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Gear ratios
http://www.policeinterceptor.com/revconforums/viewtopic.php?f=10&t=69
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Author:  Hal [ Thu Jun 04, 2015 5:57 am ]
Post subject:  Gear ratios

I'm getting ready to install a 403 into my sons 1972 Flat Nose and was wondering if I should change from the 3.55 gear to a 3.21 we also have. The 403 and 3.55's came out of a 26' GMC but the Revcon is lighter and fuel mileage is a real concern. Besides, aren't the 3.21's supposed to be a stronger gear?
Thanks for any input.
Hal

Author:  Daveinet [ Tue Jun 09, 2015 2:50 am ]
Post subject:  Re: Gear ratios

I think the answer should probably be based on if you intend to tow or not. If you are not going to tow, you can get away with taller gears, and get the mileage. However if you do want to tow, then I would change the ratio. The 403 did pull the heaviest of the Olds version Revcons, so it will do it just fine, but I think the biggest issue will be towing.

Author:  doc [ Mon Sep 05, 2016 1:32 pm ]
Post subject:  Re: Gear ratios

I don't know about the 403 but I'll assume it is similar to the 455.

The 'sweet spot' in the 455 is 2500 - 3000, thererfore you want a ratio that will put you in that range at the speed you like. My speed is 65-70, which with my 3.40, puts me at 2500-2600. Above that, the wind force starts to have an effect and the mileage will suffer. At my speed, I average 10-11 mpg.

IOW, you will NOT get better mileage by putting taller gears in.

Thom
Revcon 250
#71-0004

Author:  Daveinet [ Sat Sep 10, 2016 4:13 pm ]
Post subject:  Re: Gear ratios

My experience with speed has been that the exhaust has a profound affect on high speed efficiency. Back when I was running the stock 454, my mileage used to nosedive the faster I went. After adding headers and straight through mufflers, comparing 65 mph to nearly 90 mph, I found the mileage only went 8.5 to 8.2 - not enough to claim any difference. Normally headers and free flowing exhaust does not affect mileage that much, but most comparisons are done at more moderate speeds.

As far as gearing, the 403 does not have much power, so allowing it to turn faster applies more torque to the drive wheels at speed, so throttle response is better. There ends up being very little mileage loss as a result.

Going the other way, what I found was that taller gearing caused the torque converter to slip, so any gain was counteracted by the TC. Once I switched to a 4 spd with a lock up torque converter, then I gained about 1 mpg. All this is with the 502, I'm not sure taller gearing would be practical with a 455 or 454, as it doesn't have enough power to hold speed.

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