Alright so my '51 chevy doesn't do very well on the highway. I get up to about 60 and the engine is just screaming. I've been told that the stock gear in the car was a 4.11:1 and that in order to fix it I needed to put a 3.55:1 rear end out of a PG rear end. I had an extra rear end laying behind my fathers shop so I hauled it in and took the rear cover off. I read the side of the pinion gear and it said 39 11. Then I counted the gear and it had 39 teeth. 39 / 11 = 3.55. So I ***umed that this was the 3.55:1 rear end that I needed and that changing to this rear end would fix my problem and allow me to run interstate speeds. Well for safe measure I took the rear plate off of the current rear end and to my suprise it read 37 12. 37 / 12 = 3.08. I didn't even know that they made closed driveline chevies in gears that high. I'm totally confused, am I figuring rear end ratios wrong? If the gear is that high why am I still having problems going interstate speeds? What am I missing? HELP!?!?!
Ring gear tooth divided by pinion tooth count equals gear ratio. 3.55 ratio with 26" tall tires at 65 mph, 2970 rpms, without any overdrive. 3.08 with 26" at 65, 2590 engine rpm without overdrive. Taller tires would make for less engine rpms at the same vehicle speed. But they would look like ****.
Can't help you w/the digits posted on the rear, but if you have a 3.08 final drive ratio it is an excellent highway cruising gear. Anything up to a 3.55 is fine. The 3.08 is good for mileage, but as you move up toward the 4.00's you will be trading "mileage" for "acceleration".
See that's what I thought too. I'm wondering if there is some inaccuracy in my measurement, or my understanding.
Sounds like you are only counting one set of teeth? You need to count both the ring and pinion as was mentioned earlier. Forget about what is written on anything. Count both sets and that will give you the right figure.
To correctly calculate RPM for a given FDR and MPH you need the "rolling radius" of the rear tires. here is a real handy link for figuring FDR's, MPH, RPM, rolling radius, speedo gears etc. http://tri-5.chevyrides.com/speedo.php
Alright, I'm an idiot (see there's a "newbie" under my name). I didn't count the pinion gear, I relied on what was stamped on the ring gear. When I did manage to get a chalk mark on the pinion gear I was able to count it and it had 10 teeth on it. The ratio that is currently in the car is 37 / 10 = 3.70 not 37 / 12 = 3.08 as I had previously thought. The spare rearend that I have is 39 / 11 = 3.55 so I will be switching over to it after all. I didn't think there was a higher gear than 3.55 and luckily I was right. Hopefully when I get the switch made I will be getting better highway speeds. Thank you so much for your help everyone!
That is what I figured might have happened, thanks for letting us know. You won't see a big change from 3.70 to 3.55 ratio, but it should get you another 5 mph....
Looks like you might have a 1954 only rear ***embly in there now. I believe that was the only year where they had the 3.70 option using the 37-10T configuration. My Master Parts catalog shows a 41-11T (3.72) version, but I have not seen one. Most early cars used the 37-9T version which is hell on the highway with the low RPM six. You are lucky to have the PG unit available. You are correct that it uses 39-11T for 3.54 overall. Make sure you have the right brake set-up. The early ones used the Huck brakes. The later ones used the self energizing Bendix style. You want the laters...they make a big difference. If this doesn't work to your satisfaction, your next step is to do a T-5 ****** with an open drive-line conversion. Most use a Tri-Five era open driveline to get close on width. Tom
still not entirely sure what the original rear end ratio was (3.70, 4.11 whatever I counted teeth so many times and got either 9 or 10 each time since the pinion gears are kinda hard to count) but now that I have the new PG rear with a 3.54 my old chevy will do 90! I never would have dreamed that that old engine would be able to do that. My cousin was following me in is corvette and the speedometer only goes up to 85 on it, he was turning the needle around backwards and my old chevy was pulling away from him. I could not be more pleased with this swap.
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