Normal RPM's @ 65 mph?

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Mar 15, 2005 | 11:59 AM

newrob1

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Normal RPM's @ 65 mph?

I have a 92 with a stock 302. This is a project truck amidst a build for Baja 1000 as a chase vehicle. I recently built a custom trussed 9" rear and geared it with 4.56's based upon my 38" tires and E4OD. I have now upgraded to a built C6. Here is my challenge; I have now gone from a 4 speed to a 3 speed and now run around 3000 rpm's @ 65mph. I want to get closer to the original intended rpm's for this speed keeping in mind that this vehicle is being purpose built but needs to drive on the highway to get to it's distination. Normal speeds during Baja are 25 to 65, and right now my 25 to 50 is great but 50+ and I'm pushing too hard. Originally the 8.8 I pulled out had 3.55's but going to 38's I went with 4.56's. Anyone with "real" hands on gearing experience please respond. Thanks in advance.

Mar 15, 2005 | 12:40 PM

jdmorg

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From: just outside of Phila.

ok....well, I'm a weekend warrior at best with a bone stock '88 302 C6...2650 RPM @60mph.

Mar 15, 2005 | 06:39 PM

greystreak92

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From: Gateway to the West

Losing the OD will push your RPM's that high. In third gear @ 65 in an E4OD equipped truck with stock tires and 3.55's you are still turning about 2700 RPM. The OD is what kept your RPM's low before. The C6 with the same gearing in the axle should be giving you nearly identical RPM's as the E4OD did in third since OD automatics still use a 1:1 (direct drive) ratio for the last gear below the overdrive gear. To lower your RPM's, "shorten" your gears to say 4.11's this will drop the 65 mile/hour RPM's below 2500.

Mar 16, 2005 | 01:02 AM

justshootme84

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I think the problem is the 4.56 gears are too "short" for 38" tires, particularly with the C6. That is the ratio I calculated for 35" tires on mine with the Ford 9", so you may need "taller" gears. Something like a 4.88:1 or even a 5.13:1 to get back in the stock RPM's/powerband. WHile I'm not 100%, I think the pinion gear is weaker for the 4.88 compared to the 5.13. So if you're application is racing or prerunner, you might want the 5.13 gears.

Mar 17, 2005 | 01:41 AM

greystreak92

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From: Gateway to the West

And justshootme has the right idea now that I look this over. You have to stop thinking in "overdrive" and think in "drive" which will naturally have a higher RPM than OD will. So you will never get RPM numbers as low as the E4OD did in its top gear with a tranny that has no overdrive gearing. If you do, youwill never get enough low-end torque from the first two gears....DUH! What was I thinking.

Mar 17, 2005 | 08:05 AM

rlh

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Quote:

Originally Posted by justshootme84

With the same size tires, going to a lower gear (numerically higher) will increase the rpms at 1:1.

Mar 17, 2005 | 11:49 PM

justshootme84

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Quote:

Originally Posted by rlh

With the same size tires, going to a lower gear (numerically higher) will increase the rpms at 1:1.

I agree with that. My RPM's at true 65 mph (3000rpm) decreased with the same gear ratio of 3.50:1 when going from 31" to 35" tires (now 2400rpm). IN order to get my RPM's back up to where they should be, I need to go from the 3.50 gears to 4.56's keeping the tire size where it is at 35". THe reason for doing this, which Greystreak92 explained very well in a previous post, is to get back into the engine's powerband. If I have no power or feel "sluggish" at 50+ mph, I need to increase the gear ratio to a higher number. This is what I assumed the original question to be. The 4.56 gears for 35" tires, 4.88 for 37". The 302 reaches peak HP at 3800 RPM, while my calculations are for my 351W H.O. at 4000rpm, but both are close.

Adding taller tires really skewed my speedometer accuracy, too. Going 65mph suddenly become closer to 80mph in actual speed. SO be sure to calibrate the speedo before taking any readings on the RPM's to figure out your gear or tire ratio.