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MG MGB Technical - Miles per gallon

I have seen various posting of MGB's with and without overdrive getting anywhere from as low as 18MPG to as high as 36MPG (US gallons). How can we have the same type of car with such great differences.

Our California gas is not the greatest nor is the 87 octane wonderful but I ask those of you who claim 28 plus MPG how do you do it?
Lee

I have a 66 MGB running the original (not rebuilt) engine and gearbox (98,000+miles)using Mobil Super gas, Castrol 20x 50 oil in the engine and gearbox. Still using the original carbs, stock distributor, static timed at 10 degrees BTC, with Champion N9YG Plugs. On our last trip to Vermont and back about (700 miles) we averaged just over 31 MPG. Speeds were between 65 and occasionally up to 75mph. Car never overheated although we did have some 90 degree plus temperaturs that we were running through during that long weekend. We do this at least two to three times per year and consistantly average that kind of mileage. Sometimes it does get a bit better (up to 34 mpg) but I usually expect to get at least 30 mpg on our travels.
Hope this sheds some light on your problems with mileage.

George Herschell
George R Herschell

Lee,

I am one of those that was getting weird gas mileage. I was getting as low as 12 mpg around town.

Therefore, I made a real effort to super tune my engine - carbs, timing, etc. I think leaning out my carbs made the biggest difference.

I am now getting between 18 and 22mpg around town and 32mpg on the open road. A big difference.

I think it all has to do with the way the cars are tuned and apparently there is a big window that is acceptable for our cars and still have them running "good".

Cheers,

Robert
Robert Browning

These engines are 27 to 45 years old. They have been rebuilt a number of times and never the same way, nor using the same parts. Performance and economy will always vary widely between them. Driving style, fuel type a climate will also play a role in economy.

My '67 BGT with overdrive gets 34MPG on the highway with a light foot. It gets 17 MPG in heavy traffic or in "spirited" driving situations.
Steve Simmons

The biggest variation is who is driving it. Some peeople are naturally very inefficent drivers, jumping on and off the throttle, driving up to an obstruction then going straight from throttle to brake, and so on. Next is basic setting-up of carbs and ignition, and grade of fuel.

I get typically 33-35 UK gallons in my roadster, but 1500 miles touring in France got closer to 40 over several tankfulls. The V8 typically averages 27, can be as low as 15 knocking round town, but taking it a bit steady coming home from Le Mans one year did 34!
Paul Hunt 2

Do these cars getting 30+mpg on the highway have overdrive? If so, what would one expect for a non-overdrive car to get for highway milage. I'm averaging around 18 mpg with around town driving.

BH
BH Davis

My B, discussed earlier, is without OD. It has the standard MGB Engine as it came from the factory with no rebuilds and the same gearbox, ditto on rebuilding. The only change has been a new clutch assembly. Other wise everything else is stock. (and old like its owner)

George Herschell
George R Herschell

BH, they must!

I consistently get about 25mpg on the highway with my non-overdrive tranny, 185 tires and points. I use premium, 93 Octane gas without additives and GTX 20w50.

I sometimes wonder if slimmer tires, e.g. 165's, thinner oil and an electronic ignition would make a difference.
Philip

Just completed a 400 mile (+) weekend of competition in the Pocono Mountains at the British Marque Club News 'Triathlon VII'. With my wife driving the Road Rallye, and Hill Climb (Mt. Pocono) speeds under 40 MPH (mostly) we averaged 27 MPG. Driving up to the lodge from around sea level to 500 ft (+/-), and back down with myself driving it was just over 32 MPG with overdrive where possible. Engine and drive train stock including emission controls.

Regards,

Larry C. '74 B/GT
Larry C.

Having had a bunch of B's over the years, some have had better gas mileage than others.

Typically, mine average anywhere from 19-22 mpg around town and 27-30 on the highway during warm weather. Winter sees a drop by 2-3mpg. Most of my cars have had overdrive, but a few did not. It didn't make a huge difference either way.

My son and I did a 5k mile trip this year from Chicago through the southwest, up through Colorado and then home. That B has a 'tweeked' motor along with a Moss SC and OD. It also has 195/60/15 high performance (sticky) tires. Our worst mileage of the trip we got 22mpg (stop and go, slow, mountain driving). The best was 31mpg. Overall trip average was spot on 27mpg. The needle isn't quite right yet - it wasn't hard to keep the mileage around 27-30mpg as long as we kept the speed down. As soon as we went above 75, mileage instantly dropped to around 23-24mpg.

Some of the gas was downright awful (90 octane oxygenated) and needed octane boost to keep the engine from detonating itself to death (that's a BIG issue with a SC motor!). Most places were pretty good. We ran with 91 ocatane most of the time, but filled up with 93 when it was available.

Tom
Tom Sotomayor

From the comments I have had with other MGB owners locally, they all get around 20 in commute traffic and 24 highway. No one had been able to get anywhere near 30MPG most of us now have overdrive.

Do those of you that are getting high mileage fill to the opening of the filler or do you just allow the automatic nozzle to stop. Sometimes I have been able to get an additional two gallons after the stop to fill to the lip. What octane rating are you using?

Paul's mileage must be imperial miles per gallon. Divide by 1.2 to get U.S. equivalent.

1970 MGB GT, overdrive, new SU's, tuned to factory specs., approximately 50,000 miles since rebuild, 175x14 tires, 20/50 castrol engine and gearbox.
Lee

92 octane, 32-34 MPG on the highway with overdrive on and a very light foot. Topping up should not be an issue so long as you fill up at the same pump for each reading. taking an average over multiple tanks is even more accurate.

Also don't forget to factor in odometer error, and the fact that GTs will always get better highway mileage due to better aerodynamics.
Steve Simmons

Regular gas, Aluminum head, holly two barrel, no smog, no catalytic converter and I am running around 27. I do not really do any driving around town. suppose I could play with the carb to increase that but it is a godsend over my durango 4X4
Peter Murray


I'm getting a consistent 30 US mpg. It's mostly open highway miles. I live 25 miles from work, i.e. I drive my MGB 50 miles a day just for commuting, so I must lay down around 1200-1500 miles a month all told.

When I first got it, this car got about 20 with me driving.

Biggest mileage improvers for me:

1) I've got a Stromberg, so I cut off the horrible stock exhaust manifold and replaced it with a header. This one mod gave me loads more power and several mpg! Any stromberg owner should consider this one. Note, this is without getting a big bore exhaust - this is just a stock exhaust with a header instead of the stock stromberg manifold. I believe SU's don't have the same exhaust problem.

Speaking of carbs, max power happens at about 12:1, where max efficiency is just over 15:1. Chances are, you're running rich. So unless you're racing, don't just tune for power. Tune to stoichiometric, then let your damper oil do the enrichment. That said, I have a stromberg, therefore my carb is running rich (though as lean as it'll go), and I still get 30 mpg.

2) Overdrive. Never let anyone tell you overdrive doesn't improve your mileage. Take it from someone who pulls one hell of a lot of freeway miles in a B, without an overdrive you'll never get out of the 20s.

3) Compression ratio. The '78 came with dished pistons and an 8:1 compression ratio. When rebuild time came around, I put in some flat pistons. 91 octane is required, but it rewards me with every drive. Good for at least a couple MPG.

4) Modern motor oil. This, my friends, is going to raise some eyebrows. I'm running full synthetic 5w-30. Oh, yes I am. Really. I broke in the motor on the cheapest oil available, of course, but once the rings seated I switched to full synth and have run full synth ever since. I drive it a lot, my rebuild was a couple years back, no trouble at all, over 40,000 miles with no loss of power or mileage. I start to wonder if all the reasons for avoiding synthetic oil were actually excuses made up by engine rebuilders whose engines just flat out failed. Some say my tappets will die if given modern oil, yet my valves keep their adjustment for months at a time (like 6000 miles) so the tappets are assuredly not grinding away any more than they normally would have. They say 20w50 is the 'right' viscosity, but so long as my oil pressure stays in spec and my tappets are happy, can I seriously have a viscosity problem? I did try running 20w50, and it runs better and stronger on the full synth 5w30, and gets another mile or so per gallon. Reduced friction is not a bad thing. I do use an oil cooler, and when I was in a hotter climate I ran 10w30 (!) Anyway, as a public service, I'll keep running lightweight full synth oil until either the engine hits 100,000 without trouble or it dies prematurely, and I'll let you know the results of this experiment. So far, though, things are looking good.

Lee, I actually think you're getting pretty good mileage considering where you live. I've lived in SF - it's a gas mileage wormhole. If you're not stopped in traffic, you're climbing a hill. I had a Nissan there that got, like, 26. Leave the Bay Area, suddenly it gets 35. Go figure. If you want over 30 mpg in San Francisco, you'll probably need to get a Prius.
Sam Good

Sam,

I ran a previous '73 for years without o/d. On long trips we consistently got over 30. When I added o/d, it went up to a high of 34. Slight, but noticeable improvement. In all the years I've run MGBs, I can never remember mileage less than 20 mpg in town and 28 on the road. I've never done anything in particular to achieve this.

I do agree with Sam in that metro SF is not an ideal testing ground for gas mileage. On our recent 17-state+one-province trip, I also found California gas to be notably inferior to anything I bought on the entire trip.

The one other factor nobody has mentioned is tire pressure. Don't go by the original recommendations unless you're using 1960s tires and 1960s expectations. Just before our trip, I installed a set of new Cooper CS4s and the dealer set 'em at 35# all the way 'round. He said I could soften 'em a bit if the ride was too firm, but it wasn't and I didn't. Mileage for our trip ran 28-33 and we ran at 80 mph for about 1,000 miles across all of Montana and North Dakota. Now I wouldn't do THAT without overdrive! I know some guys would think nothing of it, but I couldn't stand the sound of the engine screaming at 4400 for two days steady.

My mileage has always varied from one MGB to another. The best was my '76 B with o/d, HS4s, a mild Schneider cam and 9:1 pistons. That car routinely ran 34 mpg on the road - occasionally 35.

FWIW,
Allen
Allen Bachelder

If you can't get 30+ on highway, OD or no, it's not right.

Sam's comments re oil are dead on - there is no need or use for 20W-50 unless the engine is tired. 10W-40 should be fine for all conditions, and far better when cold - cold is any trip under 1/2 hour on the highway under load, regardless of ambient. That's for earth oil. Along with that, the mania for excessive oil pressure - above 60 hot - is a fuel and power waster.

Allen's tire pressures are also good, and toe-in and brake adjustments have major effects.

I have many times posted the necessity of having the air filters in place for top end running with non-modified carbs; this is especially important with ZS carbs, as they are awful without the equally awful filter. However, I recently did extensive electrical work on a ZS car, which required hours of running at 2500 rpm in testing. This was perfect at idle, but at faster speeds, the thing was obviously not right, and I found it necessary to set it richer and richer to get it running right. Then I had the bright idea to put the filter back on and it went way rich. The conclusion was that even just running at a v fast idle, the filter has a major effect. If the thing is tuned with the filter off, then it will go way rich under even slight load - kills mileage AND power. These late engines are so screwed up that significant variations in timing and mixture produce very weak responses in apparent tune, while having major consequences, while the car appears to run "fine" over a wide range.

While doing this I fooled with the Ign timing too - and found that as IO got the mixture right, I could advance it quite a lot, to the point that I backed off the idle screws so much I had to reset the throttle cable twice, as I used up all the slack I had left in it. The added advance also resulted in very noticeably lower underhood and exhaust temperatures. These things will amount to significant savings, especially if you drive in traffic a lot with much idling and low speed driving.

I have also found that late cam timing, either from assembly or chain wear, causes major drops in fuel mileage with the car still running "fine".

But the biggest thing is still the driver...

FRM
FR Millmore

Fletcher,

It MUST be the driver! For years, I've been trying to improve my tuning skills, and for the same number of years I've become increasingly aware of my own temperamental deficiencies in dealing with tuning problems. Tuning, ultimately is a subjective matter. I spent my entire working life in the subjective world and MGs provided huge relief in that most of the issues I face are objective. While I'm careful to tune as well as I can, I never (save once) leave the job satisfied that I'm much more than close. So I turn a few screws, the car never idles at the same speed twice but it feels pretty good under load. I leave it alone and, presto, I'm getting 28-35 mpg on the road.

My point is: if I'm getting it, it ain't hard!!! The car runs smoothly - except at idle - but I always get the mileage. So I have to conclude from my own experience that it ain't rocket science!

FWIW,
Allen
Allen Bachelder

Hmmm, I wouldn't have said tuning (setting-up?) is subjective as the books are quite clear on what has to be achieved. Interpretation of arbitrary settings is subjective though, as is driving the thing.
Paul Hunt 2

The best I ever recorded was 27 mpg. If I just drove like an old woman and never went over 65 mph had the top up and it was mostly down hill - maybe i could get to 30... Big maybe
Frank Baker

Well, just got back from a 200 mile round trip to southern VT. A little 60 mph highway driving and a lot of 50 mph foothills up and down.

I got 23 to 24 mpg for the trip. Not as good as I'd hoped, but not that bad I guess.

It was my first long haul since getting the car and it ran terrific. Quite a fine day of playing hooky from my office! Sunny too!!!!!

BH
BH Davis

Me too!
Synthetic oils.

I'm going to try the 5W-30 Amsoil Syncromesh fluid in the gear box next.

On a road trip in June of 2005 to 'Rendezvous' in Grand Rapids Minnesota, I averaged 24 mpg driving to and from, and also participating in the 'airport using airport transfers tunbridge wells autocross, and high speed run'. I was clocked by radar at 90 mph on the runway before I had to shut down.

None O.D. but I have one one ready to go in soon.
But I will wire it to work in all forward gears, and try to remember to shut it off when using reverse...........

SF
Dwight
P.S. this is a '77 with rubber bumpers and has a '72-73 engine tweeked .
I have the Sebring front and rear valances to go on hopefully soon as well.
DCM McCullough

I strongely recommend to use the switch that limits the overdrive to 3rd and 4th gear. In the lower gears you have massive torque for which the overdrive isn't designed. Hitting full throttle in the first two gears can damage or even destroy your overdrive.
So be warned! There are some topics at this forum where this is discussed.

Rainier
Rainier

And if you forget to switch it out in reverse you will destroy the Sprague clutch instantly. Using OD in first and second causes glazing of the linings as the springs and hydraulic pressure aren't designed to cope with the torque reversals generated. That's also why V8s eventually were modified to have OD on 4th gear only.

77 and later North American spec cars having OD on 4th gear only is a different issue.
Paul Hunt 2

67 MGB GT with 85,000 miles. Do not know if PO rebuilt it but does not appear to have been.

Trip to Gattlinberg from Wisconsin averaged 25mpg (70 to 75 most of trip). Put an OD trans in the car this winter and did a 300 mile trip this summer. Average was 27. Most was freeway.
BEC Cunha

Lee, I sat with you at the MGOC picnic last week and as I told you I'm using my 74 GT as a daily driver in Northern California traffic, 70 miles every day. I was suprised at the low mileage I was getting, which was 22 mpg US gallon. I also have a 67 Roadster and I get exactly the same mileage with that. Both cars are in top condition as you saw and the engine in the GT was rebuilt 5,000 miles ago and has new everything. Now I've said that it is possible that the engine is still tight and might get better mileage when it's "run in". I thought I was getting better mileage before the rebuild but terrible oil consumption.

Andy
Andy Preston

This thread was discussed between 13/08/2007 and 23/08/2007

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