85Irocbuild

I am swapping a T5 transmission into my Iroc. The car came equipped with the 2.73 rear end gear ratio. The car is only a daily driver, so my focus is not on performance. Will the car drive fine with the T5 mated to the 2.73 gear ratio?

jmd

iTrader: (4)

You'll slip the clutch more. If it's a mild cam, it will work, but takeoff will be tougher. Assuming a V8 T5, obviously a 0.63 5th is going to be far less tractable than a 0.73 5th.

big gear head

I don't think you are going to like it. A 3.42 would be much better.

sofakingdom

iTrader: (1)

"Work", yes; "like", doubtful.

The lowest gear the factory put behind a T-5 in these cars was a 3.08, and that's already pretty much unbearable.

Formula 305

I drove a T-5 2.73 car from September until Halloween after I did my swap. Is it drive-able, hell yes. Did I stall a lot in the first week or two? HELL YES. But although the gears are not ideal, it does not render the car undrive-able. Hope this helps.
~Matt

EDIT: With the 2.73s 5th gear is useless under 55 MPH, and if you want to climb a hill, you better keep your speed at 65 or greater or you will not be able to maintain speed on a steep grade.

I did not notice a major difference in fuel economy, driveability, ect. The car ran & drove beautifully for about 1000 or so miles after the swap before it was put away for the winter time.

Johnny Blaze

iTrader: (15)

Didn't the 9 bolt t-5 equiped cars have 2.77's?

At least the 88 Formula that i parted out did.

LilSki

iTrader: (1)

My 85 is a 9bolt T-5 car and it has 3.27 gears.

85Irocbuild

Thanks guys for your help, looks like I may be swapping in a 10 bolt posi 3.23

HankL69

iTrader: (1)

Try it for a while. Depending on your driving style it may work out for you. Yes you will have to down shift on long up hill grades but since you are doing the swap to manual I don't think shifting is an issue. I'm sure the t-56 boys run into the same thing.

Ron U.S.M.C.

One of the problems with a 2.73 is not only that it kills the cars full potential and suck off the line or any were else for that matter, but changing/getting rid of them can be expensive. You can try to find a full axle/differential assemble that has what you want like a decent ratio and working posi. and you can do that and it won’t cost you an arm and a leg. Or if you want to keep the 10 bolt you have (as opposed to buying a 9" or 12 bolt) and build your 10 bolt differential from new top of the line parts that are quality, dependable, strong. (for a ten bolt)and install a top of the line posi. and is IMO a must have when you are changing the gears. Changing the axles to 28 spline also may be necessary when installing a new posi. Unit.

Side note = My car is also a DD and when I changed my gear ratio from 2.73 to 3.42 and added a posi. unit it changed the whole car. Drives nothing like it did. You’re going to love the change.
Gear ratio.recommended = 3.42 or 3.73 Fun Factor at 100 %

ZZ42Fast

iTrader: (2)

I had 2.73 with the T5. 5th is very tall and the bigger issue is with the extra strain put on the t5 case as a result of the taller gearing.

I went to 3.8's and it's a little too low now

FNFAL308

iTrader: (1)

A little late, but...

I have been driving for 8 years now with 5 speed and 2.77.

I'd have to check but I think it came with a 2.77 (may have been 3.08). You'll do better with this combo with some low end torque.

I rarely kill it taking off now... But yes I do get some awesome mpg ON THE HIGHWAY - don't ask, I won't tell. It gets great in town but not like the highway.

AssaulT/A

I have a T5 and I always thought my were 2.73, but they may be 3.08. It is a bit of a pig off of the line in 1st gear, but if you are not beating on it, it's totally liveable. But as stated above, the highway mpg is terrific. On a recent trip back to school, I averaged 29.7 mpg at 80 mph (~2100 rpm) with a carbureted 305, I kid you not! although when I upgrade to a posi I will most likely go to a 3.23 or 3.42.

Johnny Blaze

iTrader: (15)

I wonder how many here claiming impressive mpg with this combo actually have an accurate speedometer?

AssaulT/A

I wondered about my accuracy, but I verified mine while driving alongside my dad (bone stock SUV) down the road.

sofakingdom

iTrader: (1)

So IOW, you have no clue whether it's right or not...

The way to check your speedo is quite easy. It works off of the principle that the speedo ISN'T the most accurate instrument in the car, to begin with; but the odometer IS. All the odo is, is a turns counter: 1000 turns of the speedo cable = one mile. Too simple, really. And of course, essentially impossible for it to indicate wrong, unlike the speedo, which relies on the magnetic coupling between a spinning U-shaped magnet and the steel cylinder the pointer is somewhat attached to, as well as the "accuracy" that the pointer was pushed onto it at.

Drive down the freeway some convenient distance; say, 10 miles. See what distance the odometer says you went. THAT'S a number you can trust to tell you how inaccurate your speedo is, unlike the speedo itself, which has AHELLUVALOT more tolerance in it.

Johnny Blaze

iTrader: (15)

What's IOW?

sofakingdom

iTrader: (1)

In other words, IOW is, .... In Other Words.

AssaulT/A

Johnny, I believe it's "in other words".

Sofa, you may be right about speedometers having large tolerances for variation, but I just said that to make the point that my speedometer isn't so far off as to cause a significant error in fuel mileage calculations. In addition, my odometer is currently non-functional, so I have no way of checking for sure until I can convince myself it's worth it to remove my dash

FNFAL308

iTrader: (1)

Now you see why I do not post my mpg anymore?

I tested mine on two 300 mile trips from Houston to Baton Rouge and then back - 300 miles one way. Not 10 mile trips, not a 20 mile trip - 300 miles). My speed-o-meter has always been dead on based on the sheriff's roadside test stands that tell you how fast you are going. But I didn't rely on my odometer - I followed a mapquest route with the miles on it. And since this is a route that others take to get from A to B for work reasons I also asked them what was their mileage. We have to keep up with it to get reimbursed.

So, mapquest calculation may be off by a certain percent (they use lat and long and calculate, a calculation that I also use at work for predicting transportation costing) and the gas pump may be off by a certain percent, and the odometer, and of course other people keeping up with their mileage. My calculator was not off any AFAIK. Who trusts electronics anyway they are all out to get you. Fact of the matter is, it is "state of the art" as the definition pertains to legal matters (look that up before you argue - state of the art DOES NOT pertain to perfection or the best, but what is normally done - I have seen it argued before in court with my own two green eyes).

I do believe that very few people will believe when we get really good numbers, the same people that would never run 2.77/T5 because it is not fun enough. So, get a third gen with a T5, put a 2.77 geared rear in it, get some low rolling resistance tires on it (at the time it was BFG Radial T/A 215/65/15), make sure the engine is in very good condition, Fill up and take a trip. Tell us what you got.

I avg 29 back when I made the trip. Engine was almost new rebuilt 350 TPI with flat tops and a mild cam. All else was factory specs. Tuned up as well as I could get it with the matched flow injectors, new everything concerning the engine/emissions.

Want to get good gas mileage on the highway? Get your torque to come in early and put taller gears. in town is a crap shoot since you are only going to get the best numbers staying out of the lower trans gears and staying in over drive. If you drive in town a lot you would likely be better off with 3.08 or even 3.23

If you are going to refute the numbers, do it from experience. Don't tell me I can't tell how far I traveled or I can't look at a receipt for gallons. I would certainly agree I could be off by some percent of error.

FNFAL308

iTrader: (1)

And I get 25 mpg with a 1978 Blazer with 38.5" tires and 5 inch lift, 14bff and Dana 60 front.

Johnny Blaze

iTrader: (15)

Sounds like your math is pretty good.

I just figure since it's not a factory combo the speedo would likely be off on most swaps.

The speedo gear often gets ignored and people assume it's correct.

Your milage certianly seems possible under the right conditions.

Johnny Blaze

iTrader: (15)

Heck, my 69 c10 I had 3.73 and a 4 speed and averaged 16.

FNFAL308

iTrader: (1)

OK, I threw the Blazer out there as bait...

It has 4.56 gears which with the 38.5" tires is still a little tall for gears. It also has a 4 cyl Cummins 4bt engine with an NV4500/NP205. So over drive with a 4 cyl diesel with in effect tall gears. Tires are VERY low rolling resistance Michelin 11x16s.

Low end torque, tallish gears, overdrive = best mpg possible.

High horsepower + no low end torque + low(er) gears = worse mpg all day long.

You have to get your rpms down while not lugging the engine and mph up.

That's not bad on a C10. I have been looking at them as an alternative to my firebird but I keep going back to mpg and all the money I have sunk into it mechanically. Then I think put my drivetrain from the blazer in one for a 4x4 and the work involved is too much for the winter. Plus the 4bt is LOUD.

AssaulT/A

I don't believe my rear ratio has been changed, so I suppose it is a 3.08 after all. In calculating my mpg, I top off right before I leave (gas station right by the interstate) and fill up as soon as I get to campus, and note how many gallons it takes. I then divide the distance of my trip (from mapquest) by my fuel consumption. Over a 190 mile trip I used 6.4 gallons, which IIRC gives 29.7 mpg. I believe that it also helps my mpg that I drive very smoothly, and not as aggressive as around town.

P.S. With this method, wouldn't speedometer accuracy be irrelevant?

FNFAL308

iTrader: (1)

Speedometer does not matter for the calculation.

There is a sweet spot in rpm where you will get the best mpg with your setup. You were most likely there. Every engine setup will be different. I believe mine is around 1400 to 1500 with what I have.

99% of my driving is 45 mph max so I have a hard time even getting in OD unless I am going over the speed limit. Also I only drive 8 miles one way to work so I am most likely going to 3.27 or 3.42. 3.27 for mpg or 3.42 for the fun of it.

Go too fast, you lose, and go too slow you lose. Stay in OD and keep it steady. On my trip I stayed at the interstate speed limit. About half at 65 and half at 70.

I'd like to have a FI big block to play with to see what I could get out of it.

jmd

iTrader: (4)

This is one rare occasion where if you had a V6 T5 sitting around with .73 5th and a 2.7x rear sitting around, the wider ratios would work reasonably well with a 2.7x rear gear.









Holy Go post on CK5, man.

FNFAL308

iTrader: (1)

I do. From Jan 2006

http://coloradok5.com/forums/showthr...&highlight=4bt

The reason I brought the diesel into the discussion is because of the low end torque of diesels. Don't be a hater

Johnny Blaze

iTrader: (15)

Doesn't the odometer run off the speedo drive? If the speedo cable has the wrong gear wouldn't the odometer be off as well?

AssaulT/A

Johnny, AFAIK you are correct, the odometer would be incorrect. I also suspect that the difference would be exponential rather than linear compared to the speedometer variation (if that makes any sense at all). But really, this has no bearing on my calculations except that my 29.7 mpg may have been at 75, or 85 mph.

FNFAL308

iTrader: (1)

Way up there some where he said his odometer didn't work. So the speedometer being off, if it was, would not matter - he got miles traveled from another source.

I think the different would be based on a percent - if the speedometer was high by 10%, the odometer would be off by 10%. If it thinks you are going 10 mph faster than you are then you would be traveling 10 more miles in that hour. so if you were really going 90, but it says 100, you would still only travel the 90 miles in that hour. The percent would be the same.

Is the speedometer and the odometer mechanically tied together or separate? I guess either way either one could possibly be wrong and the other not.

AssaulT/A

FN, I guess that makes more sense than my idea. IIRC, I read somewhere on here that the speedometer/odometer are mechanically connected somehow. If i get bored in the next two weeks before school starts I may open up my dash to find out...

Johnny Blaze

iTrader: (15)

Of course they are connected. Otherwise what would control either of them?

FNFAL308

iTrader: (1)

"Of course they are connected" makes the assumption that they have to be connected. They do not have to be connected, they could be running independently. I do know that mechanical speedo/odo's that run with a cable are connected.

I do not know that electronic are connected. I have never looked and will not assume. I could guess...

And since I have electronic that is the viewpoint that I was speaking from.

Johnny Blaze

iTrader: (15)

If it's electronic they both would get the signal from a single vss.

FNFAL308

iTrader: (1)

Yes, but that wire could feed to two different motors. Yes it is unlikely, just use one, but I'm just saying I don't know that. If you are saying you know that it is one, then it is one. Makes sense and I would guess that.

I just did not know that from personal experience. I have never taken one apart, so I did not assume. I can say that currently mine are not connected as the speedo works and the odo does not.

Knowing that there are inaccuracies due to people changing gear ratios and just from the equipment not working correctly, there needs to be an independent verification of the miles driven anyway.

I'm really not trying to be a pain, I am just stating my position. I did not want to assume and give bad advice when I didn't know for sure. Nothing will get you stomped on a forum like giving inaccurate info.

FNFAL308

iTrader: (1)

But to get closer to on topic,

Blaze, I see that you have a T5/3.27. How do you like that combo? MPG? Fun (enough) to drive? I have a 3.27 posi 9 bolt sitting here and was going to either use it which would require a gear setup, or just get a complete 9 bolt 3.42 posi rear and put it in.

Wish you had the 3.42 or content with what you have? Have to drive far to work or close by?

Johnny Blaze

iTrader: (15)

What I am saying is there is only one point that picks up the speed, whether it's a cable or a vss. They may send their signal to multiple points but the pick ip point is the same. Ultimately it's from the transmission tailshaft and uses a gear.

If the rear end gears are changed and the gear in the trans is not, the speedo AND the odometer will be off.

I have never seen a camaro with two seperate cables or vss.

Even if they were seperate, thru would both still require some sort of recalibrating to compensate for a rear gear change.

Unless your 3rd gen came equiped with a gps based speedo/odometer.

But, we are drifting off topic.

Johnny Blaze

iTrader: (15)

Check put my build thread.

I'll let you kno. How I like it come spring, it's still in pieces.

HankL69

iTrader: (1)

For a while I had a 6 cyl. t-5 in my Firebird with a 2.72 rear gear. With the lower first gear in the 6 cyl. transmission it was no problem to drive and the .73 overdrive worked well above 60mph.
I've since switched to a 3.08 rear and a wc t-5 (v8) tranny and there was really no noticable differance .
My IROC has 3.27 behind the t-5 and is very "driveable". I have come close to getting 30 mpg but never got over the hump. I have checked my odometer on the interstate using the milemarkers as a guide. I noted the mileage as I passed a marker and chedked again 10 miles down the road and it was only off by a tenth of a mile. I would assume that would be considered a 1% error. My Firebird on the other hand has a 10% error on the odometer. It shows that I have gone 90 miles when in fact I have traveled 100. I know, I could change the driven gear and get it much closer but right now the speedometer is almost right on and I feel that is more important than the odometer. Or I could get the unit re-calibrated I guess and then both should work correctly. Lots of other things have to be done before that though.

AssaulT/A

Hank, I would tend to agree with you about the speedometer vs. odometer, especially when you can easily look up distances on the internet, on a map, etc. And I also agree that there are far more pressing issues to be dealt with.

58mark

iTrader: (3)

It's nice to see there are a couple guys here that have already done what I'm thinking about.

I have two v6 t5 transmissions already, and am getting ready for a 305 TPI swap. If I use a 2.73, the launching out of first gear would be aided by the ridiculous 4.03 first gear, and the highway RPM would be a very reasonable 2055 at 75 MPH with the .76 overdrive. I would be using the 10.5" clutch disc from the s15 truck, so I don't think slippage would be a problem.

I really don't like the 1st gear on my t5 with the v6. I understand why they used it, but I'm changing gears half way through an intersection already. I Think the 2.73 would make it a little more user friendly, and no I don't care about lighting up tires. That's not my thing

Yes, I've already bought a v8 bell housing, so unless a good v8 T5 with a 3.08 rear end falls into my lap, I think I'll plan on at least trying the tranny I already own. Won't cost me anything but the cost of a rear end and the clutch.

chazman

iTrader: (4)

I have two 3rd gens with T5s and it's interesting to compare the two. My '89 has the factory setup with a 2.95 1st gear, 3.45 rear end and .73 5th gear. Those factory engineers are sharp guys, since this is a very drivable combo. And I can also lug it around town in 5th gear without much protest if I want to.

My second T5 is a conversion for my '83 Crossfire with a 2.93 rear. It's a 3.76 1st gear and .72 5th gear which started it's life behind an '84 Iron Duke. This combo works fairly well and seems a good match with the existing rear gearing.
1st is a bit short though - it does light 'em up pretty easily, if that's what you want. About a 3.30-ish 1st gear might be better with my 2.93 rear and LU5 combo. 5th is great at speeds approaching highway speed. You can chug around in 5th gear at slower speeds, but the car is not happy there. 2nd, 3rd and 4th are just about perfect with this rear gear set.

big gear head

Better check to see if that V6 transmission will fit the V8. I think the input shaft is longer on the V6 transmission and won't work.

58mark

iTrader: (3)

I've always heard it will work with the bell housing change, but If that's not right, I need to know

big gear head

I know that some of the V6 transmission have the longer shaft and longer bearing retainer and will not work on the V8 with the V8 bell housing. I can't say for sure about the F body transmissions. Better check it out before you spend time and money on this swap.

58mark

iTrader: (3)

here's a realy good thread with lots of information

https://www.thirdgen.org/forums/tran...5-v6-bolt.html

chazman

iTrader: (4)

I'm fairly certain that any F-body T5 will work once you have the proper bell housing. As I mentioned, mine was born behind a four cylinder Iron Duke.