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Need more info. What is your tire size? Also, assume we are talking about being in 4th gear with a 1:1 ratio, and you want to know your rear axle ratio?
Need more info. What is your tire size? Also, assume we are talking about being in 4th gear with a 1:1 ratio, and you want to know your rear axle ratio?
Tire size makes no difference. Tire size only affects the ACCURACY of the speedo, but not the speedo-tach relationship, which is fixed by the speedo gearing.
In "direct drive" (1:1 trans ratio) axle ratio = RPM @ 60/760 and the speedo-tach relationship is linear so 60/70(3500) = 3000, and 3000/760 = 3.94 and 4.11 is the closest, so that's the most likely axle , but you should check your speedo with a GPS or a modern car, which typically have more accurate speedos than vintage cars assuming they have the OE tire size.
Tire size makes no difference. Tire size only affects the ACCURACY of the speedo, but not the speedo-tach relationship, which is fixed by the speedo gearing.
In "direct drive" (1:1 trans ratio) axle ratio = RPM @ 60/760 and the speedo-tach relationship is linear so 60/70(3500) = 3000, and 3000/760 = 3.94 and 4.11 is the closest, so that's the most likely axle , but you should check your speedo with a GPS or a modern car, which typically have more accurate speedos than vintage cars assuming they have the OE tire size.
Duke
Well, yeah, that's all true in theory - if the rear gears, weren't changed without also correcting the speedo drive gear and/also the tire is still original or equivalent. Based on the little info the OP provided we don't know those things to be true.
We've all seen examples where the components have been changed in an incorrect mix.
If we know the tire size none of that other stuff/potential variables matters in a 1:1 4th gear.
I just didn't want to assume original/correct parts.
SWCDuke, please label the numbers you use. I have different numbers to work with so I need to know what i.e. 60/70 means and the rest of the numbers in your equations.
You really don't need to have all the numbers and formulas if you have the correct mph (gps is easy way to get), correct rpm, and tire diameter. Just plug the numbers into any of the available calculators. Tire size does matter if you're using your car speedometer for the speed you plug in, because as Duke says, they do affect the speedometer accuracy.
The OP stated he was turning 3500 at 70 mph. He did not say that the 70 mph was indicated on his instrument cluster or if it was off of the GPS on his phone. I assumed he was referring to his phone/gps for the sake of accuracy.
If my 65 Vette is running 70MPH and the RPM's are 3500, what gears do I have?
My 66 with 3.70 rear end with 22 tooth speedometer gear run about the same as yours. It is 40 mph at 2k, 1.5k equals 30 mph etc. My tires are about 26" if I recall correctly.
The attachment is an excel spreadsheet that probably offers more than you will even need for speed vs. rpm: with inputs for tire height, ring & pinion, and transmission ratios .
Wheels and tires are: 215x55x17, and the speed is off my speedometer... The rear end is stamped AM 10 30 64 and the car was built 11-13-64, but here again I do not know if any one b4 me replaced the gears...
[QUOTE=Harry's 65 C2;1597480820]If my 65 Vette is running 70MPH and the RPM's are 3500, what gears do I have?[/QUOT
Jack up the rear and put it on stands. Rotate a rear wheel (assuming limited slip centre) one revolution and count the drive shaft rotations. That is your gear ratio for sure. No doubt. Cheers
First thing you need is ACTUAL MPH, not speedometer with those tires. They are 26.3: as opposed to 27.1" stock UNLESS the speedometer driven gear has been changed to correct reading.
The info available at this point makes buns handy, dandy pocket computer look pretty good:
So - like I said back in posts #s 2 and 5, tire size and original gearing/speedo drive do matter in making these calculations. You can’t just assume all is original and tell someone you know the answer based on theory and mathematics.
The input data points are more important than the formula itself!
Hell, I SWAGed this by using my 3000rpm at 70 3.36 axle vs his 3500 rpm and got:
(3500/3000) x 3.36 = 3.92, but that was without accurate data. Turns out that was a lot better (closer to 3.70) answer than those with fancy formulas!
Last edited by tuxnharley; 06-27-2018 at 11:12 AM.
06-26-2018, 01:25 AM
