Hi all,

I have recently purchased an 89 5.7lt Auto IROC with 24,000 original miles, a rare find here in Aussieland.

How/where do I look to find details on ratios?

I don't even know what 2.73 for example means exactly,...2 spins = .73 spins,........is it shaft to wheels?

What is the ideal ratio for fuel economy, the beast seems to drink like a lizard in the hot sun and I'm not the rev head I used to be, I have this for a daily driver.

I'm planning on a tune at some stage to make sure all that side of things are covered.

Help the idiot! Thanks.

 

a 2.73:1 gear ratio means that the driveshaft is spinning 2.73 times faster than the wheel.. but that's not your final drive ratio- you need to put the tire diameter into the equation to figure that out, and then if you've got an overdrive trans (you do), you need to multiply the final drive ratio by the over drive ratio to find out your final drive ratio in overdrive..

generally, if you have 2.73 gears you have about the best ratio for fuel economy with a small block Chevy and 26" tall tires.. a 3.23 or 3.42 ratio will raise your cruise rpm a few hundred rpm, which will probably lower your economy some but the car will seem a little faster.

 

Cheers novaderrik,
Is there a site somewhere with all these ratio details, maybe it's here somewhere, I'll have a look around later, Dad duties call for now.

 

Numerically lower gears are better for fuel consumption, due to the lower engine rpm. That said, my 2.77 geared car drinks like a fish too, undoubtedly due in large part to a carb swap from tpi.

Still, it's an old-school V8 - are you comparing the fuel consumption against your Nissans?

 

If you're not getting at least 25 MPG at 100 KPH, there's something very wrong. If you keep after it, 30 is possible, but it will be a one-time best, not an average.

 

Heh, don't know the AussieIroc's situation, but my average 12-13 mpg is dictated by the land of cold starts, short trips, and 40-50kph zones stretching for miles that I live in . It gets better on longer trips.

 

TreeFiddy.......... my comparison was loosely from memory of my old '86' 305 auto IROC from 20 years ago, I have no idea of the gearing as the cars were all new to us back then and the internet wasn't really around to pick the brains of others, but I do remember it being Fantastic on fuel,...point in question.... I followed my parents home after they bought a new Honda Accord, 800km trip, I fared better in fuel consumption than them.

Atilla the Fun........ I can't say I've recorded any figures yet, but a recent trip of 2000km I was getting around 250-270miles,...maybe the needle goes lower than it was showing but it scared me enough to want to refill, I noticed no low fuel light, not sure if it even has one, I've also noticed a tendency for the fuel handle to click off all the time nearing a full tank, I keep going and going,... twice now I have overflowed it.

The car has a few issues I'd like to address in the future but it's still giving me a chubby and it looks like it's doing 200 standing still,... I love it, I reliving my teen years .. woohoo.

 

Heh you were lucky, my teenage car was an old Leyland Marina

Your 86 must have been pretty new back then - did you import it yourself?

 

If you turn the drive shaft 2.73 times it will turn the rear wheels 1 full revolution (no matter the size of the wheel).

 

i learned exactly how johnny put it. 1 full turn on the wheels will turn the driveshaft or a majority of other rear end internals a certain number of times. lower gears are better for highway types.

 

TreeFiddy,
No I didn't import it, I saw an ad, It was L.h.drive, missing the bonnet, both front guards were damaged. I bought it for $15,000au, had it converted to R.h.drive for $8,000au, replaced the parts of course then repainted it for $8,500au. There were few examples around back then and rare to see them were I lived. It was Black/Tan/Gold wheels. Oh, the springs were cut in half by the previous owner so I had to buy them too, I had the car lowered, it used to scrape everywhere. It was missing the IROC rims too.

 

you either need to do some tuning or modify your driving habits.. the vortec headed carbureted 305 in my 86 averages just north of 22mpg in day to day driving (lots of stop and go, with 75mph interstate blasts mixed in) and averaged 24mpg when i took it on a 3900 mile trek across the center of the country from MN to TX and back in June with 3.42 gears and no torque converter lockup.. and that was with a weird misfire that i hadn't been able to figure out until last sunday- a plug wire boot had a small hole in it and it was arcing to the header tube about an inch away from time to time. it runs better how that i fixed that and i'm hoping that 30mpg averages are in my future..

 

Never!

Been tuning it for 10 yrs, this is as good as I can get it, mainly because:

I basically do almost zero highway driving, spend too much time low speed in low gears. And frequent hard acceleration from zero all the way up to 50 kph