I just took my car to an axle shop to have the rearend rebuilt and I think they put in the wrong gears. I need help figuring it out before I call them about it. I had the gears replaced. I went from 4.11's to 3.55's. When I had my 4.11's, 60mph was at 3000rpm. Well, now 3000rpm feel closer to 50mph. I swear, the gearing feels even higher than 4.11. The car seems to wind out much quicker when I'm trying to keep up with traffic. To count the ratio with the tires on, you jack up one rear wheel, and put the car in neutral. You put a mark on the driveshaft, and one on the tire up in the air. Then you spin the tire one full turn, and count how many times the yoke makes a full turn. Then, multiply that by two for a open rearend. Is this right? For one turn of my tire, the yoke spun a just a bit over two full turns. If I had to guess at a fraction, i'd say 2 1/8 turns. Times two, that is a bit over 4. Which means it's a 4 something gear, right? The speedo has always been off too. Previously, it was 10mph under. Now, it seems like an honest 20mph under. The other night on the way home, with the speedo reading 70mph, the tach was at 3100-3200. It used to be at 70mph it was 2800-2900. So, does it sound like they screwed up and gave me even higher gears than 4.11? Or is there not much difference between 3.55 and 4.11? My gut says they screwed the pooch and put in 4.11's again. What do you think?
If you were at 3000 RPM at 60 MPH previously how could you be at 2800 -2900 at 70? Your RPM's would go up not down. Anyway, it sounds like they might have rebuilt your rearend and not changed the gear ratio. You should notice quite a difference between a 3:55 and 4:11.
I'm wondering if they didn't hear you say the '3' when you said 3.55 and maybe thought you wanted a set higher in the 4's. The difference between 3.55s and 4.11s is noticable. I had 4.10's in my 57 and swapped them for 3.55's..there was a big difference it highway speed in rpm's.. I can't remember exactly, but i think it was between 3 and 400 rpm difference.. What rear end is it?
unless you had the speedo gear in the ****** changed too, the rpm now would be the same as it was before at any given speedo reading. Unless there is error with the tach or speedo.
It's a 8" Ford open rear. I mis-spoke about the RPM and speed, i'm used to translating it in my mind! I meant 70mph, not 60. Previously, I followed my girlfriend while she was doing 60mph, and it was showing as 70 on my speedo. So when I see 70 on mine, it automatically registers as 60 in my head. Makes it confusing to drive another car, that's for sure... When I was talking to the shop when I took the car in, they were going to find some 3.73, or 3.55 gears. They were able to find some 3.55's, and supposed to have put them in. I thought there should be more of a difference than what I have. I wish I had did the ratio count before taking it in. They confirmed that it had 4.11's when they took it apart. The whole carrier was wasted, the pin had a flat spot worn in it, and the side gears had extreme wear. I always thought the car had a Posi rear, it just turned out it was binding and locking up under load...
I know it's a pain in the *** to do it, but i'd pull the carrier out and count the gears to find out exactly what they are.. If the're not what you were told they were supposed to be i'd call the shop and find out what happened..It could have been an honest mistake.. You're lucky you found anything in the 3's .. I've been looking for a while and can't find ****..only carriers in the 2's.. Hell, i'd LOVE to have the 4.10's!
No need to yank the carrier, just spend a few minutes doing a gear check using the method outlined in the tech thread. It's a pretty simple check and it's easy to see the difference between 3.55 and 4.11s. Mojo, from the way you described your ratio check I'd say they did 'screw the pooch'. Definitely 4-series gears in there. Let the shop that did the rear end work remove the carrier, they'll need to anyway when they put the correct gears in.
I'm running an 8 inch rear with 3.55 gears on my coupe and I know that at 65 mph I do about 2500 rpm. I know different transmissions can make a difference, but your rpm's should have gone down, not up. My coupe doesn't have any trouble keeping up with traffic at 70.
Thanks for the opinions. I'll have to call the shop monday. I think they have got my gears mixed up with someone else's. They are a busy shop, and mistakes happen. It's just that car rental fees start to add up. I might step down even lower, maybe a 3.23 set or something. This car is a old mustang, it gets unbelievably bad gas milage. It jumps from a stop like a bear, but it's expensive to feed!
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