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speedometer readings based on rpm's ?

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bighealeysource

bighealeysource

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Quick question - I think I have read that on a typical Healey 4 cylinder or 6 cylinder with overdrive - you can double your rpm's in 4th gear with overdrive engaged and get an accurate MPH as in 3000 rpm's equals 60 mph, 3500 rpm's equals 70 mph, etc. Has anyone else read this or heard it to be true ? Trying to make sure my recently rebuilt speedo is in fact accurate.
Thanks
Mike
 
bighealeysource said:
Quick question - I think I have read that on a typical Healey 4 cylinder or 6 cylinder with overdrive - you can double your rpm's in 4th gear with overdrive engaged and get an accurate MPH as in 3000 rpm's equals 60 mph, 3500 rpm's equals 70 mph, etc. Has anyone else read this or heard it to be true ? Trying to make sure my recently rebuilt speedo is in fact accurate.
Thanks
Mike
Click to expand...

Mike,

Three things will be needed to tell you if your speedo is accurate: Final Transmission Ratio, Tire Diameter, Axle Gear Ratio, and RPM. I've sent you an email with the calculator by J. Scott Morris. Just adjust the parameters in the blue section to get your speed! The forum will not let me attach the excel file. The sample below is for a BN2 with 25.3 diameter tires and a 3.54 gear ratio.

Enjoy,
Duane

Speedo.jpg
 
Hi Mike, I have said before that with my BJ7 I am getting approx 60 mph at 2700 rpms and 70 at 3200 rpms. I am a bit surprised at this because I expected better with the 3:54:1 gears and the OD. But i don't know what my transmission output ratio is. I was told when I bought the car years ago that is was set up for rallying. Maybe the trans gears where changed. another surprise was that after I changed to the 3:54 rear gears my speedometer is right on. I checked my speedo with a GPS unit mounted in the car. They read the same from 35 mph right up to about 75 mph. I can't explain why my speedo is still accurate after changing the rear gears. The only explaination would be that it wasn't accurate before and I didn't know it. That leads me to believe that my trans output ratio is not stock. Anyway the GPS says it is accurate now. I am running 165/80-15 tires. Dave
 
Mike,

Instead of doing a lot of cipherin', why not just install a speedometer app on your smartphone (I'm assuming you're not using a GPS for that purpose because of positive ground) I have this one and it's very accurate: https://play.google.com/store/apps/details?id=com.binarytoys.speedometer&hl=en I've got 3.54 gears behind my Ford 5 spd and am turning 2200rpm at 70 mph (neener, neener, neener)
 
Vette

It doesn't really matter what gear set you have in the box as 4th gear is 1:1. What matters is the overdrive ratio, the diff ratio and the diametre of your tyres.
 
Thanks all for your comments. I should have the 4.1 rear end and thanks to Duane for his detailed spreadsheet and where to look for the numbers on the pumpkin, will check that out. Seems like every time I think to have someone driving behind me to double check their speed, it's another LBC so who knows if it is correct ! Going to try all the suggested websites and download the app Rick suggested so I can see how accurate the speedo is now. Funny thing about this, subject came up as driving back from a big Brit car show in Charleston,SC ( British Car Club of Charleston's 30th Annual Show) with six of us making the 2 hour drive back to our part of SC, the one guy with the most perfect car was traveling behind me. He owns a 64 289 Cobra, the real deal, and it is about perfect. To his credit, not a trailer queen. He was the one asking about my speedo as he thought I was running about 10 mph slow versus what his speedo showed. After asking the others in our little conga line, seems like two MGB's, one TR3, one Morgan and my Healey were all showing doing about 65 -70 where the Cobra showed us only doing about 55 - 60, so we all now think the almost million dollar Cobra was the one with the incorrect speedo !!!
 
I've obtained very accurate speed readings with an aviation handheld GPS. I don't think the GPS on my phone has this feature but I have seen it on others. That takes everything into account. It may not satisfy one's desire to know everything and be able to calculate it out but it will tell you how fast you are going to a gnat's butt, especially if you can maintain an indicated speed on your speedometer for a while when going straight.
 
Quick update - downloaded the speedo apps recommended and turns out my rebuilt speedometer is just about spot on ! Rick's idea worked great !!!
 
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