caddy83

I have a rebult tranny on my 85 GSL-SE. I've notice my gears are short, especially 5th gear. When I'm on the freeway going about 70mph, my rpm is a little over 4 or 4000 rpm. My friend has a GSL-SE and his reads about 3 or 3000 rpm's. The tranny is very smooth and is great for 1/4 mile racing. This is my daily driver and I commute. Gas is killing me. Do I have a 12A tranny or the connections on my 13B tranny not hooked up properly. I know thw 13b tranny has 4 connections and the 12a has 2 connections. Can this be the problem? Is the computer not reading correctly or did they do a bad job rebulding the tranny. Please help. thanks

louved_gen1

Thats about what my car does at 70 in fourth and 3308rpms in fifth. Im running 225/50/15s on the rear with the stock rear end, what do you have?

caddy83

I have the stock GSL-SE wheels and 205 14. The wheels shouldn't make a big difference on the rpms.

caddy83

I should be at 3000 rpm's in 5th gear

Brianhsval

I don't know what I ran with the 4:08 rear, but with the 3:93 I am at 4000 at 82 mph in 5th. I do remember that it topped out at 120 in fifth before I ripped the rear end out at the track.

louved_gen1

The wheels and tires made a 200rpm difference

Brianhsval

Yes, tires and wheels DO make a bit of difference in speed and RPM!

RotaryAXer

iTrader: (4)

Um Tires and wheels can make a HUGE difference in speed. Larger diameter tires effectivly lower your gear ratio and vice versa.

What aspect ratio are your 205 14 tires? That is what determines the diameter.

I'll assume you have the OE 205/60-14 size tires. (assume 23.7" diameter)

With the GSL-SE tranny in 5th gear (0.760:1 ratio) and the stock rear end (4.076:1 ratio) at 3000 rpms you will be going 68.3 mph.

At 4000 rpms in 5th you would be going 91 mph.

Now these speeds assume no slippage in the clutch. It is possible that you do have some slippage. That could be your problem.

Otherwise you have a 12A tranny with 5th gear that is 0.825:1 ratio. at 3000 rpm you would be at 63 mph and at 4000 rpm you would be at 84 mph.

Yet another possibility is that you have a smaller diameter tire.

Or maybe you no longer have a 4.076:1 rear gear. A higher ratio would make it rev higher for a given road speed.

Feds

Tire size will not make a difference in the reading on your speedometer at a given RPM.

The highest potential causes here are 1: wrong speedometer drive gear installed during rebuild or 2: broken tach.

Have someone drive beside/behind you to verify your speed (or use a GPS), then hook up another tach to verify your dash-tach is working.

Someone else was asking about this a while ago, and I think I did the speed calc. in my reply. Do a search

RotaryAXer

iTrader: (4)

Oh good point.

I guess I just assumed that he knew for sure that he was going 70 mph.

Yes check the speedo gear.

peejay

All good and don't forget 3: Does not have a GSL-SE transmission gearset.

GSL-SE's had really tall 5th gears. So did series 3 12As although not as bad.

Earlier 12As had much more reasonable 5ths and they will cruise at about 3500 or thereabouts at 70mph.

But given that the tachs are rarely accurate, and the speedometers are *never* accurate...

Brianhsval

Tire size, CAN make a difference. Given, that a 24.5 at 2000 RPM is at 70 mph in an undisclosed vehicle. By droping to a 22.5 the same will be traveling 68 mph at the same 2000 RPM. I do not think however that this is the case in this stuation! The BIGGER the tire the faster you travel at a lower RPM. The reverse is true for smaller tire size.

Therefore, NOT in this case I don't think, tire size DOES make a difference at a given RPM in refference to your speedo. Provided that both are fairly accurate.

peejay

Tire size won't affect the speedometer/tach relationship.

Tire size WILL affect your true speed... but it won't affect the speedometer. Which is why if you really alter your tire diameter one way or the other you need to correct the speedometer.

Here's something I neglected to point out: Compare your car in 4th gear vs. your friend's. 4th gear will always be the same since 4th gear isn't a gear at all, just connecting the input to the output and bypassing all the gears in the trans (therefore it's 1:1). This will help pinpoint what the difference is, if it's a problem or if your trans just has different gearing, or the wrong speedo gears, or something.

caddy83

Thanks guys. I forgot to mention that all gears seem a bit short. Can my differential also make my cars rpm run at 4000 at 70mph?

Brianhsval

Yes, as I previously stated. With a shorter gear you will be going 70 at a higher RPM than with a taller gear ratio.

peejay, I have allot of respect for you and your knowledge, but, from experience in the ownership of several road tractors. The shorter tire will slow you down. Nothing to write home about, and the speedo will react to it. The same as it will to a different gear ratio. 2MPH is not much difference, but the RPM will be: as previously stated the same. I think you are miss reading what I am saying. 70 at 3500 with tall rubber. 70 at ...say 3800 with short rubber. This is just a for instance on the RPM. It will not throw off the speedo or the tach. Engine speed is not road speed and is not governed by tire size or gears in the tranny or the rear. I am not saying that it is. Speedometer reacts to the tires and the gears, and as we all know the tach is nothing more than the engine speed.

I hope that this clearifies what I was trying to say. Please forgive me for not being clearer.

Blake

Brianhsval: Peejay is correct. Different gearing (either from actual gears or an effective change made by different size tires) will NOT affect the speedo/tach relationship. The tach indicates engine speed and the speedo indicates the tranny output shaft speed indexed to a fixed gear that accounts for downstream gearing. If you change any downstream gearing, the speedo won't know it! Because the speedometer measures the output of the tranny, the speedo is correct for all gearing before the tranny, so it's right for 1st through 5th gears and even reverse.

Let me give you an example from personal experience. A couple weeks ago, I changed from 3.909 ring and pinion gears to 4.875s. My speedo now reads about 80mph when I am really traveling at 60mph and the tach also reads exactly where it used to for the *indicated* speed; not actual speed.

Again, the speedo only knows the speed of the tranny output shaft and that speed is always proportional to RPM in every gear. Change anything after the output shaft and there will be NO telltales from the tach/tranny relationship whatsoever.

An easy way to estimate the discrepancy from indicated and actual speed is to use the trip odometer against mile markers. After a clicking off 5 or 10 miles, compare it to the trip odometer and the percentage discrepancy will correlate to speedo error.

Brianhsval

I do believe that I said that Blake. Not to be rude but read my statement again. I stated that gears, tires do not affect the tach! I know that speed is governed by the gears. I also know that speed is also relivant to tire size.

I beleive where I being misunderstood is that I am simply saying that to obtain the same speed : lets say to makeup the difference of having smaller tires, then the engine must be reved higher. The engine must turn higher RPM's to run 70 with 4:08 rear than it does with 3:93 rears. The same with the small tires and big tires.

That is all I was trying to say. In that way it is relavent.

As for the speedo being off.......that is another thing.

caddy83

Can I take the cover off the differential (rear) and see what is my gear ratio. Its been more than 12,000 miles since I changed my differential fluid(I use royal purple synthetic). Can i check the gear ratios at this time. I don't like going 70mph with my tach reading above 4000rpm's. When I increase my speed to 80mph, my tach reads above 5000rpm's. I feel the engine is really forcing itself. Don't like this at all. HELP

Brianhsval

You do not need to take the cover off to change the fluid or check the ratio.

Check the ratio by jacking the rear of the car off the ground. Mark a tire on the inside and mark the drive shaft. Turn the drive shaft enough times to turn the tire ten times. Count the number of times you turn the shaft then multiply by ten. Say you turn the shaft just over 40 times then you would have a 4:08, just over 39 then you could say you have a 3:93.

Here is a link for you to read:

http://www.442.com/oldsfaq/ofdif.htm

Hope this helps you.

peejay

Additionally there is no rear cover - to see the gears you need to yank the brakes, axles, and driveshaft, then drop the centersection out. Our rears use a drop-in centersection like a Ford 8"/9" amongst others.