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So yesterday I was out cruising and made note that as cruising at 70 mph, the stock tach was reading 3000 rpm. I was somewhat glad to see that low number and then began to wonder if the tach was reading correctly. So this morning, I decided to check the timing and thought, hey, I can check what the rpms register on the timing light as I rev up the engine to 3000 rpm. The readings were dang near the same. I was glad to see that. Car is a '72, 350 automatic, ps, pb, AC (non working presently).
But, tires are 26.5 inches tall...
Last edited by kansas123; May 14, 2025 at 01:21 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
none of your numbers work for your speed
225/50/15 is 27.5 inches tall not 26.5
a 26.5 inch tire at 3000 rpm would be doing 76.8 mph
a 27.5 inch tire at 3000 rpm would be doing 79.7 mph.
If your speed is correct then your torque convertor might be going bad or has a really high stall speed...it would be slipping a lot
I would find one of those digital speed trap signs that tell you you are speeding and see if your speedometer is off or have a buddy pace you on the highway. Ive done that with a buddy on hands free cell phones to get an accurate measurement....You have verified your rpm is correct, so you are definitely going faster than you think if your rear gear is correct...
none of your numbers work for your speed
225/50/15 is 27.5 inches tall not 26.5
a 26.5 inch tire at 3000 rpm would be doing 76.8 mph
a 27.5 inch tire at 3000 rpm would be doing 79.7 mph.
If your speed is correct then your torque convertor might be going bad or has a really high stall speed...it would be slipping a lot
I would find one of those digital speed trap signs that tell you you are speeding and see if your speedometer is off or have a buddy pace you on the highway. Ive done that with a buddy on hands free cell phones to get an accurate measurement....You have verified your rpm is correct, so you are definitely going faster than you think if your rear gear is correct...
Thanks Rescue. I was just now figuring my speed with the formula. And my tires are 225 70's. I just got back from a spin and tried to use an app to see what my speed was and of course I could not get it to work. Why isn't there a really really simple speedometer app available, with nothing extra on it, just the *^$&^^&(*%*(*ing speed please.
So could I create a 'slipping' condition in some way, to see if I'm slipping? Considering this automatic isn't really geared the way I would like it, what will I be able to hear or feel? Thanks for all your input.. Pardon me, I'm exhausted and so my mind is spinning.
Last edited by kansas123; May 16, 2025 at 10:19 PM.
2020 Corvette of the Year Finalist (performance mods)
2019 C3 of Year Winner (performance mods)
2016 C3 of Year Finalist
I really only know manual transmissions. From what i understand when some go bad they surge, but i had one back in the day that if you jumped on it, it wouldnt accelerate fast, it was a long build up. If you pull the dipstick and its brown instead of red or it smells burnt its usually a sign its seen better days. I would wait for an " automatic guy" to let you know whats up.
I know how you feel about all the extra bullsh@t they add to apps and such....i just like to buy one thing and have it only do that.....if you have maps on your phone, it will give you speed along with the directions. Its usually pretty close...but not perfect
I have this GPS Speedo installed, it's very accurate. Being in Canada, all our speeds are posted in KPH. The Corvette is MPH and I just got tired of doing the conversion in my head. It does KPH & MPH.
It has a compass reading, and also a speed warning beeper.
Very interesting indeed. Never even questioned the possibility it could be 3.36. But when I spun and counted, sure looked like 3 to me. I think I took a pic of the numbers on the diff, years ago, but cannot find them.... Thanks BKarol
You can’t use the tires outside dimension posted by the tire manufacturers specifications.
With the vehicles weight on the tires they could be up to an inch shorter measured from the ground to the top of the tire.
A good gear calculator will factor it into the calculation.
My 73, with a TH400, 3.36 rear gears and 245/60/15 tires ran 3000 rpm @ approximately 70 mph.
You can’t use the tires outside dimension posted by the tire manufacturers specifications.
With the vehicles weight on the tires they could be up to an inch shorter measured from the ground to the top of the tire.
A good gear calculator will factor it into the calculation.
My 73, with a TH400, 3.36 rear gears and 245/60/15 tires ran 3000 rpm @ approximately 70 mph.
I didn't use mfg info, I measured the actual tire(s) on the car. Then observed the # of rotations of the driveshaft to one rotation of the tire to validate findings. Thanks for the info. And found out I had AW on my diff., so 3.08. When I get my battery back in the car tomorrow, I'll do some more drivin' and observin'.
You can’t use the tires outside dimension posted by the tire manufacturers specifications.
With the vehicles weight on the tires they could be up to an inch shorter measured from the ground to the top of the tire.
A good gear calculator will factor it into the calculation.
My 73, with a TH400, 3.36 rear gears and 245/60/15 tires ran 3000 rpm @ approximately 70 mph.
The tire circumference doesn't change in this case. The steel belts prevent that. They will still roll at the same speed, unless they are so overloaded and underinflated that they are scrubbing.
That said, tire dimensions may change slightly between manufacturers.
My 245/60/15 BFG’s have an unloaded outside, diameter of 26.575”.
The loaded diameter of my BFG’s, recommend vehicle weight on each tire, mounted on the approved wheel width, measures 24.98”.
So roughly 1.5” difference in height.
This information can be found on BFG’s website and on most vehicle gearing calculators.
I’ve never seen any information posted on any websites showing the loaded diameter of a tire when mounted on an overloaded unsafe vehicle.
If it existed, can you please show me where?
RPMs
