How do I identify the rearend ratio?

[ubtripn]

All I know is that I run stock tires on my 68 with a 700R4 and 2000 RPM= 60mph on the freeway. Is this a good balance for fuel consumption vs power? I do mostly city driving. I do not know how to identify what rear end ratio I have.

Also, there is a lot of talk about the lack of aerodynamics causing instability but what about rigs like the Hummer. They are bricks but drive much better. There must be more than just stabilizer bars involved. (Just a learning question.)

[HotRod C/10]

Best to pull the cover and read the stamped ring gear..... Takes all of 15 minutes and is accurate.
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[ubtripn]

sounds good, thanks.

[bonnieclyde100]

mine does about the same and i have 3.73 gearing but yea opening up and looking is the only way to be sure. if you cant read numbers just count teeth on ring gear and divide by teeth on pinion to get ratio

[ubtripn]

awesome, thanks. So do you feel you have it balanced out pretty well bonnieclyde100?

[bonnieclyde100]

yes it allows me to towing a car trailer easily and still go down the interstate without burning up my engine turning to many RPM. mine used to have a 3.04 in it and that was way to high to tow much. i am only hitting about 2200 at 70mph

[ubtripn]

Good deal, then I do not see a change in my future
thx

[GMCJOE]

Originally Posted by bonnieclyde100

yes it allows me to towing a car trailer easily and still go down the interstate without burning up my engine turning to many RPM. mine used to have a 3.04 in it and that was way to high to tow much. i am only hitting about 2200 at 70mph

2200rpm is much better than what I am hitting with these 4:11. Before I tore my tach out, I was running about 3500rpm's.

You can read the axle code, but that may not be correct as someone may have changed the gears out. Pulling the cover is the best bet in knowing the correct ratio.

[ubtripn]

will do, thx.

[Fitz]

Originally Posted by ubtripn

All I know is that I run stock tires on my 68 with a 700R4 and 2000 RPM= 60mph on the freeway. Is this a good balance for fuel consumption vs power? I do mostly city driving. I do not know how to identify what rear end ratio I have.

Also, there is a lot of talk about the lack of aerodynamics causing instability but what about rigs like the Hummer. They are bricks but drive much better. There must be more than just stabilizer bars involved. (Just a learning question.)

gear 700R4 TH350 TH400
1 3.06:1 2.25:1 2.48.1
2 1.62:1 1.52:1 1.48:1
3 1.00:1 1.00:1 1.00:1
4 0.70:1 n/a n/a

The 700R4 is an 4 speed box with the last gear an overdrive ratio so comparing speeds and RPM between different trucks to guess at your diff ratio won't work particularly well unless you put it in 3rd and you have the same tires. Put the truck on jackstands and either pull the diff cover and check what's engraved on the edge of the ring gear or spin one rear tiire until the pinion goes through two complete turns and divide the number of turns on the tire by two to get the diff ratio.

As for stability, if you have good tires you can't get enough horsepower out of your engine to push that brick fast enough to check 'aerodynamic stability'.

You'll need a wind tunnel for that. Try NASA Dryden Reasearch center.

[ubtripn]

Lol - thanks for the data.

[Already Gone]

If you haven't pulled the cover off yet use this calculator to find out your rear end ratio. If you are unsure how to calculate your tires height post the size and I'll figure it for you.

http://www.richmondgear.com/101032.html