For those of you planning or working on minimalist or race car inspired builds; there is an easy (relatively) way to figure road speed pretty accurately without using a speedometer. Instead, we can read the tachometer. It's the same method used in NASCAR to make sure the drivers maintain pit road speed. Pretty much every state is going to require you to have a working speedo in order to pass inspection or title a vehicle. What you do beyond that point is up to you.... I've used this method on a couple of my past projects where I'd swapped trannies or didn't have a speedo hooked up for one reason or another. Again, we're pretty much only concerned with making sure we obey the posted limit, so finding the relative RPM at common posted speeds (I'll use 25mph as an example) should be enough to keep us out of trouble. (You can also do this just to verify the reading you're getting from the existing speedo...) With all the math weeded out as much as possible, here is an equation that tells us MPH for a given RPM, gear reduction, and tire size: MPH = RPM X Rear Tire diameter / (336.3 X Gear Ratio X Axle Ratio) It works out the same if you get rid of the brackets and keep dividing, if that makes more sense to your calculator. We could write it like this: MPH = RPM X Rear tire dia. / 336.3 / gear / axle Switching that around to find RPM, we get: RPM = MPH (desired) X 336.3 X Gear Ratio X Axle Ratio / Rear tire diameter. Here's an example, done both ways: 3000 rpm in second gear (2:1 ratio) with a 4:1 axle and 29" tall rear tire. MPH = 3000 X 29 / (336.3 X 2 X 4) = 32.34 MPH Now, let's solve it to go 25 mph with the same drivetrain: RPM = 25 X 336.3 X 2 X 4 / 29 = 2,320 rpm. Obviously, the car will be most comfortable in different gears at different speeds, so as long as you know the reduction (or overdrive) ratio for each gear in the trans. you can figure out 75 mph just as easy as 25. NOTE: If you have an automatic in your project (shame on you!), you may be dealing with some converter slippage, but this will err the speed calculation on the safe side. Also, you'll have to find out the ratios in your automatic from service info. With most automatics, you can't just spin the input and count turns to figure out reduction like you can with a stick trans. Hope this helps keep some of you out of trouble...
My 23 never had a speedometer and my 27 has one, but in 25 years I have just never hooked it up (lazy I guess) So I go by tach readings. I used a GPS to monitor my speed and it worked out pretty well on my 27 because every 1000 rpms is 20 mph.....when I am at 2000 I am going 40, 3000 is 60, etc. Good info you posted though. Don
Thanks for posting that. I'll have to save that for possible future use. I remember seeing a very basic roadster a few years ago that had labels on the dash telling the correct RPM for certain speeds in 3rd and 4th gear. It just had a tachometer and an oil pressure guage.
Yeah, they are 4.30's and are a little steep on trips but around town I love em. I've actually learned to just drive about 65 or 70 on the interstate and it runs pretty well at 3300-3500. Don
This site http://vexer.com/automotive-tools/speed-rpm-calculator is a fun way of finding your speed. Even has stick or auto option.
Thats how I have always done it. Were all model T's equipped with a speedometer when new? I am hoping my '26 will skate through inspection without one, as the dash will look more symmetrical with one large gauge in the middle flanked by 4 smaller gauges, and I never look at the damn things anyway...
Yea, next thing you know, you might be doing burnouts, getting out of shape, having fun, ect, ect, ect. Damn hot rodders anyway...
Nah, have a buddy run along behind you and honk every 10 mph while you watch the tach. Not perfect but it works okay.
That's how I have always done it in the Plymouth... 2500rpm in third = 35 mph everday surface street speed in Phoenix,2600rpm in 4th = 65mph, 2200 rpm in 5th = 75 mph avarage freeway in town cruising speed in Phoenix, 2500 rpm in 5th = 90mph freeways in AZ outside of town. I've flashed just shy of 4000rpm in 5th in this car before aerodynamics and testicular fortitude ran out...
I had to laugh. With so many threads taking about being traditional and we are using GPS and phone apps to determine how fast we go. Actually technology is traditional guys using newer engines, transmissions, rearend, steering. I use technology when it works for me.
Massachusetts does not, according to the Mass DOT website: What do they inspect on my passenger vehicle? VIN Number/Registration/License Plates Windshield Wipers/Windshield Washer Headlights/Headlight Aim Turn Signals/4-Way Flashers Tail Lamps/Brake Lights/Backup Lights/Other Lighting Devices Rear View Mirrors Horn Exhaust System Tires Body Panels/Fuel Tank Frame/Unibody Service Brake Parking/Emergency Brake Seat Belts/Air Bag Operation Ball Joints/Steering/Suspension Visible Smoke Emissions Test/Emissions Components (if applicable) Also see Mass. 540 CMR 4.04 ...no inspection or requirement for a speedometer or odometer.
That is where old cars run! You must be young and used to that new fangled o/d trans. Think 62 409 Impala 4:10 gears 27" tire! Those were highway gears in the old days. 4:88 for the track! Sorry to hijack.
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