Can someone sell me 3.80 or similar gears? (or trade)

I've been looking for a posi center section 3.70 to 3.80 gears for 8" over a year now. Good luck on your quest...Not easy or cheap to find.

 

It looks like I may have to build it myself. Just need to decide between 3.55 and 3.80. Still need highway data and tire sizes before I can decide.

 

70mph, 26" tire, 3.5 gear, =3160 rpm

70mph, 26" tire, 3.80 gear =3400 rpm

70mph, 28" tire, 3.80 gear =3200 rpm

70, 24", 3.5, =3400
so a shorter tire at the track will make a 3.50 act more like a 3.80

28" is about the tallest you can fit under a mav, in my limited experience.
the 3.80 with a 24" will tend to act like a 4.10 at the track.

you can play with the tire sizes within a limited range to get closer to what you need.

 

dont forget when looking at tire diameters & plugging them into formulae that while a 28" tire might fit the car, that tire will have a functional diameter of 1-2" less due to the weight of the car. so if the biggest you can fit is a 27.5 tire, thats a radius of 13.25 inches unloaded. on the car its more like 12.75 inches radius. that means use a diameter of 25.5 in the formulas to be safe. these arent slicks & have virtually no growth at speed. tire pressure will also affect this dimension. a wide tire like a 16-17" 50 series will be affected less by the weight of the car.

 

Dave, where did you get those numbers? If those are correct, I wouldn't be all that upset to be doing 3200RPM at 70. My current tires are 245/60r15, which is about 26.5" tall. I would like to keep these for the track because they give the car a nice stance, and have pretty good surface area for traction.

And the difference between 3.80 and 3.55 is only 250 RPM? I almost had a 3.80 center section today, but I think it got sold before I bid on it. The guy who was selling it was upset with the highway use of his 3.80, and was swapping for a 3.55. I don't see why he would bother for only 250RPM...

 

If you search in google for, car calculators, or gear calculators, even horse power calculaters you'll get a bunch of diffrent sites where you put in things like gear ratio, tire size, and speed and it'll tell you the aproximate RPM"s that your car would be doing at diffrent speeds. For the HP calculators, it goes by weight, 1/4mile time and speed and a few other factors.

Just a tip on an easy way to find out which gears work for you the best. I believe there is one that diffrent gears would give you too!

http://www.ringpinion.com/content/calculators/RPM.asp - theres a link that might help you out a bit. I agree with other people, its diffinately better to go with new gears rather then used.

 

www.extreme-auto-parts.com/engine-calculator.asp should have a calculator on the site. i used one i have off an old wolverine cams setup disk. has seemed accurate in the past, but there are always variables like air pressure, converter slippage, car weight. i have 3 different calculators for rear wheel hp based on weight and mph. and i get 3 different answers from them. these things give approximations based on data given and computation variations.

i have a fairly loose converter (stall about 3200) so my rpms are up a bit.

its more like a 280 rpm difference from 3.5 to 3.8. i rounded my figures a little, but dont forget the other variables. maybe he didnt check out the rpm difference. hopefully i'm not off & giving you bad data. but logically, it works. from 3.5 to 3.8 is just under a 10% difference. 3400 to 3160 is just under a 10% difference.

i'll give you another thought. if you're not going to be a high hp car, scrounge up an AOD trans and go with either the 3.80 or 4.10 gears. it has an overdrive & should live behind your motor with a little work. i think it also has a lower first gear than a c4 which would multiply you rear gear more. you could find out what fabrication is needed aside from probably shortening the driveshaft elsewhere on this site, as well as other pros & con s. admitttedly, this could be a more complicated & expensive route, but keep it in mind for the future.

 

I have 215/60R15 tires and a 4speed over drive tranny with a .80 fourth gear. Do you have a 3 speed?

My car is a weekend car/toy. I will probably never drive it over 100 miles one way.
mph rpm
50 1913
60 2296
70 2678
75 2870
80 3060

If i was to do it again I would probably go with 4.11 again. It would be nice if it spun alittle slower at highway speeds but I have a lopey cam. The power band of the cam is 2500-6500rpm. It is much easier to drive now with the new gear.