The Stovebolt Site Forums The Shop Area Driveline What is the best Transmission for my 54 truck?

I have been trying to decide what the best automatic trans for my 54 chevy 5 window pickup would be. I have a diff from a 2003 S-blazer 4x4, 2.73 gears and I want to use the stock 235 engine. I have been told that the 700r4 tranny is a good choice, but I then read it would be a dog. Any other ideas? I am so confused.

2.73 gears are a bit high for a 235 and an automatic. 3.90 to 4.11 gears would work well with a 700R4 and a 235. Is your 54 already open driveline? If so, the stock rear will be perfect for a 700R4.

Now what transmission is best depends a lot on how you will use your truck and your budget. Personally I like having Manual transmissions in a truck, not automatics, so my dream transmission is clearly different from yours.

Trucks and hotrods like low gears. 2.73 gears are very high on the economy side. As economy gears, acceleration will be poor at all around, and don't even think about towing anything. 3.90 gears will pull a load and accelerate quickly, but might cost you in economy. If you were running a transmission with a 1:1 final drive, the above would be true.

The 700R4 is an Overdrive transmission, with the final drive ratio being around 0.7:1 Using this transmission with 3.90 gears would give you all the benefits of the low gearing for acceleration, torque and grunt, but in OD it will also give you the economy. The 3.90 gears will affectively be 2.73 gears in OD. This would be a win/win situation.

2.73 gears and a 700R4 would not be a very useful or economical setup.

Are you sure about that ratio? GM has no listing for anything higher than a 3.08 in an S10 Blazer, and the 4x4s mostly came with 3.42, 3.73, or 4.10.

Just a thought......you also need to consider where you're going to be doing most of your driving and at what speed. The stock 235 makes it's most efficient torque at between 2000-2300 rpm which leaves you and additional 300-400 rpms useable for those spurts when you want to increase your ground speed.

Correct me if I'm wrong but the 700R's I've seen have a .63 OD. If you use this OD ratio and say, a 29" diameter tire and a 3.90 R&P ratio you'd only be turning up 1708 rpms.....a good deal on the short side of optimum engine rpms at 60 mph. You'd have to be running 75 mph to get your rpms up to 2245 where you'd be running more efficiently. You need to use the bottom calculator and crunch your numbers based on your own tire diameter, R&P ratio, normal cruise speed you desire, and your transmission top gear ratio.

http://csgnetwork.com/multirpmcalc.html

I set up my 54 truck with a built 235, .72 OD T5, 28" tall rear tire and a 3.42 R&P to cruise at 70 mph@2000 rpms and my vacuum (efficiency)doesn't come up until I'm nearly running 80 mph. I would have been much better off (closer)with a 4.10 R@P and a cruise speed of 65mph. If your average cruise speed is 55-60 mph you'd be lugging your engine down and your transmission would be downshifting....... alot!!!!

The key to having an efficient and fun vehicle to drive is to have all the variables matched up. Try crunching your numbers for a 350 transmission with a 1:1 top gear and I'll bet you'll be into the optimum torque curve at 60 mph@2100 rpms.

If by chance I'm incorrect about the 700R OD ratio and it happened to be .72, then a combination of 29"tall rear tire, with a 4.10 R&P would have you cruising at 60 mph @2100 rpms. Not great but better than the .63.

Something I've noticed is that there are a lot of 3.08 and 3.42 diffs (62" widths) on the mid to late 80's Camaros if you wanted to go with a 350 transmission. The key is to plan out your own particular needs and go from there. Hope this helps.

Dave

700r4s od is .7
200-4rs od is .67

Oops, I made a mistake, they are 3.73 gears i the rear.

Here's some formulas you can use to help get your engine rpm into the right cruise range.

60mph rpm = 20168 x Rear axle ratio x Trans OD ratio / tire diameter

If you don't have the tires to measure, you can estimate diameter with this-

tire diameter = 0.000786 x width x aspect + rim size
where width is the 215, 225, 265, etc number before the R and aspect is the 75, 65, 60 number after the R.

So if you're running 215R75-15 tires on your 3.73 rear end behind a 700R4 with 0.7 OD, you've got a 1900 RPM cruise at 60 mph.

My 235 has peak torque at 2000 so that's be about perfect for me.
In fact I'm running 215R75-15 tires, a 3.90 rear axle and a BW R10 0.70 OD. My 60 mph cruise rpm is just under 2000. Two thumbs up.

regards,
Leon