NukeC5

Ive read some threads and seen some charts now that show the approximate RPM the engine should be at with whatever gears are installed.

I have an A4 with 3.42's and I feel like my RPM's are way too high. Most people seem to be at 2000rpm at 65mph. I am at 2500 at 65mph. The freeway is killing me and I have to commute. To go 75 and barely stay with traffic I am at 3000rpm the whole way. I read so much about the gears before doing them and was satisfied when I knew it would only drop the economy by 2-3mpg. I am down from 31mpg on the freeway to 25 average.

Why is this happening?

The vehicle is in pristine condition and well maintained. No problems at all. I am mechanically inclined and have gone through everything on this car and done all maintenance items. So its not anything to do with how its running. The only change was the 3-rib 3.42 diff.

AU N EGL

10% EtOH in the fuel hurts millage quite a bit.

feeder82

So does higher RPMS you will lose more mpg once ya get the stall converter in, but the smiles per gallon will go up

WKMCD

There is a chart in this thread.

http://forums.corvetteforum.com/c5-g...-on-4-10s.html

NukeC5

That is what I'm referring too. It says I should be able to go 80mph at 2400rpm. I am at 2500 rpm at 65mph. Its at 3000rpm to go 80. This is clearly not normal.

I highly doubt the Ethanol has to do with how high my rpms are. But any info helps. Thanks.

NukeC5

Is it possible that the 3.42 diff I installed has 3.73's or 3.90's and I just didn't know it? That would be closer to the rpm on the chart I am at... I bought if form a guy that bought it from another guy and never used it. He didn't know much about it... maybe it was upgraded in the past. How do I find this out without removing it?
My car is tuned for 3.42's. Would the tuner have noticed that it was off?

NukeC5

Holy $hit, 4.10's are actually the closest to what I am getting rpm to mph wise. I'll be PO'd if I have that high of ratio behind my A4. Horrible fuel economy. But maybe that explains why I thought the acceleration increase was so insane even without a converter...

Am I on the right track checking into this? Or is there something else possibly going on?

lucky131969

...of course it's possible. You purchased a 3rd hand differential, that NO ONE could vouch for the condition/contents, and installed it in your daily driver(sans inspection)......so you can't be too surprised with the outcome.

NukeC5

If it is a higher gear, it slipped by the guy I bought it form, myself, the shop that inspected/installed it, and the tuner who corrected the shift points/tachometer. I know what I am looking at, and that it was a good diff, and the shop agreed when they inspected it. So no bad diff was installed into my daily driver. It has the GETRAG 3.42 badge on it, and its easy to assume that's what in it. Especially considering I paid $275.00 bucks for it. Thats extremely cheap for even a used stock 3-rib diff. No one considered that it was modified (If it is).

This is where I stop and wonder if its possible. How would it have been possible to tune if they were tuning for a 3.42 ratio? It shifts and drives perfect, and the shop hit it correctly on the first attempt at tuning it. So it seems like it would have been caught if the ratio was different.
All I know is that my economy went way further down than it was supposed to and I'm at around 500 more rpm at all speeds than I should be.

lucky131969

I would not call $275 cheap, that's a steal. So you are saying the shop physically pulled the cover, and inspected the internals before install? If not, the only way to compare (if you don't trust the speedo method..and don't want to pull the cover), is counting the ratio between driveshaft rotation to wheel rotation.

Supercharged111

Just because the tuner flashed you for a 3.42 doesn't mean you have a 3.42. It also doesn't necessarily mean that everything is right. Have you verified your speedo with a GPS/smartphone to see if it's off?

NukeC5

So I could lift the rear end, mark the converter or whatever is visible, put it in neutral and turn the wheels watching the mark for how many times that mark spins from one full rotation of the wheels?

Basically if its 3.42 I should see close to 3 and a half turns. And 4.10 would be 4 turns right?

NukeC5

No, this whole thing just hit me this morning. I haven't tried anything yet. Ive been driving with this diff for three months now and thought I just needed to get used to it. But now Im finding it more and more strange that its acting the way it is.

tblu92

Ok sound like your TQ converter is NOT engaging-- On my 3.42 A4 with stock tires my speedo reads 2200 RPM at 70 ( GPS verified as well)
Did you have done or did yourself a speedo correction and a shift point correction when intalling the 3.42's ?? IF so you DO NOT do a speedo correction on ant C5 on a gear change--period !! could be why your TCC is not engaing---You do however need to do a "shift point" correction in your ECM
depending on the gears you previously had the correction should be as follows:
If you had 3.15 gears you should make a - 8.5% shift point correction
If you had 2.73 gears " " ' a - 25% " " "
Try driving on a flat road with no traffic-- from a dead stop with light throttle count the shifts---You should feel 4 shifts the last one being the TCC locking--- Someimes however the 3-4 shift and the TCC lockup will happen very close or even at the same time so it's hard to tell---Another way is while it is in 4th gear if the TCC if unlocked when you punch the throttle the engine will rev up more ( maybe 300-500 RPM's) feels loose----but if it is locked it will rev up very little maybe only 100-200 RPM-- feels connected/locked

lucky131969

Yes. If you have a 4.10 rear, you will get 41 rotations of the driveshaft, to every 10 rotations of the wheel. A good point has been raised about the OD..make sure you have all four shifts, so you don't chase your tail.

NukeC5

I appreciate the detail response. But I have paid close attention to the shifting and everything is normal. After hitting overdrive the tcc locks up.
I took the car to Cunningham Motorsports near me to have the shift points tuned. I think thats all they did. They specialize in corvettes. So I doubt they changed the speedo if it wasn't needed.

I will give them a call tomorrow to explain what I think is going on and see what they say. Ill ask about a speedo change and if they did one too.

NukeC5

Tomorrow I will make time to count the rotations. Thanks for the help

WKMCD

Ryne has been doin this long enough. Have him drive your car for 5 minutes and he'll will be able to tell you whether you have a 3.42 or a 4.10. It should be VERY obvious.

bracketshark

The converter won't turn (with the engine off) so there is nothing to count. I see no way to verify ratio unless using the RPM/MPH method, like your doing, or visually checking the gears.

MikeV

Can the back diff cover be pulled and the gear teeth counted? I don't know the number difference between the two sets but I am sure someone here does.

Ex San O Nuke
Mike V