Gunny Highway

iTrader: (1)

I was originally going to go with some 3.23's, but the more I read the more I want 3.42's. The problem is, I do a lot of highway driving, so fuel consumption is somewhat of an issue. The other problem is, is like to drive somewhat spirited when I'm off the highway.

Right now at 80 MPH, I'm sittin' on 2400 RPMs. What would my RPMs be with 3.23's and 3.42's? I'm sure there's a formula for this, but I don't have it.

Also, is there THAT BIG of a performance difference between the 3.23's and the 3.42's? Anythings better than the 2.73's back there now.

Oh yeah, a SLP Posi will be going in at the same time.

Thanks Guys.

-The Gunny

4L60bliss

Blood makes the grass grow green

I originally had 2.73s, then went to 3.73s and then ended up w/ 3.42s. The 3.42's don't have quite the "pep" the 3.73s did, but traction from a stand-still is better as well as highway RPMs. Still tons better than the 2.73s. My personal opinion is that the 3.42's are a pretty good all around gear.

Another trick is a higher stall converter (if you have an automatic). The increased stall speed will raise you RPMs putting you in your powerband quicker and keeping you in the powerband, yet still has the lockup feature so on the highway its like having a stock converter (because once its locked, there is not slip). Just a few hundred RPM increase will give you a night -vs- day difference in acceleration. Just some food for thought.

P.S. where in GA are you from?

Schultzy89GTA

Gunny,
These calculations are not exact but assuming that you are running a 700R4 with the standard gear ratios and the tire diameter for a 245/50 R16 tire, here is what I get...

2.73's 80 MPH @ 2,000 RPM

3.23'S 81 MPH @ 2,400 RPM

3.42's 80 MPH @ 2,500 RPM

3.73's 82 MPH @ 2,800 RPM

The thing that confuses me is that you say that you currently run 80 MPH @ 2,400 RPM. Are my transmission or tire diameter assumptions incorrect?

Thanks,

-Schultzy

Gunny Highway

iTrader: (1)

I'm running what ever came on the Z-28's that year. I changed my tire size when I bought the Kuhmo's. . .245/55's I think. Not sure off hand.

Woodstock and you?

iroczrockz

I think the gunnies rpm at 80mph is closer to stock. My car has 2.73 gears with a 26" tire diameter and it runs about that. 2000 rpm is usually around 70-75 I think (just from memory).

4L60bliss, we're you not happy with the 3.73's? Do you prefer the 3.42 over the 3.73? Just wondering because I am aiming for the 3.73's in my daily driver. I have the 700R4 too. At some point I WILL have to drive my car across the state but that trip may come once or twice in 4-5 years. Do you think you could get better 60' times with 3.42's?

4L60bliss

Gunny, I'm in Savannah, GA

Iroczrockz, I was running 3.73s and traction was a major issue. Wet weather driving was 100% unsafe and trying to launch the car was impossible. I also felt that that was too much gear for my TPI setup. Not to mention I do alot of highway traveling, and I didn't like cruising at almost 3,000 RPM doing 80MPH. Granted, the 3.42 is only a small step down from 3.73, but it helped on the highway, I get better traction, but it also lost some of its "pep", but I think it was a good trade off. Overall, I'm more happy w/ the 3.42's then I was w/ the 3.73, but its just a personal opinion, which is always subject to change

iroczrockz

What size tires we're you running with the 3.73's? I know right now I MUST get bigger tires. I still have the stock tire size on my car. 215/65/15, the tire spins at about 30mph when I get on it with 2.73's and a "one legger rear end".

BORLAZ06

I had 3:42s. It was perfect for highway and I had good gas milage.

edit: jus saw that you had an auto. 3:42s still aint bad. good all around gear.

4L60bliss

I'm running 255/50ZR16's, however my engine is not stock

iroczrockz

Yeah, I thought it wasn't from your sig. I am just thinking ahead. I had planned on getting a couple of 17x9.5 rear wheels and running a 275/40/17. I was wondering if that would help my traction problem with 3.73's. I am sure it would but I bet they would still break loose pretty good. Anything is better than 215/65/15's.

Kingtal0n

ok man. if you like your MPG, dont go over 3.73 unless you get a T-56. 3.73 with a 700R4 is the same as a Th350 with 2.73's. really really bad RPM on the highway. I've compiled a list for people to help them with this issue... hope it helps.
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Keep in mind that the 98-up LS1 T56s will not work on a SBC.

here are match ups:

<font color="FF0000">T-56 (1994-1997) final gearing ratio, assuming .50:1, with a 255/60/R15 Tire Size:</font>
2.73 rear gear 1.365:1 at 60 MPH thats 1017 Rpms. at 80 MPH thats 1356 RPM. at 100 MPH thats 1695 RPM.
3.08 "" 1.540:1 at 60 MPH thats 1147 Rpms. at 80 MPH thats 1530 RPM. at 100 MPH thats 1913 RPM.
3.23 "" 1.615:1 at 60 MPH thats 1203 Rpms. at 80 MPH thats 1605 RPM. at 100 MPH thats 2006 RPM.
3.48 "" 1.740:1 at 60 MPH thats 1296 Rpms. at 80 MPH thats 1729 RPM. at 100 MPH thats 2161 RPM.
3.73 "" 1.865:1 at 60 MPH thats 1390 Rpms. at 80 MPH thats 1853 RPM. at 100 MPH thats 2316 RPM.
4.11 "" 2.055:1 at 60 MPH thats 1531 Rpms. at 80 MPH thats 2042 RPM. at 100 MPH thats 2552 RPM.
4.54 "" 2.270:1 at 60 MPH thats 1691 Rpms. at 80 MPH thats 2255 RPM. at 100 MPH thats 2819 RPM.
5.11 "" 2.555:1 at 60 MPH thats 1904 Rpms. at 80 MPH thats 2539 RPM. at 100 MPH thats 3174 RPM.

<font color="FF0000">700R4 final gearing ratio, assuming .73:1, with a 255/60/R15 Tire Size:</font>
2.73 rear gear 1.992:1 at 60 MPH thats 1485 Rpms. at 80 MPH thats 1980 RPM. at 100 MPH thats 2475 RPM.
3.08 "" 2.248:1 at 60 MPH thats 1675 Rpms. at 80 MPH thats 2234 RPM. at 100 MPH thats 2793 RPM.
3.23 "" 2.357:1 at 60 MPH thats 1757 Rpms. at 80 MPH thats 2343 RPM. at 100 MPH thats 2929 RPM.
3.48 "" 2.540:1 at 60 MPH thats 1860 Rpms. at 80 MPH thats 2481 RPM. at 100 MPH thats 3101 RPM.
3.73 "" 2.722:1 at 60 MPH thats 2029 Rpms. at 80 MPH thats 2706 RPM. at 100 MPH thats 3382 RPM.
4.11 "" 3.000:1 at 60 MPH thats 2236 Rpms. at 80 MPH thats 2981 RPM. at 100 MPH thats 3727 RPM.
4.54 "" 3.314:1 at 60 MPH thats 2470 Rpms. at 80 MPH thats 3293 RPM. at 100 MPH thats 4117 RPM.
5.11 "" 3.730:1 at 60 MPH thats 2780 Rpms. at 80 MPH thats 3707 RPM. at 100 MPH thats 4634 RPM.

<font color="FF0000">Th350 final gearing ratio, assuming 1.0:1, with a 255/60/R15 Tire Size:</font>
2.73 rear gear 2.73:1 at 60 MPH thats 2034 Rpms. at 80 MPH thats 2713 RPM. at 100 MPH thats 3391 RPM.
3.08 "" 3.08:1 at 60 MPH thats 2295 Rpms. at 80 MPH thats 3060 RPM. at 100 MPH thats 3826 RPM.
3.23 "" 3.23:1 at 60 MPH thats 2407 Rpms. at 80 MPH thats 3210 RPM. at 100 MPH thats 4012 RPM.
3.48 "" 3.48:1 at 60 MPH thats 2549 Rpms. at 80 MPH thats 3458 RPM. at 100 MPH thats 4248 RPM.
3.73 "" 3.73:1 at 60 MPH thats 2780 Rpms. at 80 MPH thats 3706 RPM. at 100 MPH thats 4633 RPM.
4.11 "" 4.11:1 at 60 MPH thats 3063 Rpms. at 80 MPH thats 4084 RPM. at 100 MPH thats 5105 RPM.
4.54 "" 4.54:1 at 60 MPH thats 3383 Rpms. at 80 MPH thats 4511 RPM. at 100 MPH thats 5639 RPM.
5.11 "" 5.11:1 at 60 MPH thats 3808 Rpms. at 80 MPH thats 5078 RPM. at 100 MPH thats 6348 RPM.

As you can see, the 94-up model T-56 is AWSOME for highway driving.with 4.11 gearing we are at almost the same RPM on the highway as a 700R4 with 2.73 gearing. and as for the Th350... well i wouldnt even consider it a highway tranny now-a-day.

82camaro

Are you sure those are correct? At 80 mph with 3.73's and a 700r4 with 255/60 tires I calculate 2250rpm--not 2706. Then at a 100 I get 2800rpm--not 3382. If you used .70 for overdrive it would be closer.

The other 'thing' you must consider is the rpm band of the engine. If you run a really 'good' cam that doesn't make much power below 2200rpm(not efficient), you won't want to be running lower than 2200 at your cruise rpm--mileage would be worse. However, if you run a stock TPI(or near stock) running too deep of a gear ratio can cause you to need to shift into overdrive before the end of a quarter mile and waste fuel.

iroczrockz

Those figures don't seem right with my car. I have a 700R4 and 2.73 rear and it never has been that way. At 110mph I am just turning around 3000 rpm.

Kingtal0n

the figures are calculated exactly as you see them.

the 700R4 doesnt use a .73:1 overdrive gear. thats just what i used to under estimate.

if you do that math yourself you'll see its perfectly correct for the ratio provided.

Everyone knows a 700R4 with 2.73 gears and a 255/60/R15 tire runs about 2K on the highway...
I listed: at 80 MPH thats 1980 RPM

which is damn near perfect. somtimes you think you have a 2.73 but you dont.

Trust me, the figures are accurate.
<b>At 80 mph with 3.73's and a 700r4 with 255/60 tires I calculate 2250rpm--not 2706. Then at a 100 I get 2800rpm--not 3382.</b>

How are you calculating those numbers? Im using Thirdgen.Org's calculator, not my own windows version and piece of paper.

<b>At 110mph I am just turning around 3000 rpm.</b>
dude then you dont have a 2.73 rear gear. try again.

I have a 2.73 with a700R4. 80 MPH IS 2000 rpms. Trust me, the chart is correct.

Pro Built Automatics

If you are looking to "keep" the gas mileage you have now on the freeway, and you want "better" around town performance, then go with a A.C.T. 9.5" or the Vigilante 9.5" lockup torque converter. You can pickup 3-6 tenths in ET. I would install a posi with your gears. You will not gain much in performance with just a rear end gear change as compared to what you will pick up with a good (matched to your engine) torque converter change. Do some research if this does not sound right to you or you doubt me on this. If you have any questions call me at 800 7763288.

82camaro

Well I was using the f-body.org/gears site. I tried it again and it was down. Tried it a few minutes later and the results are more like yours now. 2600 rpm if I use .7 for overdrive. When you use .73, are you assuming the torque converter isn't locked?

Gunny Highway

iTrader: (1)

How much affect does timing have on your RPMs at highway speeds?

iroczrockz

Well, I don't know what is amiss here but all I know for a fact is my car. I mean, I have owned it for 12 years and the codes in the glove box and the vin both indicate 2.73 gears. Even this website indicates that I have 2.73 gears and the bottom line is that I am not running that kind of rpm. I wonder if the calculations from the website is correct. Don't know and I am not saying your calculations aren't correct, I am sure they probably are. It is just that they both don't add up ya know. Oh well, I guess all that can be done is buy a set of 3.73's and try them out with my tranny. Maybe my tranny is in need of rebuilding. That could very well be the difference here. Thanks for the info though.

Kingtal0n

the calculations are WITH TC lockup. 1:1 tranny to engine. so are the TH350 calculations, which makes THEM slightly off (lower than they should be).

Everyone with a 700R4 and 2.73 gears will be @ 2000 RPMS @ 80 MPH. thats fact. following the same gear multiplications... thats no where NEAR 3K @ 110 MPH. thats very off. maybe YOUR converter is NOT locked up, and is slightly higher than stock stall speed? that would account for it.

Gunny Highway

iTrader: (1)

Are you talking to me? If it is indeed a fact (which seems to be the case) that at 80 MPH I should be sittin' on 2000 RPMs (700R4 w/ 2.73 Open), they why am I sitting on almost near 2400 RPMs. The timing was bumped up 8* and the tranny was just rebuilt. Can bumping up the timing really have THAT dramatic of an effect on the RPMs at cruising?? Or would that be a result of the converter not locking up on a freshly rebuilt tranny?(like 1 week old)

iroczrockz

See Gunny, I am having the same thing here and it seems it has always been that way. I had a 305 and now I have a 350 and the rpm's are the same. Timing is completely different with the two engines so I tend to believe IF there is a problem it would have to be the torque conv. or something. Did you replace the torque conv. when you rebuilt your tranny?

82camaro

Timing has absolutely nothing to do with it. Maybe your speedo is off? Correct speedo gears in the new tranny? Tach accurate? With an open rear car, block the tires, put it in neutral, jack one side of the car up(one rear tire on the ground and one off ground), mark the tire and the driveshaft, with a helper spin the tire exactly 2 turns while someone counts the turns on the driveshaft. The amount of turns is the gear ratio--should be about 2 3/4 in your case. (this won't work on a posi car)

Using light throttle and not pulling a trailer or going up a hill the torque converter won't slip much when above it's stall speed.

To try to make this as clear as possible here is the formula:

MPH = 'rpm' * 'tire diameter' / 'gear ratio' / '336'

those are what have an effect on mph
rpm = simple to figure you if your tach is accurate
tire diameter = 245 * .5 * 2 / 25.4 + 16 =25.6
gear ratio = 2.73 * .7 = 1.911
2400 * 25.6 / 1.91 / 336 = 95.7mph

2400 rpm at 80.9mph is 3.23 gears

RB83L69

For comparison's sake, my car has the stock gearing (3.73), and has made countless long road trips that way, including a couple of cross-country ones (and I do mean cross-country, as in all the way across the country). As it sits right now, 305 with XR264HR cam, low compression, stock carb & intake, it gets about 20 mpg on the highway dependably; 19.6 between here and Indianapolis the other day with my wife driving for example. With a 400, various cams all of which have been around 230° intake duration, and a 800 CFM Holley, it would get around 18 mpg on the highway. This is with a 5-speed, which has a little less overdrive than an auto. Around town, who knows what the mileage is, I don't keep up with it.

Is 18-20 on the road acceptable to you? Then 3.42 would be OK.

Stock tachs are notoriously inaccirate, since they are a trim package, not an instrument; and they almost always read higher than actual. Check it against a real test equipment tach and see if your engine is really doing 2400 RPM when your dash ornament says 2400. Until you have verified it, don't believe the number it gives you, it's probably wrong.

FYI... I checked mine, it's fairly close as these go, only about 100 RPM high at 3000; it shows 2950 at 80 mph with the T-5 .73 OD, 3.73s, and 245/50-16 (stock diameter, around 26.5") tires.

82camaro

What he said. I get about the same mileage driving purely highway miles. It's a good example of how deeper gears can HELP(or at least not hurt) mileage if you run a performance cam.
(25.6)

iroczrockz

I think I have found the problem. Gunny what size tires do you have? 215/65/15's? The reason I say, is that is what I have and that is what came stock on the car. That equates to a 26" tire diameter not a 25.6" tire diameter. That makes a difference. That is probably what we are seeing here.

Gunny Highway

iTrader: (1)

I have 245/50-16's on mine. And guys, it's definately a peg leg rear end; no doubt about it.

How do I check my tach?

iroczrockz

Oh well, back to the drawing board.

Redliner

Hey Kingtal0n
Could you possibly do a calculation like the one above for a T-5

82camaro

http://www.f-body.org/gears/
and select 3rd gen f-body t-5

SSC

Lucky people with 15/15in rims, I'm spinning 2800/2900 rpm at highway speeds with the 225/60/14's (75 mph)

To check the tach hook up and aftermarket tach and just compare the rpm at all speeds. I noticed the factory tach in my car is accurate until 4k then degrades to a 500 rpm loss at 6K.

Kingtal0n

other things besides gearing can have a huge effect on fuel economy. Converter stall speed / lockup availability are some of the biggest. cam overlap is another one. timing advance is a huge one too. so is having a slightly lean mixture on the highway.
and dont forget vehicle weight.

It comes down to many many variables. you wont know exactly until you try it out...

but there are SOME combos that you just KNOW will give you crappy fuel economy.

like a Th350 with 3.42 gears and a 2800 stall, a holley 750 DP, non vacuume advance distributor, and about 40* of overlap on the cam.

THAT combo, I can guarantee will net you less than 3 MPG on the highway. I know, Ive tried it