Reducing engine RPM with automatic transmission

[Warren]

My 1979 Fiat Spider with 3 speed auto transmission runs around 4000 RPM at 70 MPH freeway speed which is quite noisy. At 65 MPH it is much quieter. Can anyone think of an economical way to reduce engine RPM some? It has the earlier better retrofitted 1800 intake manifold, a retrofitted 34ADF carb, and stock exhaust system including original catalytic converter. I have thought about replacing converter since it probably doesn't work anyway and have considered going to an enlarged exhaust system (which may make it noiser but less restrictive). Wouldn't have problem if was a 5 speed like my Alfa Romeo but enjoy the auto transmission and 4 speed autos weren't around in 1979 especially with 55 MPH speed limits.

[Exit98]

Warren,

You can improve engine performance but without ratio changes in the gear box or rear end you'll always be turning the same RPM at any given speed. Some of us with earlier Spyders have switched to the later rear end with a lower (numeric) rear, and some 2 liter guys have put in the auto trans real which is lower (numeric) still, but there isn't anything else that I know of that you can swich to. You have the lowest rear available for our Spiders.

BTW, before I swiched rears I was turning 4200 at 70 mph with my 1800cc motor. I just swiched over to the 3.9 rear end but haven't got it out on the highway yet.

I expect to knock off about 400 rpm at 70. Meantime it is way more pleasant to drive.

[TronDD]

What are you running for tires? Taller tires will lower your RPMs.

Tim.

[Warren]

My present tires are 165/70 13 on the original steel wheels, except for one tire which was installed mistakenly as a 185/70 13 (it is on front). As suggested by Mark back in 2006 on this odd tire arrangement, I believe the best thing would be to buy another 185/70 13 and put both on the rear. Eventually I would replace the others with 185's. According to Randy's Ring and Pinion Calculator, my RPM's should drop from 3912 to 3727 at 70 MPH, which would help some. The only problem is the speedometer drive will need to be changed but since this is the GM TH180 transmission (Opel, Suzuki, and Chevette), I can probably get the correct gear from somebody. Thanks for everyone's thoughts and if you have more suggestions let me know.

[phil]

Warren,

You can improve engine performance but without ratio changes in the gear box or rear end you'll always be turning the same RPM at any given speed. Some of us with earlier Spyders have switched to the later rear end with a lower (numeric) rear, and some 2 liter guys have put in the auto trans real which is lower (numeric) still, but there isn't anything else that I know of that you can swich to. You have the lowest rear available for our Spiders.

BTW, before I swiched rears I was turning 4200 at 70 mph with my 1800cc motor. I just swiched over to the 3.9 rear end but haven't got it out on the highway yet.

I expect to knock off about 400 rpm at 70. Meantime it is way more pleasant to drive.

Looking for anyone with this problem the kick down on 124 spider / 2000 the end is chewed up have new one but cannot find way to replace except take pan off trans? Can some one tell me is this correct?

phil

[baltobernie]

The 185/70R13 modern tire is very close to the 165/R13 that was original equipment. Standard profile radials back then had an aspect ratio of about 78%. Your speedo should be pretty accurate once you have the correct rear tires. BTW, if you ever find yourself with an odd-size tire, put it on the non-driving axle. Extended use of different-sized tires on the rear of a Spider (or the front of a Corolla) will damage the axle or transaxle.