Speed vs RPM, How fast should I be going ? |
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Speed vs RPM, How fast should I be going ? |
jim_hoyland |
Apr 26 2014, 05:59 PM
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#1
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
What's the speed of a typical 914 in 5th gear at 3,000 rpm ? My speedo have been under reporting for a while.
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914itis |
Apr 26 2014, 06:08 PM
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#2
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Advanced Member Group: Members Posts: 2,892 Joined: 9-October 10 From: New York City Member No.: 12,256 Region Association: North East States |
I would say 60-65 mph
My speedo is also 10 mph advanced |
Tom |
Apr 26 2014, 06:15 PM
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#3
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Advanced Member Group: Members Posts: 2,139 Joined: 21-August 05 From: Port Orchard, WA 98367 Member No.: 4,626 Region Association: None |
Jim,
That would depend on your wheel/ tire combination. I run 195/55/15's on mine and I am doing more engine RPM's at any given speed that if I were running 195/75/15's. I believe there is a tire size/MPH calculator on this site somewhere. How did the mirror joystick work out? Tom |
jim_hoyland |
Apr 26 2014, 06:34 PM
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#4
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
I'm running 16" wheels. My iPad speedo app say 3,000 rpm in 5th is 65 mph. Just wanted to see if that's common...
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struckn |
Apr 26 2014, 07:01 PM
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#5
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Senior Member Group: Members Posts: 1,069 Joined: 9-November 11 From: South Central York Pennsyvania Member No.: 13,764 Region Association: MidAtlantic Region |
I'm running 16" wheels. My iPad speedo app say 3,000 rpm in 5th is 65 mph. Just wanted to see if that's common... If you have, or can barrow a GPS, you can read the MPH at 3000 in fifth. (IMG:style_emoticons/default/bye1.gif) |
jim_hoyland |
Apr 26 2014, 07:06 PM
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#6
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
I'm running 16" wheels. My iPad speedo app say 3,000 rpm in 5th is 65 mph. Just wanted to see if that's common... If you have, or can barrow a GPS, you can read the MPH at 3000 in fifth. (IMG:style_emoticons/default/bye1.gif) I did. My question is whether 65 mph in 5th @ 3,000 rpm is normal |
mepstein |
Apr 26 2014, 07:27 PM
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#7
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914-6 GT in waiting Group: Members Posts: 19,271 Joined: 19-September 09 From: Landenberg, PA/Wilmington, DE Member No.: 10,825 Region Association: MidAtlantic Region |
I'm running 16" wheels. My iPad speedo app say 3,000 rpm in 5th is 65 mph. Just wanted to see if that's common... If you have, or can barrow a GPS, you can read the MPH at 3000 in fifth. (IMG:style_emoticons/default/bye1.gif) I did. My question is whether 65 mph in 5th @ 3,000 rpm is normal Mine was around 75-80 (GPS) on my way to Hershey at 3K but my spedo was reporting about 4 under. The tires are skinny but tall. |
Hine62 |
Apr 27 2014, 05:53 AM
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#8
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Member Group: Members Posts: 132 Joined: 4-October 12 From: Binghamton, NY Member No.: 15,000 Region Association: None |
My speedometer is also off because of tire size. I used gps on mine and found that the speed advance changes as the car goes faster...
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toadman |
Apr 27 2014, 08:11 AM
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#9
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Member Group: Members Posts: 168 Joined: 26-December 05 From: Cincinnati, OH Member No.: 5,316 |
If the tires and transmission on your car are completely stock, and you have the owners manual, you can look in the manual for this information. I have a 1976 car and the owners manual, too. In my manual there is a transmission diagram that shows engine rpms and ground speed information for each of the five forward gears. The diagram specifies a pinion to ring ratio of 7:31 and a tire size of 165R 15. The ratios for each gear are also listed. According to this diagram, for my car, 3000 rpm in fifth gear equals a ground speed of 70 mph.
Changes to tire size or transmission modifications affect the accuracy of the diagram. If this is the case for you, I think using a GPS to measure ground speed is a good method. |
jim_hoyland |
Apr 27 2014, 11:11 AM
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#10
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
Just checked my manual. Says 3,000 rpm in 5th = 70 mph, for 165 R 15 tires. If I have 205 R 16 tires would 65 sound right ? The tire calculator wasn't working. In any case, the GPS on my iPad reads 65.
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r_towle |
Apr 27 2014, 11:47 AM
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#11
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Custom Member Group: Members Posts: 24,574 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
165r15 are 25.4" in, diameter
205 60 r 16 are 25.685" in diameter. It matters exactly what tire you are running. 205 50/16 are shorter and have a smaller diameter than 205/60 Once you know the diameter, use a calculator to determine how far off from stock your speedo is. So, 25.4/25.685= 0.988 About 0.9% off. If you have found a tire you like, north Hollywood speedo, or Palo Alto speedo can redo your speedometer to be correct based upon your tire diameter. 165r15 rotates 207.8 times per miles 205 60 16 rotates 205.5 times per mile You are very close to correct, so if your speedo is reading wrong, it's just wrong and may need to be repaired. I bet they were all off.... |
jim_hoyland |
Apr 27 2014, 04:01 PM
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#12
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
Does the car go faster with smaller tires, given the same rpm ?
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Big Len |
Apr 27 2014, 04:07 PM
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#13
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Senior Member Group: Members Posts: 1,747 Joined: 16-July 13 From: Edgewood, New Mexico Member No.: 16,126 Region Association: Southwest Region |
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r_towle |
Apr 27 2014, 04:11 PM
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#14
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Custom Member Group: Members Posts: 24,574 Joined: 9-January 03 From: Taxachusetts Member No.: 124 Region Association: North East States |
Does the car go faster with smaller tires, given the same rpm ? Think that one through based upon how many times a larger diameter tire goes around over a mile versus how many times a smaller tire goes around. So with larger diameter tires, you go faster at the same rpm. You are effectively changing the final drive ratio by changing the tire diameters. This is why we put 205/50/15 tires on autos cars. Lower gear ration, quicker acceleration. Down side is 4500 rpms on the highways.... |
pvollma |
Apr 28 2014, 07:34 AM
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#15
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Member Group: Members Posts: 205 Joined: 12-May 13 From: Camp Hill, PA Member No.: 15,862 Region Association: MidAtlantic Region |
What's the speed of a typical 914 in 5th gear at 3,000 rpm ? My speedo have been under reporting for a while. I've attached the chart I created for my 914 with 195/65-15 tires. 914PerformanceChart.txt ( 1.88k ) Number of downloads: 82 Let me know the complete size of your tires (width/section-diameter) and I'll run one for you. |
Elliot Cannon |
Apr 28 2014, 10:56 AM
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#16
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914 Guru Group: Retired Members Posts: 8,487 Joined: 29-December 06 From: Paso Robles Ca. (Central coast) Member No.: 7,407 Region Association: None |
It doesn't matter what size tire you use or what size wheel you have them mounted on. When it comes to speedo accuracy, it is based on the outer diameter of the tire mounted on the wheel and inflated. I have always had 17" wheels on both my 914's but I chose a tire that gave an overall diameter of just over 25 inches. I found a chart on the Tire Rack web site that gives combinations of wheels and tires and what their overall diameter is. It helped me a lot when deciding on what tires to order when I went to a five lug on my car.
Cheers, Elliot |
jim_hoyland |
Apr 28 2014, 06:19 PM
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#17
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Get that VIN ? Group: Members Posts: 9,296 Joined: 1-May 03 From: Sunset Beach, CA Member No.: 643 Region Association: Southern California |
What's the speed of a typical 914 in 5th gear at 3,000 rpm ? My speedo have been under reporting for a while. I've attached the chart I created for my 914 with 195/65-15 tires. 914PerformanceChart.txt ( 1.88k ) Number of downloads: 82 Let me know the complete size of your tires (width/section-diameter) and I'll run one for you. Fantastic ! (IMG:style_emoticons/default/smile.gif) I began the same today using my iPad. Our figures for 5th gear above and below 3,000 rpm are very close. Mine would round up to the next whole number. At speeds below 45, my speedo and iPad app were in agreement . My speedo reads 5-10 mph after 65 mph; can a speedo be re calibrated when the accuracy is mostly in the higher speeds ? |
ndfrigi |
Apr 28 2014, 11:05 PM
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#18
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Advanced Member Group: Members Posts: 2,928 Joined: 21-August 11 From: Orange County Member No.: 13,474 Region Association: Southern California |
Drove my 75 1.8 F.I. last weekend with 185x65x15 tire size and it shows 70 mph at 3,000 rpm. But my phone apps speed show 66 mph. I believe the difference of 4 mph is from the current tire size which is not the factory size. Well hoping when I got my new tire 195x60x15 or 205x55x15 will have more accurate speed.
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dlkawashima |
Apr 28 2014, 11:53 PM
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#19
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914 Guru Group: Members Posts: 9,749 Joined: 1-October 10 From: San Jose Member No.: 12,234 Region Association: Northern California |
Drove my 75 1.8 F.I. last weekend with 185x65x15 tire size and it shows 70 mph at 3,000 rpm. But my phone apps speed show 66 mph. I believe the difference of 4 mph is from the current tire size which is not the factory size. Well hoping when I got my new tire 195x60x15 or 205x55x15 will have more accurate speed. You really need to use the tire size calculator because you're setting yourself up for disappointment. Use 165/80-15 (or 165/82-15) as the original size tire and input the three tire sizes above to see the differences. http://www.tacomaworld.com/forum/tirecalc.php |
ndfrigi |
Apr 28 2014, 11:56 PM
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#20
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Advanced Member Group: Members Posts: 2,928 Joined: 21-August 11 From: Orange County Member No.: 13,474 Region Association: Southern California |
Drove my 75 1.8 F.I. last weekend with 185x65x15 tire size and it shows 70 mph at 3,000 rpm. But my phone apps speed show 66 mph. I believe the difference of 4 mph is from the current tire size which is not the factory size. Well hoping when I got my new tire 195x60x15 or 205x55x15 will have more accurate speed. You really need to use the tire size calculator because you're setting yourself up for disappointment. Use 165/80-15 (or 165/82-15) as the original size tire and input the three tire sizes above to see the differences. http://www.tacomaworld.com/forum/tirecalc.php Yes sir I already did use that tire size calculator and thanks again! |
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