greatskiiiier

iTrader: (10)

so it is conclusive- well for me anyway

did a rear end swap in my car, went from 3.42 to a 12 bolt with 4.11s

a couple of hours on a major highway (~75 actual mph whole way) (my speedo was reading ~90mph because it was calculating my speed off of my RPM and 3.42 gears) this is what ive found-

my car had 4.11s in it, but still tuned for the 3.42 so my tripometer said i went 267.5 miles (this is what the mileage would have been if i had 3.42 and was at the same RPMs)

my friend was driving with me the whole way (in a Cavalier) and his tripometer said 223.3 miles (this is the accurate mileage)

when we stopped for gas, my car too 9.884 gallons

The Calculations:

if i had the 3.42 still and were going ~90mph (2300rpm) my gas mileage would have been 27.064 mpg (267.5/ 9.884)

with the 4.11s actually going ~75mph my true gas mileage for the trip was 22.592 mph (223.3/9.884)

SO going from 3.42 to 4.11 your HWY mileage going ~75mph will drop ~4.472 mpg- again this is strictly HWY

as a reference point i did do 3-4 whole shots on the trip and im sure my friend in his cavalier did too just trying to keep up- his gas mileage was ~30.1

bearcatt

iTrader: (41)

Good job. Wow! that's a big drop in gas mileage.

I'm not a drag racer ( just a putter ) lol so I'm happy with the 3.42 gearing.

I can understand wanting 4:11s though.


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wrd1972

iTrader: (12)

Yup 4.11's kill the MPG's.

greatskiiiier

iTrader: (10)

i wasnt too upset- i was going a decent speed and did ~3 hole shots, 22mpg on the highway is better than most V6 and V8s
i was upset when i saw i would have got 27mpg but def not upset with 22

ps- anyone know where/ how i could get my speedo fixed now lol

bearcatt

iTrader: (41)

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These cars are pretty much a win/win situation. Good performance, good gas mileage, lots of parts, lots of aftermarket, lots of support, easy to work on and a great community.

And 22mpg is better than my Toyota and Nissan 4 banger work trucks.


I bet the 4:11s make it a blast to drive. I'm now running a MN12 T56 with the slightly different gearing. Can't tell much of a difference other than it now breaks the tires loose in 2nd.


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therobman

iTrader: (39)

a tune or handheld and wow thats a huge drop and with my mods in sig and 3.73's i get abot 25-27 hwy

greatskiiiier

iTrader: (10)

ya im trying to get my hands on a handheld on here.... my friend told me his dads car (same mods as mine but 4.10 in a 10 bolt mine are in a 12 bolt and hes tuned for em already) gets 27mpg on the highway.... hmmm...

2002_Z28_Six_Speed

iTrader: (19)

I didn't lose that much and I went from 3,42 to 4,56. Something might be up.

greatskiiiier

iTrader: (10)

your speedos were all calibrated for the new gears right?

qwikz28

iTrader: (12)

i don't drag race (much or yet) but i went from stock 342s, to 410s in a 12 bolt and now 373s. i'd like my 373s more if i had a steeper first gear like that of a GTO transmission, but its definitely a good sacrifice for a cammed daily driver. 410s simply required too much shifting and killed my highway mpg. 342s with a cam would be murder though. a part of me is thinking that 390s would have been a happy medium but i am more than happy with 373s for now. sorry for the ridiculous writing, i'm really tired

RevGTO

iTrader: (12)

I know it's off-topic, but I had to give a thumbs up to this absolutely spot-on statement. Only thing I'd add, is "reliable." On top of all the rest, these cars are just hands down day after day reliable, as much as any Honda or Toyota would be.

bearcatt

iTrader: (41)

Good call. Yes these cars are reliable too. You have to love em.

Good point.


lol


Another good point.



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99TransAmLS16Spd

iTrader: (8)

The only flaw I see is that you're not taking aerodynamics into consideration. Not that it's going to throw your numbers completely off, but they would be different if you were actually going 90mph vs theoretically going 90mph.

But it's close enough, and your mpg loss sounds about right. Some engines actually have the potential to make better gas milage with steeper gears depending on the set-up.

greatskiiiier

iTrader: (10)

ya gas mileage isnt going to change if your speedo is saying 90mph at 2100rpm with 4.11 (even if your actually only going 75) now if the speedo was calibrated and it was saying 2100rpm, 75mph and you calculate you gas mileage to will be 2100- heck, im jealous! lol

- aerodynamics arent changing from a 98 camaro with 3.42 to a 98 camaro with 4.11s
- gas mileage can improve if yours shifting more around town (in 5th when you would have been in 4th or even 3rd)
- on the HWY if your at 1700rpm with 3.42 or 1700rpm with 4.11 your gas mileage hasnt changed, but instead of going 75mph your only going ~67mph... if you wanted to maintain 75mph with the 4.11s youd have to be at 2100rpm which will drop your MPG

deelong4002

iTrader: (8)

I dont think your calculation is right. How can you calculate mileage with 3.42's if you have the 4:11's in there. The only calculation you can do is your Gas used to your friends mileage shown on his cavalier. Something doesnt seem right here. That 267.5 isnt how many many miles you would have gone on 3.42's its how many miles your car thought you went because your drive train is spinning alot more for every x distance traveled. The only calculation that is right here is your calculation of your gas used, to your friends miles read (assuming his trip is accurate)

greatskiiiier

iTrader: (10)

i knew id have someone calling me out... our speedometers calculates how many miles our cars go based off of our tachometers and the whole equation through the drive line until it gets a final number- its all computer based, so the miles my tripometer read are the miles the car would have gone with 3.42 (assuming my tripometer hasn't been off the past 11 years)

his tripomter (assuming it hasnt been off for X amount of yrs) is calculating his mileage based off a stock car, not one change down to tire size, so unless Chevy's are notorious for having horrific tripometer readings (which to my knowledge they're not) then my calculations are corrects

again this is all theoretical, but now-a-days whats not?

bearcatt

iTrader: (41)

It will be interesting to see what the actual results will be once your car is tuned for the 4:11s.

There was someone selling a Crane tuner in the parts classified the other day.

I have one of those myself and I'm pretty happy with it.



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Tuner@Straightline

Mileage and speed is based off the vehicle speed sensor (VSS) on the tailshaft of the transmission. Gearing and RPM's are independant of the VSS. In a manual, the computer has no idea what gear you're in. The VSS counts the pulses from the reluctor on the output shaft. The computer knows the tires size and the final drive ratio, from these two constants, it calculates a "pulse per mile" signal. X (this number is a constant and is determined by the mount of teeth on the reluctor) amount of pulses is one rotation. Now the computer does simple math to figure out that Y (amount of drive shaft rotations) is Z (distance).

rollinna65

iTrader: (18)

I got 27-28mpg at 75mph when I had 3.42s.

Now that I've got 4.10s (with the speedo corrected) I can't do better than 22.5mpg at 75mph.

4.10s definitely lighten the wallet quicker but damn are they fun.

02NBMWS6

iTrader: (22)

Your statement "if i wouldve had the 3.42s and been going 90mph" is wrong. Your car wouldve been under much more load at 90 mph from aerodynamics and also more load from the shorter gears. You wouldve used considerably more gas then you did at 75 with the 4.11s

There is more to mpg than just what rpm/mph youre at. According to that theory then, the lower you go in gearing (2.0, 1.0) the better gas mileage your calculations would yield. Thats incorrect