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Old 06-15-2013, 08:11 PM
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Whats the equation for speed calculation

I'm trying to figure out what RPM it will take to maintain 65MPH in my car.

1988 Monte carlo SS, 15" rims, 255/60/15 rear tires, 3.73 rear gears. Th350 trans, 3rd gear = 1 to 1.

Basically i want to know what kind've RPM ill be running at highway speeds, next year i'm moving back home and that's 900 miles away. I'm driving my car and considering dropping the rear gears down to 3.42's for the drive.

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Old 06-15-2013, 08:50 PM
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http://wahiduddin.net/calc/calc_speed_rpm.htm
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Old 06-15-2013, 09:07 PM
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Tire Size, RPM, Speed, and Differential Ratio Calculator
3012 / 3.73 = 65.0 mph
2761 / 3.42 = 65.0
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Old 06-15-2013, 09:53 PM
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Thanks that's exactly what i was looking for. I'll probably swap to the 3.42 gears just for the trip, and put the 3.73 back in when i get home.
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Old 06-15-2013, 10:04 PM
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I punched in the info on my daily driver, 95 geo metro 3 speed automatic. the final drive for that car is 3.79, tire diameter ends up being 21.543, 3842 RPM to maintain 65mph. However 3rd gear in my little metro is 0.95:1 , its geared awkwardly. is there a way to do that equation on paper and enter more variables?
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Old 06-16-2013, 02:25 AM
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All the calculators you could want:
Calculators from Wallace Racing
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Old 06-16-2013, 09:08 AM
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Thanks guys. It's kind've sickining to know my daily driver has to turn 4000 RPM to keep highway speeds....
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Old 06-16-2013, 10:57 AM
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There are a couple of things to remember when using these calculators. First is that an automatic trans, unless it has a lockup torque converter, will have some converter slippage. That means that the engine RPM will be higher than the driveshaft RPM (assuming a 1:1 top gear in the trans). Second, all these calculators assume a theoretical tire diameter. The reality is that tires deflect under load and the loaded rolling radius is always less than the theoretical unloaded radius. That means that the tire is effectively smaller in diameter than you think, so the RPMs will be higher than the calculator suggests. Truck tires for 18 wheelers actually have a published revs per mile number that takes this into account.
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Old 06-16-2013, 01:38 PM
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The rpm will only be 8% lower at a speed than with the 3.73's.

Not worth the change for 1 trip. You are not going to save gas.

Wouldn't it make a lot more sense to just install a taller rear tire for the trip?
a tire that is 8" taller will have the same effect on the rpm.

Such a tire would be a P265-70-15

A 275-70-15 (30" tall 3" taller than the tire you got) will drop the rpm as if you changed from 3.73 to 3.27:1

Now you might actually save some gas but still no tnough to pay for the tires or a gear change....so

save your mony and drive 8% slower (60MPH) > It will only take you 8% longer to get there and you will save money. And save a lot of work, done twice. for nothing. You are planing to spend many hours and money to SAVE 1 HOUR TRIP TIME..

Think about it. 1 friggin hour.. I cannot chage a ring and pinion in 1 HOUR You can't either.

It is not even worth the 1/2 hour to change a set of tires.

Depart 1 hour sooner ...

drive the 900 Mile trip @ 60MPH and enjoy the scenery FOR 1 HOUR. save a lot of time and wasted money.

Last edited by F-BIRD'88; 06-16-2013 at 01:45 PM.
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Old 06-16-2013, 05:00 PM
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I guess my question is, are you doing this to save gas or to cut RPMs on the engine? If it's only the cost of gas, making a change just for this trip isn't cost effective. If it's wear and tear on the engine, I wouldn't worry about it. If you're really concerned, get off the interstates and see the world.
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Old 06-30-2013, 10:18 PM
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Joe,

I used to think that the tire deflection under load resulted in a smaller effective diameter. But, someone once pointed out that the circumference of the tire remains the same and each revolution of the tire travels one circumference whether it is fully inflated or flat.

Invison the treads on a tank. They have a flat oval shape but the tank travels the distance of the full circumference (length) of the tread with each tread revolution.

John
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Old 07-10-2013, 01:20 AM
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I can't imagin changing rear end gears for a trip. Just slow down a bit and enjoy the drive. For my street rod even changing gears for a little better economy is not justifiable. It costs 2-300 dollars just for parts then a whole after noon taking the rear end apart and setting up a new ring and pinion. I will only drive 4-5k miles this year and if it got 1 mpg better I'd be surprised. It would take 3 years just to pay for the gears. Not worth it.

I drive 58-60 mph with my dually all the time. It's the most efficient rpm and speed for the Cummins diesel. I get 21 mpg all the time on the highway and 19-20 in town. Just take your time...leave a little earlier and stay in the right lane. I could probably get even better mpg with a taller gear but at $1000 just for parts it would take forever to justify the change.
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