Rear axle swap and MPG change

[jameshoward]

This is slightly self-indulgent, so forgive me, but I wanted to record somewhere here the change in MPG when I change my rear axle from the 4,08 to the 3,46 perhaps to help others decide whether the expense (not inconsiderable, so far) and time (very considerable, not all of mine, either) is worth it. The car currently has the 4,08 and today I drove from the SW UK to central Germany (moving house; beautiful weather; very dull drive). I kept the car at a steady 70 MPH pretty much throughout the journey, which is rare given the tendency of this route to grind to a halt periodically. At 70MPH with the 4,08 the car is doing about 3800 RPM (measured off the tacho; the speedo is accurate). The road was pretty flat; traffic light, but the car was reasonably heavy (fully loaded, but only one passenger) with hardtop. The car is pretty well set up, thanks to Joe's linkage tour, and runs very well. It's on the black coil and uses points. I was using Shell 98 octane unleaded fuel.

Whilst I need to do one more fill to get a better picture of the MPG, a snap shot covering 209 motorway miles shows that the car returns 32.08 MPG (209.1 miles and brim to brim (absolutely completely full) was 29.63 litres.

I always knew that the car was pretty economical compared to some other cars of fellow members (as I found out on the last Euro Tour), but this figure surprised me. Oddly, it's more economical than my MY12 Land Rover diesel (8 speed ZF box!). I should add, though, that these are purely motorway miles and no stop/go. Mostly covered in 4th gear, also.

Anyway, once I've re-assembled all the brake parts and have drilled a new hole for the breather tube (as per Dan's suggestion) I'll post the details of the MPG with the new axle. Hopefully it'll be of Prius standard! (Or not).

[dwahi]

Wow, thats very good MPG! Im going to try mine too one of these days as the weather is good here in the UK. Im wondering if there is much difference in driving with the soft top or the hard top, although i think the latter is probably better for motorway miles and long journey?

[Cees Klumper]

I have not noticed any change in the fuel consumption of my car after the swap from 4.08 to 3.69, although it stands to reason there would be some impact. The biggest difference is probably due to highway/speed/city driving factors. For exMple, my 7.4 liter V8 Camino gets almost the same mileage as the Pagoda on the highway, but worse driving around town.

[Benz Dr.]

Which axle did you change to?  32 MPG seems unusually high.  I'm getting about 27 to 28 MPG ( IMP ) with a 3.92 and the ZF 5 speed gear box which give me a final drive ratio of about  3.35. Your car must be a 230SL?  If you have a 3.27 I suppose over 30 MPG is possible.

[dwahi]

Good point! It seems to be a 230SL indeed (from the signature!)

[Larry & Norma]

I think he is still running a 4.08 and is planning to change to 3.46 at some later date!
So that is remarkable consumption figures.

[Benz Dr.]

Maybe he should check the battery in his calculator. 

[ashley]

I suspect recalibrating the speedometer would have an adverse affect on the mpg results 

[jameshoward]

Yep, 230 with a 4,08 giving 32mpg. But as I said, this is solely a motorway run at a steady 3800 rpm and no start/stop business, so not a reliable overall figure, but a pleasant surprise all the same.

[DaveB]

32 mpg UK is equivalent to a US mileage of 27 mpg (a UK gallon is 20% more than a US gallon). In metric this is 13 km/L or 8 L/100km. It's still exceptionally good, as the official figure in the technical data book is 10.2 L/100km, established at 110 kmh (70 mph). I don't doubt the mileage as he said he already knew it performed better than other cars. So James' car and tune, and driving, are somewhere between perfect and miraculous - what's the secret James?

[jameshoward]

 

I'm not generally (i.e. ever) associated with the terms "perfect" or "miraculous" so spare my blushes, please, help me preserve my entirely mediocre reputation and let's settle for diligent application of the advice offered in Joe's linkage tour. That, combined with magic dust from Naj, and many, many hours spent messing about with the points, timing, linkage (again), idle air screw, filters, and a rebuilt FI pump all help to ensure that the car runs nicely, thank you.

Personal consultations aside, I've learnt pretty much all I know from this site over the years.

However, I need to do one more fill to the brim to get a good idea of what the overall mileage is like, given that the last stage of my journey was more like normal driving in Germany: stop/go in large traffic jams, interspersed with brief but enjoyable periods of 80+MPG dashes past 18 wheelers when road conditions allowed.

Incidentally - and in the lonely 12 hour drive with not much else to do - I can offer the findings of my additional research, which shows clearly that the German driver is most likely to demonstrate publicly his (only the men, I'm afraid) delight at seeing a pagoda tootling along the highways and by-ways of mainland Europe (3 drivers). Next are the Brit's who, who were the only other peoples to acknowledge with a hearty toot and 'thumbs up' (brashness is excused; he was probably from Essex; and there was only him so I can't be too picky). Alas, neither the French, Dutch or Belgians offered any recognition for our little cars. There was a Polish loo attendant at the Esso garage just outside Gent who waved admiringly, but that was only after we'd exchanged words about his less than competitive tariff for availing oneself of his facilities. I think he was currying favour, the blighter. I discount his response as it lacked spontaneity and any feel of genuineness.

So, 4 friendly waves in 12 hours. One in the UK, and 3 from our German comrades.

[Benz Dr.]

Odd thing, this noticing orthers noticing you while driving an old car. I used to pay much more attention when I first started driving these old cars but seldom notice anymore. When I did, I usualy ended up nearly hitting something so decided to let them look and be on my way.
 However, there are times when one can afford the time to notice. Small children come to mind as they're not afraid to yell out '' nice car '' as you stroll down a quiet back street. You wave or toot your horn in acknowlegment.

 Or perhaps the pretty girl in the lane next to you at a stop light, or the one filling up her car at the gas station? However ( again ) it's mostly like an old dog chasing a car. He'd never catch it and wouldn't know what to do with it if he did. :'(