gearing questions

[bigboi328]

i have a 65 c10 with a 292 i6-4spd with granny gear trans and a 12 bolt-4.11 diff. my question Is what gear ratio do I need to hit highway speed. in this gear ratio I top out at 55mph. anyone have any ideas.

[TwoFiftyShifter]

Had a 4 speed creeper gear tranny hooked to 3.42 gears and another with 3.08 gears. With the 308 it was 2000RPM at 60MPH, nice on highway but the gears are gapped like crazy!

[66Submarine]

Depends on tire size, what "highway speed" means to you, and what you consider an acceptable cruising rpm/engine speed.

My '65 C30 has 4.10 gears with a 32" rear tire, and it's the truck I drive everywhere; I went somewhere today and drove with traffic in the left lane going 60-65MPH the whole way. It'll run 70+ without any trouble, and I figure the actual top speed is probably around 100. The engine is spinning 2,900RPM @ 65MPH though, and many today would call that "screaming"; my father's '66 with tall gears and tires is going nearly 85MPH at the same engine speed.

FWIW, the 3.07 gears are commonly used as a swap.

BTW, are you sure you actually have 4.11 gears? Almost all of these trucks (at least IME) came with 3.73's from the factory.

[GVDobler]

There is an excel spreadsheet floating out there that will figure it out based on input such as tire size, etc. Tirerack or somebody has it.

The key is usually to have an overdrive transmission. A regular trans has a 1 to 1 gear ratio as high gear. An overdrive transmission or 4-speed auto trans will have something like 1 to .7 as high gear.

That is why people put 2-speed rear ends in trucks. That way they get to get to keep granny low and still get a decent highway drive rpm.

Tall tires (higher than 27" standard), high ratio rear (3:08) and a standard trans will give ok highway rpm but low end grunt suffers, unless you have higher HP engine to power through.

It is all math, period. The devil is in the details of what size everything is.

I like 2000 rpm at 65, but you have to match everything up to get it.

[bmacneal]

There are many calculators out there. just google "gear calculator".

a basic one is on summit's website.

http://www.summitracing.com/expertad...tio-calculator

[GVDobler]

wallaceracing.com


This data input was with 27" diameter tires and 1 to 1 high gear
Engine RPM 3.08 rear----3.73 rear-----4.11 rear
8000 RPM 208.64 MPH 172.28 MPH 156.35 MPH
7500 RPM 195.60 MPH 161.51 MPH 146.58 MPH
7000 RPM 182.56 MPH 150.74 MPH 136.81 MPH
6500 RPM 169.52 MPH 139.98 MPH 127.03 MPH
6000 RPM 156.48 MPH 129.21 MPH 117.26 MPH
5500 RPM 143.44 MPH 118.44 MPH 107.49 MPH
5000 RPM 130.40 MPH 107.67 MPH 97.72 MPH
4500 RPM 117.36 MPH 96.91 MPH 87.95 MPH
4000 RPM 104.32 MPH 86.14 MPH 78.18 MPH
3500 RPM 91.28 MPH 75.37 MPH 68.40 MPH
3000 RPM 78.24 MPH 64.60 MPH 58.63 MPH
2500 RPM 65.20 MPH 53.84 MPH 48.86 MPH
2000 RPM 52.16 MPH 43.07 MPH 39.09 MPH
1500 RPM 39.12 MPH 32.30 MPH 29.32 MPH
1000 RPM 26.08 MPH 21.53 MPH 19.54 MPH

[bigboi328]

I have 29 inch tires. and I counted the gears. they are 4.11. I have a complete diff from a 67 c10 that has 3.73. what I mean by screaming is at 60mph it will not go any faster and you hear it whine. I will later change out the complete drive trane to a 5.3 from a Silverado-5spd manual- and new gears

[66Submarine]

It has bad problems (or the throttle linkage is adjusted improperly and the throttle isn't opening all the way) if it won't go over 60MPH with the throttle wide-open; like I said, my truck has 4.10 gears with a tire that's only 10% (or so) taller than yours.

The 3.73 rear will give you a 10% change, which pretty much means 60 will become 65.

[bigboi328]

thanks ill check my linkage

[66Submarine]

Yeah, if it actually won't go over 60 something is wrong; you shouldn't even be making max peak/rated net HP until something like 75. It'll be wound up nice and tight there, don't get me wrong, but it should do it.

I'd personally probably also swap in the 3.73 rear if it's not a big deal for you to do so (after you fix whatever's making it weak, of course). Not a giant difference, but it'll help out some.