Differential question

[mrsprite]

I know this has been hashed out before and I thought I knew the answer, but I need something more concrete than my memory, I guess. What I need to know is, what differential ratio came in a 1978 Midget (built 5/78)?

I have been led to believe that all '78 & '79 models have the 3.7 ratio, but is this true? I know.....look at the stamp in the case. Yes, this will answer it for me, but the car in question is many miles away and I don't want to waste the gas driving there just to look at the diff. (although I may wind up doing that anyway).

So.....<span style="font-weight: bold"><span style="font-style: italic">do</span></span> all '78 & '79 Midgets have the elusive 3.7 rear end???

Thanks!

 

[mrsprite]

Here's another question......I currently have a 3.90 diff and my speedo reads about 4.5-5% fast (based on those radar things on the sid eof the road....you know, "your speed is X"). So, if I put in a 3.7 diff., which is about 4.8-5% taller than a 3.90, should that negate things and make my speedo pretty darn accurate?

I've crunched the numbers and it makes sense that it would, but it's also late and I may be completely off track (wouldn't be the first time!).

Thanks again.

 

[Morris]

Those stupid radar things always seem off to me. They give me weird readings in whatever car I am driving.


Okay... I know there is a joke there, but I really wasn't trying to be funny.

 

[mccalebr]

Lay your (or borrow one) GPS on the front seat. Run up to 2500 in fourth gear, then stop and check the MAX SPEED on the statistics screen. Record it. Then hit RESET. Do the same for 3000, 3500, etc.

That should give you a pretty accurate estimate.

Ray

 

[Sarastro]

Your accuracy check should be based on the odometer, not the speedometer. Speedometers in 60's-era cars always read high. 5% is not half bad, actually. They usually read high in modern cars, too, but perhaps not as much. The odometer is hard-wired, so to speak; each so many turns of the drive cable results in so much of distance indication, but the speed indication is a soft, magnetic coupling which is not as precise. So, as suggested by Ray, I'd get/borrow a GPS, check the odometer against it, and decide if it is OK. (Be sure the tire pressures are right when you do this!) Then, if that checks out, worry about recalibrating the speedometer if you think you need to.

My odometer is right on, but my speedometer is something like 12% high! Someday I'll dig into it, but for now it's OK. I've mentally recalibrated it.

 

[mrsprite]

The accuracy of my speedo really wasn't the gist of my post.....really. I want to swap over to a 3.7 rear end to gain a bit of fuel economy (might be able to hit 50 MPG on the freeway!).

I believe I figured out my second question about whether or not the 3.7 diff will correct the speedo/odometer discrepancy (it won't.....I had things backwards). Just to let you know, my speedo reads about 3% slow and my odometer is about 5% slow. I had it backwards....it's not reading fast, just the opposite. When I am at 30MPH I am really doing ~31MPH......when I have gone one mile, it reads .95 miles on the odometer (checked using the milemarkers in the interstate).

Still trying to figure out if a Midget built in May 1978 has the 3.7 diff though. Anyone?

 

[Guest]

Mine's post 5/78. I'll check on it tomorrow while I have the thing up on a jack.