MG MGB and MGB GT Tech Talk
All this talk of O/D's has me wondering, what rpm do you guys do if your steady at 80mph? That is if you have a 3-synchro non-O/D. I'd be curious to see the diff. between the 3 synchro non-O/D and O/D. Anyone HAVE a B w/ a 3-synchro tranny anymore?
I do about 3500rpm in the Audi for steady 75-80mph. I thought that was high. I wonder what B's do.
Checking time between mile markers, mine shows ~64 MPH (63.54) at 3500. 175 tires. Thanks to a post by one of our illustrious gurus JDW and his time/speed/distance calculation.
Stock MGB, non-overdrive with standard size tires clocks out 17.9 mph /1000 rpms,or at 80 mph, about 4470 rpm. The O/D reduces the output drive ratio by a factor of .81 (depending on year), so at 80 mph with overdrive and stock diameter tires the rpms drop to about 3600.
MPH RPM RPMw/OD
55 3072 2519
60 3351 2748
65 3631 2977
70 3910 3206
75 4189 3435
80 4469 3664
'nuff said.
H.
all right, who's the wise guy that got their B up to 80mph?!
80 can be a comfortable but expensive cruising speed if you have overdrive and a fat wallet. Without overdrive it sounds like the engine is about ready to come apart thus drawing the attention of any of the local constabulary
My '64 is very happy cruising at 3500rpm and 65mph. That's the speed limit, so why convert to OD? When all I can see in my rearview mirror is an SUV bumper, I slow down and force it to pass. If the driver flips me off on the way by, I flash him a big cheesey smile.
I'll remember the "big cheesey smile" tip. That's great.
Pete
Do not forget to do this with the top down..
They cannot se you with the top up...
A (Five finger)wave will do then..
With my 195 tires it's a hair under 3500 at a clocked 80 and she seems real happy and not too hungry there. I get 32 mpg at 75 which is the speed I usually run on the Interstates, like to Memphis and back. Well, 75 going down (daylight) and 80 coming back late at night. 90, no problemo, still rock steady and accelerating on a two lane when I quit watching the speedo and started calculating which slot I had to use to get back on my side of the road. The oncoming traffic was closing fast at the speed I was going and I needed to get around one more car to catch a slot long enough to slip into safely.
By the way, If you want to clock your speed by the Interstate mileposts:
Seconds post to post MPH
65.45 55
60 60
55.38 65
51.42 70
48 75
45 80
42.35 85
40 90
After that you're on your own.
Something changed when I posted that. The last two didgets are your speed.
John,
Just for clarification, I'm assuming that you're running with overdrive?
H.
ps. just don't want anyone confused....
Right! That's with OD. I turned about 4 grand at 70 with the four speed in.
The formula for the time/speed table I posted is.
60 divided by the speed you want to check multiplied by 60. Looks screwy, but it works.
John,
The variant of the formula that I use is
MPH = 3600 / Time for one mile in seconds.
It's the same, just put a little different.
If you're turning 4000 at 70 in 4th without O/D, then your tire size must be fairly close to nominal.
H.
My tire diameter is real close to stock but they're somewhat wider.
