but it feels more comfortable at 60 or 65, to be honest.  The only out of town trip I made (which was last weekend) was about 50 miles (each way).  I feel I could run at that speed indefinately.

The 3:55 is a great all around gear.  You almost can't go wrong with it.  And if you don't like it for some reason just sell it.  It will hold its value well.

I hear that...been looking for one for a while.

Hoping i can find a setup at Carlise this year.

[HP2]

Take your car to a shop that has a chassis dyno. There you can get a graphic illustration of the power band of your engine and discuss how these changes will effect the driveability of your car.

FWIW, my truck get better mileage and cruises smoother at 3500 rpm than it does at lower rpm ranges. But, after an hour on the road, it does get tiresome on the ears and the conversation.

[efidart]

I just ditched my 3.55's last month for a set of 2.76's. Didn't regret it for one second!

I run 255/60/15s on the back and with the 3.55's at 60MPH I was at around 2700 rpm, that is extremely annoying at speed revving that high. If it was only for 20 minutes or so you could cope, but the car just feels so much better at high speeds!

Now at 60 mph im at about 2100 which feels great and you can cruise at 75mph with ease.
Might even switch to 275/60's just to fill the wheel well more, if I goto a 5 speed 3.55's would be proper.

[SBDave]

I'm trying to figure this all out as well.  I think it's probably best if you have some idea of what your whole drivetrain package will be (engine, trans, diff, tire size).  I have a 318, 904 with what I assume is an open 2.76 and 205/60R14's.  My axle is begging to be overhauled, so that's what I'm starting with and was recommended the 3.55 true trac.  I talked to Dr Diff and told him I planned on doing some road racing and freeway driving but no drag racing.  So my plan is to start with the 3.55 true trac, then figure out wheels/tires/brakes and then later on I can figure out my engine/trans combination.  I want to stay small block and want to convert to a manual transmission.  I plan on doing some longer freeway trips so I'm guessing I'll need either a 5speed or 4spd OD.  Oh, and I don't have a tach but my car seems fine cruising mid to high 70's right now.  I know that'll change with the 3.55 and new tires, but I figure I'll get cruising speeds back when I change transmissions.

-Dave