for the longest time i thought i had 3:55 gears until i rode in my neighbors 01 the other day. i know for a fact he has 3:55 gears because the tag inside the glove box said so. his truck is identical to mine but his is an 01 with 285 tires and an uncorrected speedo. mine is a 99 with 305 tires and an uncorrected speedo. tire height's the same but mine are wider for those who don't know. cruising down the freeway his truck is right aroun 1900 to 2000 rpms at 70 mph. at 70 on mine it is almost at 2400 rpms. keep in mind that this is an uncorrected speedometer. when my truck is almost at 60 mph, i am at 1900 to 2000 rpms. my truck does not have the tag inside the glove box but i can see where a previous owner must have ripped it out. i have not called the dealer and gave them my vin number to see that the gear was during manufacture but that is not to say that a previous owner might have changed the gears without me knowing. so does everyone think i have 4:10's? what rpms should i be at with 4:10 gears? any info will be helpful, coming close to new tire time and i am thinking of getting 315's to lower the gear ratio since i am driving 55 miles each way down the interstate to my new job now. thanks

 

Not sure what RPMs 4.10s would put you at, but 70 MPH with 3.55s on an uncorected spedo (calibrated for 245/75/15) is right over 2k RPM. Sounds like you have 4.10s

 

Jack up the rear end, put it on jackstands (safety first). Mark the driveshaft at either end, near the yoke, just so you have a solid reference point. Mark the tire near another reference point, rotate the tires 1 revolution while counting driveshaft revolutions. The driveshaft will spin just over 4 times with 4:10 gears, or about 3 1/2 times with 3:55's, or about 3 3/4 times with 3:73's. There should be a tag on the housing, but if the gears were changed, and the tag wasn't, it won't matter.

 

Is that the Goodyear Silent Armor 305/70/16? If so go here and put in 633 (revs per mile)

http://xj.cdevco.net/auto/rpmchart

If you put the tires in there it'll tell you exactly what RPM you should be at...

 

Is that the Goodyear Silent Armor 305/70/16? If so go here and put in 633 under Manual Revs per mile. Put 4.10 on 1 side and 3.54 on the other side.

http://xj.cdevco.net/auto/rpmchart


It'll tell you exactly what RPM you should be at for each. Print it out and bring it with you!

Since your speedometer is uncorrected that kind of makes things confusing. You might have to do it on the highway with milemarkers and time yourself until 1 mile is exactly 60 seconds. I am beginning to like racer9's method better

 

cool the online calculator and tires test both confirmed 4:10's. thanks ya'll

 

I have 4:10s in mine and at 2000 rpm I'm running 60 mph in 5th gear, 80 mph at WOT.

 

Hm, all these higher readings make me suspect I just have smaller tires and 3.55. Ive got 70mph and 2250rpm indicated.
Ill look into what tires I have and see if I can read the sticker under the hood and post it here.
Im with Plow though, Id rather mount up some large tires and lose a bit of low end for the sake of better economy.

 

Turbo dreams, your RPM correspond to 4.10s.

Kawi600, you should have 3.55s with the smaller 245 tires.

You can go as big as 285 to help MPG, but that might not be possible with your OEM wheels. 285s on 6.5" wide wheels are pushing it....

 

yeah I just checked under the hood and Ive got 4.10's with 265 tires mounted. that gives me about 71mph at 2200rpm
I dont get much benefit from increasing tire size. I may look into ring and pinion sets