"New" '86 C20 need advice...

[pdxmac]

by granny tranny I assume you mean a 4sp with a granny low and no overdrive, correct?

[Roper]

Originally Posted by pdxmac

Really? 4:56s ? I am game, I just dont want it toi be wrapped up to 3000rpm+ at 65-70mph while I'm going down the highway. Is there a way to figure this all out? as far as rpms at speed etc?

..

There web site is down today http://www.bowtieoverdrives.com/index.shtml
But from the graph I have printed out a 700r4 with 4/10 gears and 33" tires at 65mph your motor is only turning 1900rpms which is to low.
My th350 is turning 2700rpm with same 33" tires <that actually measure 31" mounted and under the load of my truck and same 4/10 gear ratio. I haul horses and this combo. It works great for me but if my truck was even a bit lower spead that would be even better. Really what I should have is a Gear Venders overdrive.
With the 308 gears the truck sucked gas and had no power.

[Roper]

If I was you I would start looking around for a good deal on a second hand Corporate 14 bolt FF 10.5" ring gear with 456 gears. Your probable only going to be able to find one with 4/10s but thats better than what you have. Those axles sell pretty cheap<think $150-$200> and seals and bearings dont cost that much.
This would give you more bang for the buck than modifying your motor.

[MylilBowTie]

Here is for the speedometer gear calculator.
http://www.tciauto.com/Products/Tech...eedo_gears.asp

Here is a tire size calculator.
http://www.miata.net/garage/tirecalc.html

Here is another link to check out.
http://www.ringpinion.com/Calc_RPM.aspx

I think 4.10s would be fine for the 3/4 ton. Unless you find a 14 bolt ff that has 4.56 gears. The 3.73 4.10 gear ratios are pretty common. Having the 700r4 is going to get you off the line a little better vs other automatics.

I would think about running an external transmission oil cooler. It will help keep the some of the heat build up and stress of towing down.