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Speed Reading vs. RPM Reading

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pkmh

pkmh

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Hi all,

I have not investigated the actual specifics noted on the Healey's speed in each gear and in what rpm readings, accordingly.

However, I have had two Healeys so far. Before Hurricane Sandy claimed my first one, I do remember driving my her up to as much as 80 or even 90mph and the rpm gauge read at about 4000 as a close estimate. That is from memory or best I can recall. Does this make sense so far?

Well, as a more empirical note, today, I am driving on the highway performing for a real first time, a high speed testing of my rebalanced wheels in my second Healey. Results...Driving on level ground, in overdrive, my speedometer is reading 45mph and my tach at 3000rpm. If I were to try to drive up to 65/70mph, I "feel" I would be nearing the red line. I believe the Healey is suppose to reach about 110mph.

Anyone else have the same results or what should the rpm read at if say traveling at 45mph and in 5th gear?

Would one agree this is a low mph vs high rpm?

Thoughts, possibilities???

Thanks,

Paul
 
As a general rule of thumb, 3000 rpms should get you about 60 mph. I say that because I know a couple MGBs that is what they do. One Bugeye with a Toyota 5 speed get 60 at 3000rpms. My 3000, w/ 3:54 gearing gets 60 at 2700 rpms.
I'd say your speedo is off. Also I believe tire size will have a significant impact. 155 verses 185 can be as much as one inch difference in diameter. Dave.
 
Thanks for that info Vette. I do have 185r15's on this Healey whereas on my old Healey, I had 155r15's. This can serve as a significant difference? Maybe my Speedo is off (or my Tach).
 
pkmh said:
Thanks for that info Vette. I do have 185r15's on this Healey whereas on my old Healey, I had 155r15's. This can serve as a significant difference? Maybe my Speedo is off (or my Tach).
Click to expand...
I have 195/65-15s on mine, with a 3.9 rear end. I was doing 60 mph @ 3,000 rpm. The speedo has been adjusted for the tire size and gear ratios.

Changes in wheel size will make a difference, but not what you're seeing.
 
John Turney said:
I have 195/65-15s on mine, with a 3.9 rear end. I was doing 60 mph @ 3,000 rpm. The speedo has been adjusted for the tire size and gear ratios.

Changes in wheel size will make a difference, but not what you're seeing.
Click to expand...

Yeah, I am kind of at a loss. Can't find fault with engine in any way. Maybe differential was changed. Can't figure out what else could cause. Guess I will have to start methodically look into this on my own. Thanks for the input.

Paul
 
pkmh said:
Yeah, I am kind of at a loss. Can't find fault with engine in any way. Maybe differential was changed. Can't figure out what else could cause. Guess I will have to start methodically look into this on my own. Thanks for the input.

Paul
Click to expand...
If you have a GPS, you can use it to check your speedometer. I have an old dwell meter that I can use to check my tach (sort of).
 
Find your tire diameter, either from the manufacturer or measure from center of wheel to top of tire and multiply by 2. Confirm your tach readings against a decent dwell-tach (last I checked about $40 from Sears). Plug the numbers--including your rear-end gear ratio--into a calculator like this one:

https://www.angelfire.com/fl/procrastination/rear.html

You should be able to get within a couple MPH at higher speeds.
 
Thanks Bob. Will do. My present tires on the car are marked 185/70 r15. I just bought a 155 r15 for the trunk as a spare. On my old Healey, those are what I original had on all fours. Guess another approach would be to just stand the 155 up against the 185 and see what dimensional difference I come up with. I would also try your approach as see what I get. I've already been informed about this possible change in type elsewhere as having an effect on speed vs. rpm, And yes, I believe my tach reading is out, maybe the speedo as well. I will just live with it for now.

Thanks,

Paul
 
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