Rear diff gear ratio?

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Sep 5, 2018 | 12:13 PM

Tommy K 47

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Rear diff gear ratio?

Can Anyone tell me what rear diff ratio I have by knowing my RPM at a certain speed? I was told it was the stock 3.08 but not sure. I have a 78 L48 with a th350 and at 3000 rpm I’m going just under 75 mph.
Thanks

Sep 5, 2018 | 12:39 PM

forman

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From: Randolph nj

What’s your engine suffex... first 3 letters

Sep 5, 2018 | 12:40 PM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

Quote:

Originally Posted by Tommy K 47

You have to know the actual tire diameter. Give me that and I can tell you the gear ratio.

If I make some assumptions and assume you have a 27 inch tire with about 5% converter slip, your rear end ratio is the stock 3.08.

Lars

Last edited by lars; Sep 5, 2018 at 01:19 PM.

Sep 5, 2018 | 02:48 PM

HeadsU.P.

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From: Cool Northern Michigan

Quote:

Originally Posted by forman

What’s your engine suffex... first 3 letters

Those code numbers prove nothing but what was in there 40 yrs ago. Who knows if it has been changed.

Sep 5, 2018 | 04:01 PM

Tommy K 47

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Quote:

Originally Posted by forman

What’s your engine suffex... first 3 letters

thanks Forman but I can’t currently see the number and that wouldn’t help me anyways because I already know it came stock with a 3.08. I’m wondering if the previous owner might have changed the ratio and I’m not currently able to turn the tire/wheel one complete revolution while counting how many times the driveshaft turns or I could figure it out that way.

Sep 5, 2018 | 04:26 PM

Tommy K 47

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Quote:

Originally Posted by lars

Lars

Thank you Lars, yes they are the original p255/r60 15’s and I believe they were 27”. I’ll have to look them up to be sure. They’re the Firehawk Indy 500 brand.
I thought maybe the rpm seemed a little high for that speed but I guess not. I’ve read posts where guys have run the 700r4 with a 3.73 and with the 1:1 3rd gear they were at 3500 rpm at 70 mph and with the overdrive it dropped to 1900 rpm at that speed? I know the overdrive really helps but I didn’t know to what extent. Does that sound right to you? Thanks Lars 😊

Sep 5, 2018 | 04:48 PM

lars

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From: At my Bar drinking and wrenching in Lafayette Colorado

Quote:

Originally Posted by Tommy K 47

Thank you Lars, yes they are the original p255/r60 15’s and I believe they were 27”. I’ll have to look them up to be sure. They’re the Firehawk Indy 500 brand.

I thought maybe the rpm seemed a little high for that speed but I guess not. I’ve read posts where guys have run the 700r4 with a 3.73 and with the 1:1 3rd gear they were at 3500 rpm at 70 mph and with the overdrive it dropped to 1900 rpm at that speed? I know the overdrive really helps but I didn’t know to what extent. Does that sound right to you? Thanks Lars ��

The 255/60R15 tires are 27" as I assumed, so based on the numbers you provided you have a 3.08 rear end - no doubt about it.

Keep in mind that the 700R4 has a lockup converter, so there is no converter slip - you can't compare that tranny to your TH350. You have about 5% converter slip which raises your rpm at any given speed. Yes, a 700R4 in overdrive with the converter locked will run under 2000 rpm at over 70 mph: I had an '85 C4 with that tranny, and that's what it ran.

Lars

Last edited by lars; Sep 5, 2018 at 04:51 PM.