High RPM's?
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
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From: Southern California
High RPM's?
Ok so my 74 highboy (C6 auto) sits on 35's. The rear D60 has 373 gears. At freeway speed at 65-70, im running about 3200-3400 rpms. Why does this seem so dang high.....
Lead Driver
Joined: Nov 2006
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From: Pa.
Something is not right .. @65 it should be 2328
Gear Ratio and Tire Size Calculator - 4x4 and Jeep Ring & Pinion
Gear Ratio and Tire Size Calculator - 4x4 and Jeep Ring & Pinion
Moderator & parts slinger
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From: S/W Missouri, Polk county
Is that 65-70 off the speedo? If you did not change the cable gear in the trans to compensate for the bigger tires I believe you are actually going alot faster than indicated.
Do you have a GPS/tom tom to ck the actual speed?
Do you have a GPS/tom tom to ck the actual speed?
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
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From: Southern California
Yea i used the GPS and at 70 mph, its at about 3400..... Makes no sense what so ever. Ill make sure its set for 8 cylinders right now though.
Edit- the tach is set at 8cylinders. So idk what to think....
Edit- the tach is set at 8cylinders. So idk what to think....
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Lead Driver
Joined: Nov 2006
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From: Pa.
Are those straight speedometer readings or calibrated for increase in tire size over stock ? and the tranny is in high gear? manual or auto trans?
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Thread Starter
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
Likes: 5
From: Southern California
I got the speedo from the auto parts store and just wired it like the instructions said sooo idk what that means....... And i have a c6 auto 3 spd
EDIT!!!!! Not speedo but TACH^^^^^idk why i said speedo....
Lead Driver
Joined: Nov 2006
Posts: 5,217
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From: Pa.
Have a look here at the speedo difference if your using a stock speedometer and I'm not even sure of what would have been stock tires 245/75-R16 vs 345/70-R16 Tire Comparison - Tire Size Calculator
Now look at this chart again http://www.alloyusa.com/gear-ratio-calculator
Now look at this chart again http://www.alloyusa.com/gear-ratio-calculator
Thread Starter
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
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From: Southern California
Ugh idk why the heck i said speedo.... I meant tach! 
..... Anyway, its standard aftermarket tach. Makes no sense cuz its shifts into all 3 gears fine... So im not riding in 2nd on the freeway which the numbers seem to indicate
..... Anyway, its standard aftermarket tach. Makes no sense cuz its shifts into all 3 gears fine... So im not riding in 2nd on the freeway which the numbers seem to indicate
Thread Starter
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
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From: Southern California
Ok ima go out and check. Take a test drive cuz i swear i feel it shift into 3rd.....
Lead Driver
Joined: Nov 2006
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From: Pa.
The point is if you are doing
Speedo 55 mph (88.51 km/h) Actual 63.21 mph (101.72 km/h)
With a stock speedo and using the tire size difference im this example
245/75-R16 vs 345/70-R16 Tire Comparison - Tire Size Calculator
Point is : read your tach at a speedo reading of 55mph and that will be your actual RPMs at 63.21 mph with my example .. To dial this in to be more exact we have to know the exact tire size the truck came with originally
Speedo 55 mph (88.51 km/h) Actual 63.21 mph (101.72 km/h)
With a stock speedo and using the tire size difference im this example
245/75-R16 vs 345/70-R16 Tire Comparison - Tire Size Calculator
Point is : read your tach at a speedo reading of 55mph and that will be your actual RPMs at 63.21 mph with my example .. To dial this in to be more exact we have to know the exact tire size the truck came with originally
Thread Starter
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Fleet Mechanic
Joined: Jun 2011
Posts: 1,804
Likes: 5
From: Southern California
The point is if you are doing
Speedo 55 mph (88.51 km/h) Actual 63.21 mph (101.72 km/h)
With a stock speedo and using the tire size difference im this example
245/75-R16 vs 345/70-R16 Tire Comparison - Tire Size Calculator
Point is : read your tach at a speedo reading of 55mph and that will be your actual RPMs at 63.21 mph with my example .. To dial this in to be more exact we have to know the exact tire size the truck came with originally
Speedo 55 mph (88.51 km/h) Actual 63.21 mph (101.72 km/h)
With a stock speedo and using the tire size difference im this example
245/75-R16 vs 345/70-R16 Tire Comparison - Tire Size Calculator
Point is : read your tach at a speedo reading of 55mph and that will be your actual RPMs at 63.21 mph with my example .. To dial this in to be more exact we have to know the exact tire size the truck came with originally
Gotcha. Well i know the speedo is off by about 10mph so 60mph on the speedo is actualy 70 haha. Ill put the gps back in it and go down the freeway and then see what i find out. Im also gunna make sure its shifting into 3rd...