calculating RPM and Tire size.

[Logans Run]

http://www.grimmjeeper.com/gears.html

OK so I used the link above to figure my numbers.

I have a good 360 with an Edelbrock 2 plane manifold and 650 CFM carb
Edelbrock shorty headers with full duel exhaust exiting through cherry bombs
A TH400
A D20 transfercase (stock gears I think )
D44's with 3.54 gears
and 35" tires (which actually sit about 33 inches tall ... that made me scratch my head when I just measured them)

Assuming I have enough HP to push my rig through the wind at 75MPH
I should be turning around 2,700 RPM with everything in high...

Does this sound right? It seems very easy to do ... did I miss something?

[joe]

Sounds about right to me. Below is what I use:
RPM =(MPH x axle ratio x 336) / tire dia

[addicted]

That sounds about right. derf's calculator is pretty accurate and probably the best i've used out of the many on the web. Mathematically it is correct, but real life is always going to be a bit different.

[Billygoat]

The size on a tire is considered (being such in name only)
Most companies list the true od.
Also with load tires squat, radials more than bias ply.
I found that if you measure from the ground to the center of the hub (axle) and double that for your tire size you will get really close to actual rpm's
Auto's tend to fluctuate more from a calculated figure because of the torque converter slip.

[derf]

Originally Posted by joe

Sounds about right to me. Below is what I use:
RPM =(MPH x axle ratio x 336) / tire dia

For highway driving using an automatic without a lockup torque converter, the general rule of thumb is using 355 in place of 336. That usually comes close to accounting for slip.

[Tigger4X]

It sounds like the calculator did the correct math with the numbers you fed it. The only thing that got me was the 2700 RPMs. maybe its cuz you are at 75 MPH at that RPM or that you plugged in the supposed tire size of 35", but for me I geared myself to be running at much lower RPMs at that speed.

I re-ran your numbers with 33" tires and it shows you are actually running 2475 RPMs at 65 MPH and 2856 at 75 MPH. IF you are running a physical 33" tire and have 3.08 gears you'd be running a much more comfortable RPM and save a buncha cash at the wallet killing pump : 2154 RPMs at 65 and 2485 RPMs at 75 MPH

In order to get your desired RPMs you will have to decide if its more cost effective for you to change out your gears or physical tire size. Gears may cost a bit more up front than new tires, but you do it once and you're done. IF you had to go up in tire size you could be looking at waaay more than gears to get there or some other mod to pull them off. Or you may end up going down in tire size and depending on your wheeling preferences you could be losing valuable ground clearance. It all comes down to what you want your rig to be capable of and then make the best plans from there whether they be short or long term.


BTW ... I'm not that far north of you in Ventura County by the "infamous" Camarillo Premium Outlet mall so if you need a hand wrenching or whatever gimme a shout!

[Logans Run]

Originally Posted by derf

For highway driving using an automatic without a lockup torque converter, the general rule of thumb is using 355 in place of 336. That usually comes close to accounting for slip.

My understanding is the 336 is the constant which takes into account pi (3.14159), 60 minutes in an hour and 63 360 inches in a mile.

I would think there would be a better way to account for slip.

[Logans Run]

when I said 2700 I was just rounding off. I should probably use the 33 inches instead of the 35 that I plugged in as well ... I just wanted a rough estimate ... that is until I started looking at the formula lol

[JERSEY JOE]

MPH= RPM X TIRE DIA (INCHES) X 2.96
_____________________________

GEAR RATIO X 1000


EXAMPLE:


3000 X 35 2.96 310800
------------------ = -------- = 75 MPH.
4.10 X 1000 4100

[derf]

Originally Posted by Logans Run

My understanding is the 336 is the constant which takes into account pi (3.14159), 60 minutes in an hour and 63 360 inches in a mile.

I would think there would be a better way to account for slip.

The 336 works if you have a lock up torque converter or manual trans. That factors in no difference between the engine and transmission input shaft speed (in RPM).

If you want a fully accurate way to account for slip you have to factor in vehicle weight, the grade up or down you're driving on, rolling resistance of the tires, aerodynamic drag (including head or tail wind), vehicle speed, specific configuration of the converter itself (stall speed, etc), condition of the transmission fluid (including viscosity at current temperature), parasitic drag of all the drivetrain components.... and that's just what I can think of off the top of my head.

Or you can just ballpark it with 355 like everyone else does.

[Tigger4X]

ummmm ... what DERF said

[Logans Run]

Originally Posted by derf

The 336 works if you have a lock up torque converter or manual trans. That factors in no difference between the engine and transmission input shaft speed (in RPM).

If you want a fully accurate way to account for slip you have to factor in vehicle weight, the grade up or down you're driving on, rolling resistance of the tires, aerodynamic drag (including head or tail wind), vehicle speed, specific configuration of the converter itself (stall speed, etc), condition of the transmission fluid (including viscosity at current temperature), parasitic drag of all the drivetrain components.... and that's just what I can think of off the top of my head.

Or you can just ballpark it with 355 like everyone else does.

... I was trying to make a joke with this ... the 336 or 335 in this case is the only REAL constant ... and its the thing that is changed so it is not the only real accurate part LOL ... it seemed to be funny to me.

[Logans Run]

I punched in 33 for tire size ... since that is what my 35s measure at
and came up with 2,856 RPM at 75MPH still not really all that bad.
do they make an overdrive that would bolt into my set up?

[joe]

Originally Posted by Logans Run

I punched in 33 for tire size ... since that is what my 35s measure at
and came up with 2,856 RPM at 75MPH still not really all that bad.
do they make an overdrive that would bolt into my set up?

You're correct using actual tire size and not going by what the tire says.
Workin the formula with 75mph, 3.54 gears and 33" tires I get 2703 for RPMs.
PS: the 336 in the formula is the constant for an AT allowing for slippage.
Non OD trannies have a high gear ratio of 1:1 If you work the formula for an OD trans you have to first multiply the axle ratio by the OD gear ratio before solving.