The 1947 - Present Chevrolet & GMC Truck Message Board Network - View Single Post

69GMCLonghorn · 07-14-2009, 11:59 AM

It is a .78 overdrive. So here's the formula:

RPM= MPH x Gear Ratio x 336, divided by tire diameter.

So if you're running 31/32" tires and a 4.10 rear end, this is where you'd be at when doing 70 MPH:

Top gear 1:1, non O/D: 3050 rpm
Top gear 1:1, with O/D: 2350 rpm

The other advantage that they seldome talk about is that you can split all the other gears, too. So on the highway, you could have 3 gears, instead of one like I do. You'd have 3rd O/D, 4th, 4th O/D.

07-14-2009, 11:59 AM	#2
69GMCLonghorn Registered User Join Date: Dec 2006 Location: Porter Ranch, CA Posts: 978	Re: Gear Vendors Question It is a .78 overdrive. So here's the formula: RPM= MPH x Gear Ratio x 336, divided by tire diameter. So if you're running 31/32" tires and a 4.10 rear end, this is where you'd be at when doing 70 MPH: Top gear 1:1, non O/D: 3050 rpm Top gear 1:1, with O/D: 2350 rpm The other advantage that they seldome talk about is that you can split all the other gears, too. So on the highway, you could have 3 gears, instead of one like I do. You'd have 3rd O/D, 4th, 4th O/D.