'89 BII Engine RPM miscalculation

dispatchrobb · Mar 30, 2013

I'm trying to find my actual engine rpm and have looked at the calculators that should figure this out for me and have done the calc myself, but something is wrong since I'm not actually getting what they say I should. I have the 2.9L engine, I'm running 33 inch goodyear wrangler mt/r, I have the m5OD-r1 trans, and what I was told were 4.10 gear ratio but I really think they're 3.73. So at 70 mph my engine rpm should be 2288 give or take. it's not it's about 2900 rpms. Even with 4.10 it should be 2515 rpms with the .79 overdrive. I'm clocking my speed with a gps so I know that part is right. I'm getting my rpms from my tach.

What am I missing? I don't want to be running my engine at 3000 rpms on the highway if I don't have too just to keep up with everyone else and seem like I'm gonna burn my engine up.

Thanks

adsm08 · Mar 30, 2013

If I understand how you are doing this, then I don't think you have 3.73s

A 33 inch tire with a 4.10 ratio at 70 MPH will produce an engine speed of 2900 RPM. In 4th gear.

The way I read this (and correct me if I am wrong) your numbers were taken in overdrive. At .79 this should give you roughly 20% drop in engine speed or require a corresponding 20% drop in rear end ratio. This means you probably have 4.56 gears since those are the closest ones available to what my numbers are coming up as (4.88, but I don't think anyone makes 4.88 gears for the 7.5).

To be honest though, the 2.9 hits its peak MPG between 2500 and 3000 RPM, and will do better if you can hit that number around 60 to 65 MPH.

dispatchrobb · Mar 30, 2013

Yeah, I'm doing the 366.1355/33 = 11.095 * 4.10 = 49.480 * 70 = 3184 * .79 = 2515 RPM. With the 4.56 at 70 in 4th is 3541 rpm and in 5th is 2797 rpm.
The individual I purchased this from said he had swapped axles into the 4.10 8.8 but maybe he was mistaken. I don't think it's an 8.8. There's no tags.
Is the RPMs somehow calculated by the speed sensor by some chance, I don't have the correct gear in so when it says 60 I'm actually 10 mph faster on the gps.

Also, running for 2+ hours straight at 2800-3000 rpms on the highway really shouldn't worry me?

dispatchrobb · Mar 30, 2013

Found a tag in the glove box that has "SL10A 75 6C06" I know the actual axle is a 7.5 based on the diff cover (previous owner said it was 8.8), so I'm gonna go out on a limb and say the rear is a 7.5 Limited Slip with 4.10s, if that's the tag for that axle. I'm still confused because driving at 70 on the gps I'm still off by about 400 rpms. Maybe my tires aren't 33" anymore? The front is a 28 and not sure why someone would change gears to 4.56 instead of putting a better axle on it.

adsm08 · Mar 30, 2013

That is not the tag that tells you the real axle info. I think that one is more of a casting number.

As for the engine speed being calculated by the vehicle speed sensor. NO.

You have a sensor in the distributor that give the engine speed data, you speedometer is cable driven.

I'm wondering if the speedo drive gear is incorrect by a lot.

4x4junkie · Mar 30, 2013

dispatchrobb said:
Also, running for 2+ hours straight at 2800-3000 rpms on the highway really shouldn't worry me?

I have 5.13s and 35" tires (same as 4.88s and your tires) and I think it's absolutely perfect. I've driven it all up & down California, to Oregon, touching Washington, down to Arizona, Nevada and more.

With 4.10 gears you would actually be lugging the motor in 5th as adsm said.
It's possible your tach is off too (mine's about 10% fast it seems)
What I would suggest is jack up a rear tire and give it exactly two rotations while counting your driveshaft's rotations to verify what your actual gear ratio is (3¾ turns = 3.73, just over 4 turns = 4.10, 4½ turns = 4.56 and so on).
This assuming you don't have an aftermarket rear locker... If you have a locker then you'd have to jack up BOTH wheels and turn them once while counting the driveshaft turns.

dispatchrobb · Mar 31, 2013

I'm going to take a wild guess and say it's a combination of having 4.56 gearing (that the previous owner didn't know) and my tach is off a bit, but I will jack it up and check when it's in the garage. I took it out on the highway yesterday and checking my speed with a gps I wasn't anywhere near the rpm's I should have but I was closest with the 4.56 calculation.
My speedometer is way off too, when I was doing 70 on the gps, my speedo read about 57. It's a progression, the faster I go the wider the gap.

'89 BII Engine RPM miscalculation

dispatchrobb

Well-Known Member

adsm08

Senior Master Grease Monkey

dispatchrobb

Well-Known Member

dispatchrobb

Well-Known Member

adsm08

Senior Master Grease Monkey

4x4junkie

Forum Staff Member

dispatchrobb

Well-Known Member

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