yet another tranny question, but a good one

Hello every one, I have been scanning most of the threads and I have not been able to find an answer to this question. Probably because every one is doing the opposite, And I appologise for yet an other tranny question. My 78 cj5 is almost restored to vintage and I will not be off roading with it any more What I have is a 258 with a t-18 1B trans and I can find no marking on the diff. ( I assume it is stock) What I need is to decrease the rpms on the highway, It crys out for a 5th gear or overdrive. Is there a transmission that I can use for this, I can give up low 1st. Can one rebuild existing trans with different gears?

 

No, you cannot change the transmission gears. Only overdrive transmissions actually have a gear for the top gear - all others are just straight through, a direct connection from input to output.

What's your axle ratio? There are stampings on a M20, but I'd have to search for them. The Dana 30 will either have a tag or you can read the tooth count off of the side of the ring gear.

You could change to a T-5 (a light-duty overdrive transmission) and Dana 300 transfer case from a later Jeep, but that's really a downgrade. Plus, it won't be a "drop in" replacement. You'll likely have to move the crossmember and change the driveshaft lengths.

1980 was the end of what was known as the 'great transmission era' at Jeep. You currently have what's considered the best and most desirable transmission ever put in a CJ by the factory. You'd be foolish to change it IMO.

Jeeps are noisy. The drivetrain is noisy. The floors have no insulation. The tires are noisy. IMO, unless you want to go well into ticketable speeds on the highway, you won't be near stressing the engine.

Do you have carpets? Is there insulation under the carpet? Do you have a tachometer so that you know what speed your engine is really turning at?

 

I would just change the axle gearing. Leave the t18, it is a great tranny, and you never have to worry about breaking it.

 

for a 78 most likly 3:54 gears or a option of 4:11

 

Well it's a start, I don't have a tack, but I can hear the high revs. I have searched the differential and axle and can find no markings or tags, the only markings are a triangle surrounded with Numbers 1, 2 and 3. Will changing the diff/axle make much of a difference? I heard tires that are big in circumference will help, is there any truth to that? Thank you

 

Indeed. How tall are your tires?

 

Google is your friend.

M20 letter codes: http://www.jeeptech.com/axle/amc20.html
Stamping location: http://www.vintagejeepparts.com/axle_id.asp

 

This is good info, I will go home and measure my tires. So you'r saying bigger (taller) will change the rpms? I will search again to look for numbers on the rear axle.

 

More like it will make you go faster for the same RPMs. A taller tire is bigger around, so you travel further with each revolution. If your speedometer reads 65 mph at, say 2500 rpm, then it'll still read 65 with larger tires, but you'll really be going faster than 65. 10% taller means 10% faster at the same engine speed, and so forth.

 

Here's a pic of the AMC 20 letter code location. In this case, its a side wards "A" for 3.54 ratio, open diff.