Is my 4L60E in overdrive?

[Lawrence]

I have a LT1/4L60E in my 68 Camaro, which I started driving this week for the first time. It's mostly motorway (freeway) miles that I do in it, and it's running at about 2350 RPM at 70 MPH. If I kick it down, it's only coming down one gear. Is there any way of determining what gear you're in easily, and would the wrong gear and tire size set in the PCM stop it from changing into OD? Any help appreciated, i'm only getting around 12 MPG and it's costing me a fortune here in the UK

[Steve Chryssos]

Lawrence,
You should very simply be able to lauch the car from a dead stop and count the number of gear changes. Use moderate throttle. As for your second question: Not likely. If the PCM is being "fooled", I would think that shifts would occur sooner or later than stock as a result of the mods. But would they be eliminated? I don't think so. You more than likely have a some sort of linkage, tps, or internal problem. I don't know what you have for a final gear ratio or tire diameter, but 2350 rpm on the highway is not so far out of line that the car is in 2nd gear. Without more specs, I would guess that at highway speeds, revs would be much higher in 2nd gear.

Edit: another thought is that your lock up may not be functioning.

[Lawrence]

Thanks for the reply, I believe it's in 3rd and kicking down to second on the motorway. When I first had it I had problems with the VSS and my Autometer speedo and it wouldnt change up till the revs got really high. (I only drove it up and down my street like this and realised there was a problem), so thats why I suspected a PCM problem rather than a physical transmission problem. I'll connect my laptop up and take it for a drive and see what speed the PCM thinks im doing vs the speedo.

[Steve Chryssos]

Great. Also eliminate the silly stuff. Get under the car, disconnect the shifter cable, move the detent through it's entire range to make sure all gear positions are accessible and there is no binding. Then repeat with the shifter cable connected.

[Lawrence]

Hi Steve, I've already done the shifter as the car had the original horseshoe one on when I got it, and now it's got a Hurst one. It changes all the way up and down and there is no slack at either end of the shifter throw. I counted the shifts as you suggested, and it's only in 3rd at 70MPH. I took it on to 90MPH and it still didnt change up a gear. Guess I should get one of your shrifters to "test" it eh?

[Fuelie Fan]

I have a 700-R4 in my Nova with 3.42 rear gears and a 26 inch tall tire, and in fourth gear lockup I am running (coincidentally) exactly 2350 RPM at 70 mph. If oyu supply your rear end ratio and tire height I could calculate (or there are plenty of calculators on the web) RPM versus speed versus gear.

12mpg, is that around town on the highway?

[Lawrence]

12 MPG is total highway and round town, but 90% of my miles are highway miles. I'm going to get a friend of mine who works with me to sit in the car tomorrow and read what speed the PCM thinks vs what speed I'm travelling. Hopefully I can then work out what rear gears I have roughly without having to take the axle to bits and count the teeth. (it's an autometer VSS driven speedo that you can calibrate which matches my friends Audi as a guide).

Tire height is 25", and the donor car, an Impala SS had 27" tyres and a 3.08 final drive I believe. I'm guessing I'd need a final drive around 3.00 to make the highway RPM more like the impala which I calculate at 1975 with the 3.00 rear vs 1878 in the Impala.

If I've got my maths right, should that make it close enough to get some MPG back, or would anyone recommend different gears?

[Fuelie Fan]

Something's fishy here. In order to be going 70 mph at 2350 rpm in third gear with a 25" tire, you'd have to have a 2.50 rear end ratio, which seems unlikely but possible I guess. Also, regardless of what gear you're in, if you're turning rpms that low and spending most of your time at that speed on a highway, your mileage should be better than 12. My POS gets 18 on the highway. Having your friend ride around with you using a scantool will of course put all of this to rest, let us know what you find. I think you'll find more than one error code...

[Steve Chryssos]

Guess I should get one of your shrifters to "test" it eh?

No. You would more than likely you would experience the exact same problem.
Anyway, there's no need to get the thing up to 70 or 90 mph. That's not helping. To the contrary, try testing with jackstands under the rear axle and the rear wheels about two inches off the ground. Chock the front wheels. This will remove load from the equation. Shifts will be based entirely on vehicle speed. It will be easier for you to follow data on the laptop under these controlled conditions.

Oops! Don't forget to reapply the brakes before shifting back into park.

[Lawrence]

I replaced both o2 sensors and a maf on the car last week, which made it much better than it was, and I also did a set of new plugs on the weekend and disconnected the battery for half an hour to start from scratch. Yesterday I managed to average 19mpg total driving the 60 miles I cover going to work and back. Will get it on the ramps at the garage across the road from work and get real numbers on RPM and shift points. Thanks for all the advice and suggestions so far, I still think I should convnice myself to get a shrifter.....

[Steve Chryssos]

We'll be here when you're ready.