first drive....trying to do speed limit?

[derek_m]

Took my 1950 busi coupe for the first drive today...seems like anything over 35-40mph and the engine is running too high of rpm? Its a basically stock 230, rebuilt couple years ago. Straight pipe to lake pipes. 3 speed trans stock rear axle. G78-14 tires. Talked to a previous owner, he said it will do 50-55 but pretty high rpms.

I understand stock gear ratio is 3.90? Does anyone sell different gear sets for these?

[Young Ed]

Different rear gears are around but not reproduced so you are looking for used or old stock. First steps I would take are to get the proper size tire back on there. The 14s are increasing your RPM at a given speed. Also get a tach either permanent or temp and see how many RPM you are really turning.

[TodFitch]

The first place I found that had a diameter for G78-14 tires listed that at 26.5 inches. A Mopar L-6 engine in good shape should be able to run 3200 RPM everyday all day. With a 3.9 rear end that works out to 65 MPH. If you get back to a stock tire diameter around 28 or so inches mounted tire diameter then that would be around 70 MPH. You can play with the numbers at http://www.ply33.com/Misc/speed#calc

 

Until the gas crisis of the 70s many American cars were geared to be running 3000 to 3600 RPM when running at speed on the highway. We aren't accustomed to that nowadays and it seems brutally fast on the engine. Thus all the effort people put in to changing rear ends and/or putting in overdrive transmissions. But that is what they were designed to run at and with modern lubricants and a fresh rebuilt they should be good for it.

 

When "getting there" is my primary concern, then I'll run my '33 at 62 MPH (speedometer and GPS agree fairly closely) which with my stock tires and stock rear end works out to 3250 RPM. Did that from Tucson to Sunnyvale (about 900 miles) last year. Did that to and from the BBQ the other week (about 70 miles of freeway each way plus some 20 or so miles of 55 MPH or slower roads). My first year 190 cu. in. 70 BHP engine doesn't cut it for maintaining higher speeds on the steeper hills so those are slower, just keep the pedal to the metal and crawl up the hill in the truck lane. Your later 120 BHP or so engine shouldn't be faced with that issue as often.

 

And an older engine that hasn't been run at those RPMs may have developed wear patterns that can cause issues if you start pushing it now. So I'd only recommend running that high if you have a fairly fresh rebuilt. But it sounds like that is what you have.

[greg g]

As noted above your 14's are too short vs stock. According to an on line calculator at 3200 rpm you should be rolling at 62 if your 3.90 diff ratio is correct. A 28 inch tire would raise that to 68 at the same 3200 rpm. Rule of thumb is that 80 to 85 percent of advertised max HP rpm (in your case 3600) is a safe and efficient cruising rpm. So if you do 3200 at cruise you are right in that zone. I run 225 75 15 on my 46 coupe with a 4.11 rear gear set. The calculator puts my set up at 3280 at 65. I have run y car on several long distance trips and have never experienced a problem. I have about 20k on a rebuild.

Also remember you are hearing a lot of other noise that you aren't used to when driving a modern car, fan noise being one of the biggest culprits.

[busycoupe]

Check the actual speed of the car with a GPS unit. My 48 Dodge seemed to be revving high when the speedometer said 50 MPH, but my GPS said that the car was actually going 60 MPH. You may just have sluggish speedo.

[derek_m]

Thank you for the responses. I went for a cruise today with my buddies and their c10's. We did figure out that my speedometer is slow. When it reads 35-40 I'm actually doing 45-50. The comments about running higher rpm back then does make sense, it seems high revving but I have more throttle to go.

I will be adding a tach, do 6v require a different tach than 12v?

[TodFitch]

Might be just me, but I'd get the speedometer to read correctly before I worried about a tachometer.

 

Maybe its just that I associate the only real need for a tachometer with needing to know the maximum RPM I can safely take the engine to when racing and I don't race my old Plymouth. But I do like to know if I need to be keeping an eye out for the local law enforcement officials and a working speedometer does that.

[DJ194950]

50 bus. cpes.should have had a 3.73 as stock unless it was changed.

Time to go under and clean the 3rd. member and see!

They all sound like the overreving compared to any newer car.

Best

Doug

[55 Fargo]

Thank you for the responses. I went for a cruise today with my buddies and their c10's. We did figure out that my speedometer is slow. When it reads 35-40 I'm actually doing 45-50. The comments about running higher rpm back then does make sense, it seems high revving but I have more throttle to go.

I will be adding a tach, do 6v require a different tach than 12v?

Use a GPS or the interim to keep an eye on your actual speed. yes you require a 6 volt tach, shop around, do some searches on here for vendor sources. And at some point get the speedo fixed, and working accurately. If it is the stock diff and gears, no trans swap with some other speedo drive gear, then it may just be the speedo itself. Those smaller tires should get that speedo reading higher, if all is functioning correctly, but lower reading indicates it is not.....good luck

[Lumpy]

I would vote for some taller tires. It sure wouldn't hurt. Taller tires sure look better.

 

My car loves 50mph, feels fine at 55, and that's where I usually drive it. If I do have to get on the freeway for a short hop, it will do 65 okay, but the revs are getting up there. Really the only time I drive it 60 or faster is if I have traffic behind me. After a while I'll pull over and let it go by, and then go back to a nice 50mph cruise.  

 

It is a matter of perception. Sometimes Lumpy's engine seems to be winding at 55, but other days she'll feel sweet at 60, and other days 45 is what feels right. All things considered, I think these cars like to be in the 55mph zone.

 

ken.

[55 Fargo]

I would vote for some taller tires. It sure wouldn't hurt. Taller tires sure look better.

 

My car loves 50mph, feels fine at 55, and that's where I usually drive it. If I do have to get on the freeway for a short hop, it will do 65 okay, but the revs are getting up there. Really the only time I drive it 60 or faster is if I have traffic behind me. After a while I'll pull over and let it go by, and then go back to a nice 50mph cruise.  

 

It is a matter of perception. Sometimes Lumpy's engine seems to be winding at 55, but other days she'll feel sweet at 60, and other days 45 is what feels right. All things considered, I think these cars like to be in the 55mph zone.

 

ken.

I agree Ken, 55 mph with a set of 3.73, 3.90, 4.10 gears seems to be a sweet spot, with bursts to 60-65 mph.

Yes somedays slower speeds seem right, a strong head wind, hills, rough road surface etc. I am really lucky up here, cuz many of the smaller highways you can get away without super high speed traffic. This province drivers are much slower than other regions of Canada, with multilane highway speeds of 65 mph is very common. Traffic does not run at 75 mph , as in other places, and it suits me just fine..

[Lumpy]

Indeed, location has a lot to do with it. I live in a rural area, outside of a small city, so I really never have to get on the freeway, and if I do it's just for a short hop. Mostly I have nice paved rural roads, secondary highways, etc. that go all over the place, the state, and the state next door. (Idaho) Idaho drivers also seem to be slower drivers than Washington state. 

 

On the other hand, if one lives in a large city, the choice is between the freeway (where 75 is the norm these days) or stop light to stop light street driving. I can see where, in that situation, our cars could leave something to be desired.

 

ken.

[50desotocoupe]

The one and only time I have been able to drive my  Desoto on the highway it seemed to do just fine at highway speeds.  My speedo was off.  I kept it between 50 and 55 but was still keeping up with traffic.  I was following a buddy in a 59 Chevy truck.  Turns out we were cruising down the highway right around 70 (his speedo is dead accurate)...no wonder we weren't holding up traffic.  I was actually surprised that it would be that solid at that speed....  Can't wait to get it back from the interior shop and actually be able to drive it on the road instead of staring at it in the garage.

[Lumpy]

Interior would be nice. Lumpy has no door panels, or headliner, carpet is bit's and peices that used to be in people's living rooms, rear seat has a "Southwest" type blanket on it and front seats are some nice buckets I found at the junkyard. Lumpy don't care and I enjoy her just the way she is. But...I'd like to have a nice interior. !!!

 

k.