MGB: Non OD 4 speed

Buying a '75 B without overdrive trans. What kind of RPMs can I expect at hiway speeds (70-75) and what would they be with OD trans? What would be appx cost to buy an OD trans? Thanks

4k to 4200 with overdrive 3400 @ 75mph. Cost wise between 5 to 1k for the tranny 200 for a new clutch another 100 in might as wells plus labor.

Figure on these conversions, your results may vary;
Top gear speed per 1000 rpm
standard 18 m.p.h (29 kph)
overdrive 22 m.p.h (35 kph)
RS

There is no way to financially justify buying an OD. There is no substantial fuel savings. Do the math and you'll find that you would have to drive many tens of thousands of miles, all at highway speeds, for the OD to even start to pay for itself.

If you want one, buy it just because you want one and/or you like it to be a bit quieter at highway speeds. Keep in mind that the major benefit of ODs is just the volume level of the drivetrain at highway speeds. It's not going to do anything significant as far as saving fuel or reducing wear on your engine. These engines are happy going 4500 RPM all day long. (But it's loud!)

If you want a little quiet and you've got the dough, go for it! If not, don't sweat it! :beer:

Thanks All - answered my question.

Just did the numbers and calculated that we have to drive about 2500 highway miles for the gas savings to pay for the OD. We are currently getting about 25 mpg city and highway and 31 highway. We have already driven about 500 highway miles, so only 2000 to go!

The OD adds a touch of British sportscar driving that can't be justified by dollar signs. It adds a vintage fell to the car and allows the driver to "split" gears. I installed one 20 years ago and still marvel at the enjoyment it brings when I drive the car. RAY

There is an OD on Craigslist up in Litchfield (I think). Been there a bit.

Kathy wasnt too fired up about the OD when I first installed it. She was in love with her new cruise control (that I put in). She was a little unsure about it the first couple of trips we took. Fast forward to now.......she loves it!! On the highway now she waits about 30 seconds before engaging the OD, then the cruise control and sits back to enjoy the ride.

[quote="Kats Bee"]
Just did the numbers and calculated that we have to drive about 2500 highway miles for the gas savings to pay for the OD. We are currently getting about 25 mpg city and highway and 31 highway. We have already driven about 500 highway miles, so only 2000 to go![/quote]

Wait a minute - you can't compare city to highway mileage to get your numbers. You have to compare highway mileage with OD[/u] versus highway mileage [u]without OD[/u]. The difference then is a fraction of what you're calculating - probably something like 29 or 30 MPG versus 31 MPG. Remember that there's not that much fuel consumption difference between, say 3500 RPM versus 4000 RPM. And remember that the fuel consumption at 3500 RPM isn't simply 88 percent of your fuel consumption at 4000 RPM even though 3500 is 88 percent of 4000. It isn't a linear progression. The fuel consumption at 3500 is probably 93-95 percent (give or take a bit) of the fuel consumption at 4000 RPM.

I don't recall the calculations I came up with before on this forum, but it's more like 15,000+ miles [u]at highway speeds in OD[/u] to pay the cost of an OD, clutch, etc. YMMV.

There is no way to justify the cost from an accounting point of view. It does [u]not make fiscal sense - just get one 'cause you like the luxury of it! :beer:

Has anyone give a thought to the 5 speed conversion kit at moss? Is it too crazy expensive?

I've got 2 cars. My 78 B with an OD and an MGA with a B motor and a Riverside 5 speed tranny conversion. (Much cheaper). The RSide uses a a Datsun Z 5 speed tranny (about $275 - 300) and the adapter Riverside kit (about $800). Less than 1/3 the cost of the Moss Kit.

The justification rationale is NOT gas savings. It is comfort and noise and vibration reduction. Obviously the engine runs at lower revs and you could make a case for improved engine life. The big advantage is being able to cruise with traffic in the middle and left lanes instead of playing Mr. Magoo in the right lane. You'll find the car actually keeps up with traffic safely and comfortably.

PS - Although the OD is nostalgic and original, the Datsun/Riverside tranny is actually a little better and geared a even lower and is effortless. Fifth gear is a breeze down the highway.

[quote=mikem67]
Has anyone give a thought to the 5 speed conversion kit at moss? Is it too crazy expensive?[/quote]

Moss has offered one for years. It's about $3100+ See it here:
Moss 5-speed conversion kit

I don't think it is as well thought-of as some of the other conversions and performance stuff Moss makes.

[quote="Kats Bee"]
Just did the numbers and calculated that we have to drive about 2500 highway miles for the gas savings to pay for the OD. We are currently getting about 25 mpg city and highway and 31 highway. We have already driven about 500 highway miles, so only 2000 to go![/quote]

Sounds a little low to me....I would have thought a LOT more miles than that to cover the cost of an O/D install.

I find 3200 is about 55 mph and 3600 is about 65 mph.

I usually drive at the lesser speed - the engine just seems to the over-reving at anything faster than that..

[quote=wa6rxm]
I find 3200 is about 55 mph and 3600 is about 65 mph.

I usually drive at the lesser speed - the engine just seems to the over-reving at anything faster than that..[/quote]

These engines are designed and made to be happy as clams going 4500 RPM all day. You'd end up with some high-end hearing loss after many days of that, but the engine won't mind it a bit!