YOu may know that I've been fighting insanely bad fuel economy for quite a while. I recently installed new throttle shafts, KN needles, and jets in my SU HS6 carbs, and coupled this with an M41J taken from my old 1800E.

The 1800E has a 4.30 rear, while the 130S has a 4.10 rear. It's great having an overdrive with a 4.10, but it has gotten very difficult to figure my mileage since the upgrade.

After all my brake issues, this week was the first that I took the car to work each day. I filled up my tank Monday afternoon leaving work, noted the odometer, and checked the reading again when I arrived at work Friday morning.

After 4 round trips at 30 miles each, I had logged 120 actual miles, but my odometer read almost exactly 100 miles

The difference appears to be that the odometer reads .833 miles for every actual mile traveled. Does that sound correct? That would mean I'm also traveling that much faster than my speedo registers?

So 50 mpg is really 60 mpg? It does not "feel" like I'm that far off, but I do think I'm going faster than the speedo tells me.

I had planned to avoid all this trouble by using the 1800E's gauges and rear axle in my wagon along with the M41J - a complete drivetrain transplant. it hasn't worked out that way since the wagon needs some new panels welded in before I can paint and re-assemble it.

Anyway I'm going to wait until the fuel gauge drops some more before fueling up and figuring actual mileage, but so far, I put 135 miles on the car and my gauge has just dropped to 1/2 - putting my fuel use closer to 6 gallons.

If correct, that would put me up over 20 mpg... I'm crossing my fingers...

16mpg has been my max until now...

My 63 wagon with J-type M41 records about 5% error on the Odo and 10% error on the speedo. Error is such that an indicated 110 km/hr has me travelling at 100 km/hr. I discovered the extent of the error with a GPS after I did the O/D upgrade. I only regret that I didn't check with a GPS when it had the original setup. I have just learned to live with it. My passengers think I am reckless driving 10% over the speed limit everywhere.

I use a spreadsheet to calculate mileage and have just put a 5% fiddle factor in to compensate.

The odo is hard geared, whereas the speedo relys on slippage from a magnetic coupler working against a spring. The speedo will not necessarily be accurate to the odo. If you wanted to get things exactly right, you'd need to sort the gearing from the M41 (I figure a 22 tooth wheel in the J-type would be perfect for my setup), then pull the speedo out and calibrate that to suit. I have read of this being done by adjusting while driving along - using the services of a helpful co-driver with a GPS.

Why not use the speedo drive gear from an M40. Just today I sold an M41 and checked the drive gear against one from an M40. Both were the same part number and yellow in color. I know the M41 had been installed in an M40 car, so the previous owner had made the switch. Many Interstates now have mileage markers every tenth of a mile--and they are easier to see than the old signage. I do a speedo/odometer check on each "new" car I use when I'm out on the road. Take a reading between what your odometer says and the markers--drive 10 miles (actual) and compare the results.

I think you must be looking at an M41D. The J gear is quite different from the gear in the original M40.

Generally sedans use yellow and wagons use green. The gear in the M41J is black and differs in size and shape. I checked when the M41J went into my 67 wagon and did not think it would fit.

I kinda wish I has not already modified the 1800e's rear to fit into my wagon. It'd be a bolt-on replacement for the 68 sedan and I'd know that the gear and rear were matched perfectly.

A clarification...
I know that my commute is 15 miles each way - verified with mapquest and with the trip odometer (before that part quit). Until I switched gear boxes, the main odometer has been spot-on and I have no doubts about its old accuracy.

I calculated a 0.83/1 mile error by dividing the known commute distance into the distance shown on the odometer since I changed gearboxes.

What I'd like to know is if 0.83 is on par with the error that would be expected if you took a 15" tire/M40 gearbox/4.10 rear and replaced that with a 15" tire M41 gearbox/4.10 rear end. The M41 was geared for a 4.30 rear, not a 4.10.

So does the "missing" 0.20 account for the 0.83 odometer error mathematically?
I'm not sure how to run the equation here.

Thanks for all the responses so far.

Of course you are correct about the difference between the "D" and the "J" --my bad--it had been too long since I've examined that area of my formely M40 '69 with it's M41J and 4.10 rear. When I first put the car on the road in '89 I installed an M41D and made the gear switch. Then at some point I installed the "J". I've had lots of wheel/tire combinations--15's and 14's--the measurement of the circumference is generally within 2-3" difference, meaning approx. 2-4% difference. I don't recall the combo used the last time I drove the car a long way but the difference between actual distance and odo reading was 10-12% based on my ten mile mileage checks with the odo reading high. In other words I'd driven ten miles, odo registers 11 point something. I do think you have to take into account the fact that your 15 mile (or 13?) commute is not very far. You are spending a great percentage of your trips in the warming up mode--coolant temp comes up right away but the mechanical parts of the drivetrain and the oils they carry take much longer to full running temp. I think you should not be too concerned with comparing your mileage with that claimed by others, but rather in relative terms to what you are getting and the improvements made along the way.

If the M41 “thinks” you have a 4.30 rear end (i.e. the speedo gear is correct for a 4.30), and you couple it to a 4.10 rear end, the speedo/odo will be in error by approximately 5%. You will be traveling 5% faster than the speedo registers and the odo will register 5% less distance than you have actually traveled. You get this by dividing the new ratio (4.10) into the old ratio (4.30), giving ~1.05. This assumes everything else is equal, particularly tire size.

For you to see a ratio of .833, there must be something else at play.

Cheers,

--
Barry -- 1967 122S 'Betty'

oh bloody 'ell.

Let me think about this "out-loud"

My calculated commute each way is about 15 miles.

Filled my tank Monday afternoon, Odometer reads 49945
drove car home Monday - 15 miles (15 est. total)
drove car to work tuesday, home tuesday - 30 miles (45 est. total)
drove car to work wednesday, home wednesday - 30 miles (75 est. total)
drove car to work thursday, home thursday - 30 miles (105 est. total
drove car to work friday, home friday - 30 miles (135 est. total)

Odometer now reads 50057

50057
-49945
=112 odometer miles

vs. 135 estimated miles

112/135= 0.8296...close to my estimate at 100/120 miles.

But...

135/112= 1.205

Ah, that's pretty close and based on an imperfect estimate, I think I performed the calculation in reverse.

If the mechanical error is more like 1.05, then my "estimated" commute is a bit off and my real distance traveled is 112*1.05 = 117.6 miles and my daily commute is closer to 13 miles (117.6/9 one-way trips). That is possible. Until I break out a GPS, I can't say for sure.

It does not bode well for my mileage however, which would be less since I'm traveling a shorter distance with the same fuel consumption...

I'll post when I fill up next...

Unfortunately for us Volvo never made an M41/4.10 combination but they did do at least 3 different speedo gears at the gearbox end. I think I've read of black, green and white gears with a different number of teeth. Unfortunately I've never seen a definitive chart of what you use with each combination of gearbox/OD and axle. Someone may already have it written down and there maybe one that can get closer to being correct. Sounds like another project for Ron Kwas?
It's possible to get a small gearbox that fits inline with the speedo cable. They can be custom made to correct the error.
http://www.speedometershop.com/ratio.html
On another site talking about fitting Datsun gearboxes into MG TD's, the price of one of these corrector gizmos is $60. Cheap when compared to a speeding ticket!

FWIW, I took the following from Part 9 of my 122 green manual

Tire size Axle ratio Teeth – large gear Teeth – small gear
165S 15 (Sedan) 4.10:1 (10/41) 5 16
4.56:1 (9/41) 5 18
640 x 15 (Wagon?)4.55:1 (11/50) 5 17

Cheers,

--
Barry -- 1967 122S 'Betty'

Hello,

The most accurate way to calculate mileage will require the use of some sort of portable GPS unit.

I do not own one, but I do have the luxuary of having a friend who owns one and he loans it to me when I ask nicely.

Fill the tank, reset the GPS total mileage to zero and go at least 50 miles before returning to a gas station to fill up again to determine how much gas you have consumed.

You can also determine what speed you are actually going by viewing the speed readings from the GPS and compare this to your speedometer to determine your speed reading offset.
--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90501

Hello,

Go ahead and do the math, but I still recommend that for ultimate accuracy that you employ a GPS.

Did you know that your odometer actually goes backwards when you back up?

You won't have that problem with a GPS as it will just add up your total distance traveled.

And when you combine all the variables, I guarantee your math will not be as accurate.

With the data from a GPS, you will able to accurately calculate your fuel mileage and more importantly you will be able to determine what offset to add/subtract to your speedometer readings so you know how fast/slow you are actually going down the road.

You probably know someone that has a GPS.

Some pilots have one or know someone who does.

People who go hiking, hunting, fishing sometimes have them and off and onroad recreational vehicle users might have a GPS.

--
Eric
Hi Performance Automotive Service (formerly OVO or Old Volvos Only)
Torrance, CA 90501

You can also sometimes use Mapquest to calculate your mileage for your commute to and from work.

Mapquest tells me that the route I take to work everyday is 5.1 miles each way, for instance.

If I were to install a GPH meter I could get pretty accurate readings of my daily fuel economy.

Bill