Nearly 15MPG with 4.57's and no OD in a C30!

[66Submarine]

A combination of a tach error in the truck and a speedo error in the pace car had suggested the truck I'm driving had 4.10 gears, but the numbers didn't quite agree with the speedo app I downloaded onto my phone (they also didn't seem to agree with the numbers for 4.57's).

I ran it up to a little over 3,100RPM yesterday and the numbers pretty much agreed with 4.57's like I originally thought it had--65MPH.

Checked it out again and there we go--4.57's. The tach apparently reads low at the speeds where I normally drive.

The '63 I bought actually does have a 4.10 third member in it, so I'll swap it in shortly. I've been driving with the flow of traffic, so I expect I'll get a little fuel economy boost with the reduced engine speed of the taller gears.

I got just a touch less than 15MPG driving across Atlanta to Conyers the other day. Not too bad for an old one ton truck IMO!

More to come after I swap the taller gears in...

[DLW]

What height is your tires? That's some good MPG for that rig.

You can always double check your findings also by doing the tire rotation vs DS revolutions check and see if you come up with around 4.5 revolutions, it's pretty accurate actually, but then again you probably already know this ..

[franken]

Might want to pop the cover and look at the numbers, or actually test by spinning and counting before getting too excited...

[66Submarine]

Originally Posted by DLW

What height is your tires? That's some good MPG for that rig.

32" remolded 235/85R16 mud tires.

Originally Posted by franken

Might want to pop the cover and look at the numbers, or actually test by spinning and counting before getting too excited...

That's what I did; 4.57's in this one.

I actually (very casually) did it when I bought the truck and came up with 4.57's, but when the tach and late-model car speedo suggested 4.10's I figured I must have messed up when counting it somehow.

I had the cover off the '63 and saw the 41:10.

[BigDan3131]

Going to 4.10's will only lower that rpm by about 100. I'm saving up to go to 3.54's or 3.73's after I do the OD swap.

[66Submarine]

Who told you that? 4.57/4.10=1.11--3,000RPM on the highway now will be roughly 300 less/2,700RPM with the 4.10's.

I have at least one set of 3.73's for a 14 bolt, but I don't think I want to gear it that tall. I'll probably go that route if I drop a 454 in it, though (unless I end up with an OD).

[markeb01]

Those 32" tires are helping keep the rpm's down at highway speeds. I'm running 4.10's in mine and spin 3,000 rpm's at 60 mph on 29" tires (when new). A swap to 4.10's should make a noticeable difference.

[Old Truck Man]

The old 64 we used for a wrecker had 410 gears. I ran a set of really tall 19.5 tires . with a four speed and a one BBl intergrated head 250 It got exceptional fuel economy. not much power in high gear if loaded It needed to be downshifted on any steep grade. I once blew the rear in a 66 3/4 ton. I took the chunk from a 1950 one ton. It had 538 gears. you could feather the clutch and take off in high gear.

[66Submarine]

Yeah, the tire diameter is certainly a factor (it actually had 34.5" tall 7.50-17's on it when I bought it!).


5.83's? That's a rare bird! Only available as an option in '63 P30 step vans AFAIK.

[Old Truck Man]

Originally Posted by 66Submarine

Yeah, the tire diameter is certainly a factor (it actually had 34.5" tall 7.50-17's on it when I bought it!).


5.83's? That's a rare bird! Only available as an option in '63 P30 step vans AFAIK.

5.38 not 5.83. I pulled that chunk from a 1950 chev one tom that has single wheels and has a 9 ft 6 inch stepside bed.

[hopped up]

Wow! I only get 12 mpgs with 3.73s and some tall whitewalls (29inches?) tall

[66Submarine]

Originally Posted by Old Truck Man

5.38 not 5.83. I pulled that chunk from a 1950 chev one tom that has single wheels and has a 9 ft 6 inch stepside bed.

Thought it must have been a misprint. 5.38? Never seen any mention of that ratio before. Any extra info on it, like maybe the tooth count? No mention of it in the 1950 GM info kit (just the normal 5.14).

[factorystock]

Originally Posted by 66Submarine

I had the cover off the '63 and saw the 41:10.

What series '63 is this, a C20 or C30? A C 20 uses the HO52 pumpkin while the C 30 uses the HO 72 pumpkin. Are they interchangeable? First year for the optional 4.10 HO72 was 1968.

[66Submarine]

It's a DRW C30, but it's not the original third member (it was stamped '70 or something IIRC).

The third members are supposed to interchange between the 52/72, but I'm not what the difference is between the two--or if there even is any (I don't have any to check and they'd need to be the same era as mine, anyway).

There is some level of variation, though; AFAIK any third member bolts into any 52/72 housing, but the internal parts differ through the years. I.E., I saw someone the other day that tried to swap just the gears (why???) and found out they wouldn't work in his third member and the driveshaft had to be shortened for the new one. With a production run of 1947-1972 they certainly had time to change stuff up!

I know a little bit about them, but not really that much. There are a few 4x4 people that are pretty into them that know more about all the differences.

I figure if I ever manage to blow one up I'll just stick another third member in or swap the whole deal for a 14 bolt.