Rear end gear ratio - making my head hurt

[bikeguy]

Ok, need a little help with this decision. I've been researching for the past couple of days and can't make my mind up on which gear ratio I should change to.

I restored my 1970 Chevy 4x4 truck a few years ago. The engine is a 350 4 bolt main, when I rebuilt it I ended up boring .30 over, and installing a mild cam. Originally equipped with an SM465 4 speed and NP 205 transfer case, converted it to a TH350 with NP 205. Front and rear ends are non-posi (open) and 3.08 ratio. Currently running 35/12.5/15 wheel / tires.

Driveability is ok, a bit sluggish off the line, but does ok when cruising at 50. Today my son and I were driving and verified with his GPS that at ~55 MPH engine was turning at 1850 RPM's.

From what I've read the optimal RPM range should be a bit higher. I've been thinking that I need to go to at least a 4.11 gear or maybe a 4.56? I like the 35" tires so I will be staying with that size tire. Also this is a city driver, not much time on the freeway, but I do go on longer trips occasionally. I don't tow much with this truck, just like to cruise.

So, the question is, which gear ratio do you guys/gals think I should go with? Also, here's a pic from when I finished the restoration.

Thanks for your help.

[67c10rustbuket]

3.08 has to be pretty sluggish with those tires! id guess 3.73 or 4.11 would be a good all around gear ratio for you. my 93 Z71 has 3.42 gears with approx 32" overall tire height i think it needs more power or a little more gear in the city but it kicks butt at around 72 mph and 1800ish rpm while getting 16 mpg on the highway.

That truck looks very nice ! good job!

anybody else running big tires have an opinion ????

[BlazinC5]

Try playin with #'s on this calculator.
http://www.richmondgear.com/streetcalculators.html

When I was running 35's on my Blazer with a TH350 4:11's seamed pretty good... I had at one point 38's and 4:56 felt pretty good with those tires.

[Ackattack]

If you are somewhat happy with 3.08s, then 3.73s I think would give you what you want without buzzing the engine too high.

[gearhead1985]

here is a chart telling tire size and highway rpm
http://www.4lo.com/calc/geartable.htm

the lower the gear the better off the line take off

also instead of doing a gear swap it might be cheaper to do an axle swap for the gears a lot of the 1 ton trucks come with 4.10's and 4.56's so you could get the gears you want and a stronger axle

[72 Z-28]

I've got a 77 with a 4speed and 205. It used to be a th350,203. When it had those it was sitting on 35's with 4.56's and it would turn something like 3500 rpm at 75-80. I cant remeber what it did at 55 this was 5 or 6 years ago. If it were me and I was going to be driving it on the street more then on the tralis I would go with 3:73. But if your going to be wheelin it I would get some 3/4 or 1 ton axels with the lower gears and have a BEAST haha hope this helps.

[mikajo39]

With those tires you're going to need more gear. With gears it's basically how you want to drive it, if you like cruising, step up to a moderate gear like 3.73s. You'll notice a big difference off the line and it'll probably keep the rev's up a little bit for the mild cam you have. If you wanted to give up some freeway driving but wanted some real zip off the line in the city drop in the 4.11s. It's basically how hard you want to drive it, just match it to your driving style.