Questions on Highway speeds/RPM and rearend ratio

[morganater]

I am not sure of the rear end ratio in my truck but i know it has been gone through recently . I have 295/50/15s and at right about 70 MPH (ESTIMATE) i am at 3,000 RPM's in my 460. NOT a highway truck IMOP. Can anyone estimate my rear-end gears (are they high, or is it my tires that are so short) I know there is a calculator for this somewhere or does anyone know

[ak-1970-360]

i couldnt tell ya the gears but i just asked the same thing and if ya look where i asked it could help ya

[heep70]

Lift the reareand of your pick up off the ground. Turn the tire one full rotation and count how many times the yoke turns. The last spin the yoke makes will more than likely not be a full rotation. You will have to guesstemate that part. Example: 3-3/4 rotations of the yoke is a 3:73 R&P

If the rearend is open there is a chance that you will need to only lift one side of the pick up and spin the tire. If the yoke spins more then 5 full revalutions then do only one side off the ground.

Let us know what you find.

[willowbilly3]

Simple math. Mile in inches (63360) divided by tire circumfereance in inches (probably about 78 for your tires)=812 tire roations per mile. Divide 812 ( or your actual value) into your engine rpms per mile(your rpms at 60 mph) and you have the gear ratio.

[NM5K]

Should be a 3.50. I'd verify though...MK

[DuckRyder]

http://www.4lo.com/gearratio.htm#

You need exact numbers (use 1.00 for the tranny and crawl) but based on a nominal 295/50/15 from

http://www.powerdog.com/tiresize.cgi

it is a 3.39. (one would expect that your numbers are off and it is actually a 3.25 or 3.50)

Get your exact RPM with a tach and exact speed with a GPS or calculate from stopwatch between mile markers and the calculator will get you close enough to figure out which ratio it is.

You could also get your tire size closer by looking up your brand and size on tirerack.com if they sell it.

[morganater]

Sounds like a 3.50 to me. I do have a tach so those #'s are pretty accurate. hmm this gives me more reason to install some nice 4.10s with a powertrax locker

[tcb1969]

Actually - it gives you a reason to install a 2.89 rear. 4.11's will run you at 3500 rpm at 70 mph. A 2.89 would put you at 2500 rpm at 70 mph.

But who the heck wants a 2.89 rear? Leave the rear alone and get taller tires - or a new transmission - or a gearvendors overdrive unit! :twisted:



John