3.5 REAR END---Worth it???

Is it worth it to replace the 3.8 rear end on a 59 BT7 with a (3.5) rear end that allows for lower RPMs on the highway.

Thanks in advance for the feedback

Will Harrison

Will

I think it depends on the type of driving you intend to do.
If it is a lot of freeway cruising at a fairly high speed then yes I would recommend the conversion.(Do you STILL have a 55MPH speed limit?)

If you intend to use secondary routes with lower speeds then probably not.

I intend to fit to my BJ7 this Winter.

Cheers

Alan

Will,

I converted to the 3.54 on my BJ7 this spring and was concerned about the drivability because we have some very steep hills in Laguna Beach CA (one street looks like it belongs in San Francisco and the road to my house freaks people out all the time) Anyway, it has been no problem. I use first now for starting out but the motor has plenty of torque in 2nd for everything else. What has been great is the car is quieter at highway speeds and gas milage has improved. I think the 3.54 ratio should have come standard ... I don't see any downsides. If you do go this route, make sure you have someone qualified to set the gears up properly and make sure all the bearings are replaced. I even had my gear set micro polished which reduces the noise, temperature and increases the power but thats probably more than you wanted to know!

-John

How do I determine which rear-end ratios my 60 BT7 has? What is involved in the conversion if I decide to go that route?

There are numbers stamped on the Differential Case (the part right behind the drive shaft). The numbers stand for the number of teeth on the crownwheel and the pinion. You divide the first number by the second i.e. 43/11 = 3.909 or 39/11 = 3.545

Basically the conversion requires pulling the drive shaft, differential (pumpkin),axles, having someone set up the new gears and reinstalling with new seals and bearings as needed. It's pretty straighforward but you should get a workshop manual to see if its within your mechanical scope.

I did the pulling and reinstalling and still spent around $700 for the new gears, set-up, bearings, gaskets, micro-polishing and shipping.

Hope that helps,
John

The lower engine RPM would be nice for cruising and would make the car less "buzzy", but be sure you understand the value of this conversion from a "moneywise" view.

The lower (numerically) ratio will give you about 8% better gas mileage and a similar increase in engine life. For a car driven about 3000 miles a year, that's a gas saving of less than $20 a year. Might save a quart of oil too. Hard to say how much the 3.54 ratio would increase engine life....some, I'm sure, but not a dramatic increase.

If you want a car with "long legs", go for it....just don't expect any financial payback for a good many years.

Doug, There is a cool speed calculator at
https://www.teamhealeytexas.com/ With my tire size of 165x15 = 25.3 Dia. it came out at follows (approx):

@70mph the rpm changes from 2840 to 2600
@60mph 2430 to 2200
@50mph 2000 to 1800

I figure a 9 1/2% reduction with the 3.54 rear end. As aeronca65t points out that does not save you a lot of gas money. It is nice however to go close to 10% farther on a tank of gas and cruising along at 75 to 80 (typical California speeds)is much more relaxed.

John

Just in case anyone is curious how/where to order the 3.54 crownwheel and pinion set. Contact Michael Lempert at mdlempert@worldnet.att.net Michael put together the first production run and said he has been getting enough requests to do a second smaller run at a slightly higher price. He should be able to answer all your questions.

Cheers,
John

So John, how much did RPM drop with the new ratio? signed; 55k@65mph!