3:73 gears and RPM question

kfellers

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Mar 31, 2005

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Oct 31, 2005

#1

• Oct 31, 2005

• #1

Recently had 3:73 gears installed. Here in DFW area traffic flows at about 65-75 MPH. My Stang is now taching about 2800-3000 RPM at 75ish. I know the stock tach and speedo aren't the most accurate, but, is this too much RPM for a constant 20-30 mile drive? Let me know what you think. Thanks in advance.

 

T

tjm73

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Oct 31, 2005

#2

• Oct 31, 2005

• #2

Not an issue. Don't be concerned at all.

What size tires are you running to get 2800-3000 rpm at 75mph? I turn about 2500 at 73ish MPH with 26" tires. RPM seems high for the speed.

 

srothfuss

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#3

• Oct 31, 2005

• #3

tjm73 said:

Not an issue. Don't be concerned at all.

What size tires are you running to get 2800-3000 rpm at 75mph? I turn about 2500 at 73ish MPH with 26" tires. RPM seems high for the speed.

Click to expand...

My car runs 80MPH at 3,000RPM. I have made the trip from my home to my families house at 80 (about 90 mile drive) on a few occasions over the past couple of months without any problems. If my car can do it, so can yours.

 

0

00-Saleen

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#4

• Oct 31, 2005

• #4

75 in 5th gear right?

 

FORCED2DV8

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Oct 31, 2005

#5

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• #5

At 60 mph in 5 th gear your rpms should be 2005. Yes driving with the engine constantly revving higher will cause more wear than normal. So you may have to slow down to 60ish on long drives, or be prepared to replace things earlier than if it was stock.

 

T

tjm73

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#6

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• #6

FORCED2DV8 said:

At 60 mph in 5 th gear your rpms should be 2005. Yes driving with the engine constantly revving higher will cause more wear than normal. So you may have to slow down to 60ish on long drives, or be prepared to replace things earlier than if it was stock.

Click to expand...

The increase in wear will not ammount to anything to be worried about. You'll be looking at the difference between getting 150,000 miles out of the engine and getting maybe 180,000 miles. Long before it wears out or even wears enough to matter, you'll probably rebuild anyway.

 

2000xp8

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#7

• Oct 31, 2005

• #7

Judging from the pics looks like you have turbines for rims, so i assume your tire size is 225/55/16 or equivalent diameter.
If this is true at 2800 you'd be going over 80mph.

So i'd say your tach is way off.

 

FORCED2DV8

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#8

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tjm73 said:

The increase in wear will not ammount to anything to be worried about. You'll be looking at the difference between getting 150,000 miles out of the engine and getting maybe 180,000 miles. Long before it wears out or even wears enough to matter, you'll probably rebuild anyway.

Click to expand...

I agree, I was just stating that it does have some affect on your vehicle.

 

kfellers

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Oct 31, 2005

#9

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• #9

The tires are 225/60/15's on original turbines.
So would my tach be reading higher than actual RPM.

Plus I think with the exhausts I have it's pretty loud and droney, this maybe part of the reason I was concerned. Haven't driven it enough to get used to the sound.

 

2000xp8

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#10

• Oct 31, 2005

• #10

Here you go.
Comes up slow as hell, hopefully it hyperlinked the actual page. Wait it out and then scroll down.
http://www.car-videos.com/tools/speedrpm.asp?Car=Select&Num1=225&Num2=60&Num3=15&AxleRatio=3.73&Ratio1=3.545&Ratio2=2.11&Ratio3=1.448&Ratio4=1.088&Ratio5=.68&Ratio6=&Ratio7=&Redline=6250&Increment=100&B1=Recalculate

 

93GTosu

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Oct 31, 2005

#11

• Oct 31, 2005

• #11

i tach about 3000rpm around 75mph. but i'm almost 100% positive my stock gauge is off a few hundred rpm.

i drove from medina to columbus (about 130ish miles) ranging from 60-80mph. Didnt have any problems but i was worried about keeping it up that high for too long so i'd slow back down...

 

Mavrick

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Oct 31, 2005

#12

• Oct 31, 2005

• #12

I wouldn't be concerned with turning 3k on long trips.. there are many 4spd auto cars out there that turn that kind of rpm at that speed.

 

kfellers

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#13

• Oct 31, 2005

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Thanks guys, I now will roll with piece of mind...and a radar detector.

 

Michael Yount

Mustang Master

Apr 10, 2002

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Charlotte, NC

Oct 31, 2005

#14

• Oct 31, 2005

• #14

fellers - the revs won't hurt anything if the engine is well maintained (My wife's car runs 3900 rpm at 80 mph), but something's inaccurate. It shouldn't be turning over that high at that speed. Did you change the speedo gear when you did the rear gear change?

 

M

macx

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#15

• Nov 1, 2005

• #15

It'll hurt your mileage likely a few mpg.

Here's something you can use to calculate
your mph & rpm's.

http://www.smokemup.com/auto_math/index.php

 

Michael Yount

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#16

• Nov 1, 2005

• #16

I don't think it will necessarily have a significant negative impact on fuel mileage. In fact, if you keep your foot out of it, city mileage will likely improve. The torque multiplication provided by the gears means the car can be accelerated with less throttle opening.

And on the highway -- well with 3.73's and a shorter tire than the Stangs use (think Stang with 3.90's) I just averaged 26.5 mpg on a 650 mile round trip. Mine has a mild H/C/I combo -- I can't imagine it getting too much better mileage on the road with lower gears. It got 23 mpg on the road when the motor was stock and the car had 3.54 gears in it.

 

srothfuss

Last night I stabbed the same guy 7 times in a row

Oct 17, 2004

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#17

• Nov 1, 2005

• #17

Michael Yount said:

I don't think it will necessarily have a significant negative impact on fuel mileage.

Click to expand...

Somehow we always end up talking about accelerated wear on the engine and MPG when a rear end gear thread shows up. Coming from a guy who only has done gears, I can say that my motor hasn't blown up and I still get near 300 miles on one tank full. Nothing changes when you swap gears except your car becomes much funner to drive.

BTW: Someone asked about the inaccuracy of the tach. The needle could have been taken off in the past and not re-installed in the proper position. Or it could just be the horrible inaccuracy of the stock gauges.

 

Michael Yount

Mustang Master

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#18

• Nov 1, 2005

• #18

srothfuss - yup, I'm with ya. Now, go shoving 4.30's or 4.56's with a stock height tire, and yes, you'll probably see a bit of a highway mileage drop. But 3.73's? Virtually no difference except for the fun factor.

If he corrected his speedo, then the tach must be off. He should be turning over something in the 2200-2500 rpm range when running 65-75 mph with a standard T5 or an AOD and standard height tires. Running the T5Z would drop those about 150 rpm.

 

ninety15.0

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#19

• Nov 1, 2005

• #19

your stock tach is way off. I have an autometer and it shows 200 rpm difference from the stocker. With 3.73's and a stock t5 your speed vs. rpm curv is not acutally a curv its linear and you gain 30 mph with each additional 1000 rpm...that means 3000rpm = 90mph and 180mph at 6k. this means that at 75 you should be turning almost exactly 2500. Which means your tach is off significantly. check this relationship on a gear rpm calculator if u like then get a good tach!

 

kfellers

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#20

• Nov 1, 2005

• #20

Yes, when I traded out the old gears, I had the mechanic put in the red speedo gear. Lot's of good info. The car is well maintained and I'm pretty sure it's just the wonderful stock tach.