2500 RPM at 65MPH??

[adam45356]

thats weird i had my speedo gear changed its on the dot according to the sign on park street hear and when i hit od its like 2000rpm then the tc lockes and falls to a nice grown at 1500rpm i have all terain t.a 31x10.50 15 with no tread if that makes a diff

 

[Jason94sport]

Your stock gears, & the auto or 5 speed make a difference as well. Also keep in mind the stock tach isn't the most reliable thing.

 

[Maniak]

According to the math I just did..

An a4ld transmission in 3rd gear at 2500 rpms with a 3.54 rear end with 31" tires will go 65mph.. In lockup OD it should be at 96mph.

I used the following site to help figure it out..
http://www.grimmjeeper.com/metric_gears.html

You can do the math here also... http://www.stangsource.com/calculators.cgi?calculator=1

4th gear in an a4ld (w/ lockup) is .67:1
The xfer case is 1:1
The rear end is 3.54:1 (or 3.55 depending on who you ask)..
I used a tire height of 31" (its a < 5% difference between 30 and 31" actual height)

Either way, 2500 rpms with 31" tires with an a4ld in od/lockup with a 3.5x gear ration is NOT 65mph..
65mph is what you would get in 3rd gear (1:1)..

Soo, if your really getting 2500 rpms at 65, your not going into 4th gear/lockup assuming the info I used about the truck is correct.

~Mark

 

[Rustang©]

Hey, thanks all for the information, and I did have it recently serviced, i had the trans re-built, but i just had it looked at, the od/and lockout are working properly, i also was told i have a larger planetary and a 1600 stall speed converter, im not sure if that makes a diff, but thats what i was told. I was even doing 2500 before i had the trans done, im suspecting it might have something to do with my Exhaust, as i have no cats and no back pressure my truck seems to onyl wnat to move at 3000+ RPM!

I did also check my odometer and speedo, both do not register correwcty, the odometer is off by 1/10 a mile and the speedo by about 7 MPH, as for the gas mileage, im still getting 19 MPG, i traveled 220 on 11.5 gallons of gas, so im guessing my trucks not in the bad a shape!\

hey man, the website's great and all,. in theory my truck should do 96 at 2500, but i dont think my truck can 96!

Thanks again for the information!

Kia Ora
-Dan

 

[adam45356]

yea my stock tach died

 

[3DFanatic]

My 5speed runs about 2400 at 75...iuno what gears I have...haven't got around to looking yet...just thought I'd throw that out there PP

 

[Maniak]

Either the assumed information is incorrect (more likely) or something is wrong with your truck.

There is no way an explorer with an a4ld and 3.55 gears with 31" tires will be turning 2500 rpms at 65mph in OD with the converter locked up.

We can assume the tach is somewhat close (its rare to have it read an incorrect value while looking like it is working correctly)
We can assume the tires are 30" tall (31" tire with no tread)
We know an a4ld in OD and in lockup is a .67:1 gear ratio (stall doesn't matter becuase the converter is locked)
We can assume that your indicated 65mph is actually 72 (your said it was reading off 7mph.. and I'm going to assume that was at 65 since a speedo will not be off the same amount at all speeds.. its off a certain percent)

Until we figure out which piece of information is incorrect, the number just don't add up..

On to your subject about 3000 rpms in town...
That is caused by hp/torque of the engine and the stall speed of the converter and the gear ratio/tire size

A 1600rpm stall converter for an explorer, in an explorer should flash stall less than 2000 rpms.. That means if you stomp on the gas it should go up to about 2000 rpms (w/out the truck moving) then start climbing as your speed increases... If it flash stalls higher then either your converter is slipping more than it should (for a 1600 stall) or your tach is reading off (it is possible, just never heard of one reading the wrong numbers but otherwise looking like it is working)..

A heavy truck (like the explorer) and a motor with a bunch of torque (4.0 ohv has as much torque as a chevy 305) will affect the stall (bring it up) but if the converter is a 1600 stall and designed for an explorer than it should stall somewhere around 1600 (usually a little higher).. If you are low on torque your converter will stall earlier (less rpms to get moving) which will hurt your performance.. But too much stall and your have to get the rpms up to really get moving.


~Mark

 

[Jason94sport]

I'd say that site is wrong. My numbers aren't close to what I actually get. If you throw the gear selector into D when at 65 do you RPM's go up? If so the TQ lockup is working.

 

[Maniak]

You can check multiple sites, or even do it by hand.. Here is how to figure out your rpms at 60mph..

Take the tire diameter (30" in this case) and multiply it by 3.1416(pi)
I get 94.248

94.248 is how many inches the tire goes in 1 revolution..

Now divide the number of inches in a mile (63,360) by how far the tire goes in 1 revolution (94.248)
I get 672.26

Now you need to figure the final drive ratio.. the a4ld is .67 and the rear end in this case is 3.55.. so multiply them..
I get 2.378

Soo.. the motor turns 2.378rpms per turn of the tire.. Soo.. you multply them together to figure the rpms at 60mph..

I get 1598.. That means according to the math a 3.55 gear ratio with a tire that is 30" tall w/ an a4ld should be turning just under 1600 rpms at 60mph.. no where near the 2500.

You can look all over the 'net and find many calculators and ways of doing the math and see that 2500 rpms won't be 65mph, it should be just over 1700 rpms.


~Mark

 

[Rustang©]

Just the other day i took a long trip, 220 miles, i know the odometer is off and i knwo the speedo is off, i did infact try the shift into D, and when i did sure enoiugh the rpm whne up to about 3000, or so my tach says.

The 1600 stall is an upgrade fomr the stock 1400, its a boss hog if that makes and difference.

As for the tourque, i know I have plenty, i can do a burnout with fully treaded 31's, and i have no problem driving on our local beaches, Hummers get stuck so do wranglers, and oddly explorers are the most common to get stuck "usually the 95 and ups" so im assuming torque is no problem for me and my motor runs fine, im guessing the tach is off or somehting, idk.


Thanks for the help and if nay new ideas hit ya let me know thanks.

 

[Big Red]

75mph @ 2100RPM
80mph @ 2350RPM
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http://photos-171.ak.facebook.com/ip002/v67/109/16/66501169/n66501169_30487171_8004.jpg
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