265/70/17 & 3.73
in overdrive with the convertor locked:
60mph is about 1775rpm
75 mph is pretty much right on 2000rpm

 

I am also close to this with my 3.55LS and 285/75 series tires. Mine doesn't have the power to pull OD on the highway, so I leave it out of OD 99.99999% of the time...

 

My truck has P275/60R17 tires and 3.55LS rear end in it. At 70MPH, the engine is turning at 1940 RPM.

 

i'm thinking i got 3.55's

 

Did you mean to type 72mph@2000rpm? You and I have the same rear gear and size tires and my 70mph@1940rpm would be closer to this than 82mph.

 

Mine was running up the mountain side @ 90 mph when the mean guy with dark sunglasses pointed the radar gun at me =/

Mark

 

I have 4.56's with 265/70/17s. Runs 2320 rpm @ 70mph with OD and locked convertor. With the O/D off it's 3000 rpms.

 

Could someone do me a favor and please explain the gear ratio to me? I have been searching for a while and cant find anything. I know one number is the tire turning and the other is the driveshaft turning. Im not sure tho. Thanks.

 

take a 3.55 ratio.....the driveshaft turns 3.55 times to one revolution of the tire/wheel.
Rich

 

Ah ok thanks 96f150

 

darkimage,
there is a little more to it than the reply you got.
Here is it.
In third gear, your ratio thru the transmission is 1 to 1 so the 3.55 would be correct as the reply stated, but in overdrive, the transmission ratio is
.7 to 1.
What that means is the 3.55 x .7 = a final ratio of 2.485 rounded either way you choose.
The tire "diameter" determines how far the truck will roll in distance for one revolution so affects the speedometer reading and the engine RPM at any peticular speed.
See that even the tire pressure will affect the rolling diameter and affect the RPM and speed registered even though small.
Note that the speedometer most always is not accurate from the factory, to boot..

 

so how can i tell what ratio i have on my truck? its an 01 supercrew automatic.....im running 315/75/16 on my truck with a 6 in lift! how can i tell if the rear end has been worked on? i notice alot of silicone around the bolts! at 2000 rpm my speedo is showing 70mph but people have told me im going faster! HELP.....

 

rulydog,
here is a way to tell what you have.
Select some reference point near the drive shaft to mark the drive shaft for starting point with chalk.
Mark one wheel the same for starting point reference.
Turn the driveshaft until the wheel has completed one complete revolution back to the wheel's starting mark.
Count the number of times the drive shaft had to be turned (including the fraction of a turn such as 1/4, 3/8, 1/2 etc.
This should provide the gear ratio in the rear end
using the following to judge by:

standard ratios for these rears are 3.55, 3.73, 4.10, 4,56..
Note the fractions will show up as a partial drive shaft revolution if you pay close attention.
For example you can certainly tell a half revolution (.55) from three quarters of a revolution (.73), so it will indicate the rear gear by adding full revolutuions and the fraction to see the rear gear ratio installed.
Tire size has has no meaning for this test. One revolution is the same for any size tire. The only difference is when it's on the road surface and rolling, it's how far it will roll in distance per the diameter that counts.
This is why changing tire size affects the speedo and tach reading.
.
The only way to see the engine RPM more accurate is on an electronic dash, it can be setup so the mileage digital indicator will show engine RPM directly, aside from the analog tach.
How it's done is a seperate discription of how the dash is used to do testing and see some other functions as well a diagnostics related to the dash and indirectly the engine, for some functions.
Good luck.

 

thanks bluegrass!!!! but cant i tell by looking at the tag or vin?

 

If a gear change has been done, you don't know what you have.
except to do the test discribed.
Otherwise the gear "code" is on the door jam tag.
You need to see it then get the code too gear translation.
Good luck.