Speedo? - The 1947 - Present Chevrolet & GMC Truck Message Board Network

MobSled · 01-13-2010, 06:11 AM

Here's the deal guys...this past Saturday I decided to lower my truck, 5/6 drop. I was running a 255/70/15 tire all the way around which we all know is too tall for a lowering job like that. Decided to run 245/60 and 275/60/15 in the rear. Now for some reason I lost my mph. My speedo shows 70 when in reality I'm doing 55-60.

Any inpuit would be greatly appreciate...btw its a 1977 c10 LWB.

My 2 questions are what caused this, how do I get my top speed back?

MobSled · 01-13-2010, 11:23 AM

Not sure if this help guys but I can safely rule out the cable because while my mph gauge is showing 70mph my rpm gauge is at 2300 2400rpm. In reality I'm only doing about 60mph. I will get a more accurate reading on my way back home using my gps which is 99% accurate in all my other cars untill then let me know what you guys are thinking, if anything comes to mind.

THX.

MobSled · 01-13-2010, 11:26 AM

willett · 01-13-2010, 03:27 PM

my spedo cable has done some funny things in the past while on a straight stretch of road. it has started to "jump" 0 - 160 and then to 100 and then all the way around and then back down.... etc.

i would yank you spedo cable and make sure that there are no crazy bends, as well as lube er' up with a touch of graphite.

bigger tires in the rear would suggest that the spedo should be reading lower speeds if anything....

MobSled · 01-13-2010, 03:34 PM

Well that's just the thing I went from a 255/70/15 tire in the back to a 255/60/15...I guess I could be missreading my rpm gauge. On a 350/350 combo with a stock 77 rear diff, while cruising down the highway at 65mph where should my rpm be?

TwoFiftyShifter · 01-13-2010, 03:46 PM

Depends on the axle ratio. If its a 373 it might be 2800 rpm or so.

MobSled · 01-13-2010, 03:52 PM

So 70mph 2300rpm doesn't sound right at all?? Sorry for all the noob questions fellas I got into the muscle scene about 3months ago. Another question is what's a healthy rpm I can take a bone stock 350 up to?

TwoFiftyShifter · 01-13-2010, 04:00 PM

No it really depends on your axle ratio, what I said is close for 3.73, but if you have a 3.07 ratio that would be correct.

TwoFiftyShifter · 01-13-2010, 04:01 PM

4500 RPM is pretty safe max rpm.

3r!c84 · 01-13-2010, 04:37 PM

you went down in tire diameter so it will throw off your speedo.just like changing your rearend gears

MobSled · 01-13-2010, 05:03 PM

So how do I go about recalibrating my speedometer cable its showing 9mph faster then what I'm actually driving.

3r!c84 · 01-13-2010, 05:06 PM

get a diffrent size speedo gear.its on the tranny on the drivers side.at the tail of the tranny.you insreww that and a gear comes out.you can go to gm or a parts store might have them and get a size smaller or bigger(i dont know what way to go)but ask them they will know

01-13-2010, 06:11 AM	#1
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Speedo? Here's the deal guys...this past Saturday I decided to lower my truck, 5/6 drop. I was running a 255/70/15 tire all the way around which we all know is too tall for a lowering job like that. Decided to run 245/60 and 275/60/15 in the rear. Now for some reason I lost my mph. My speedo shows 70 when in reality I'm doing 55-60. Any inpuit would be greatly appreciate...btw its a 1977 c10 LWB. My 2 questions are what caused this, how do I get my top speed back?

01-13-2010, 11:23 AM	#2
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Re: Speedo? Not sure if this help guys but I can safely rule out the cable because while my mph gauge is showing 70mph my rpm gauge is at 2300 2400rpm. In reality I'm only doing about 60mph. I will get a more accurate reading on my way back home using my gps which is 99% accurate in all my other cars untill then let me know what you guys are thinking, if anything comes to mind. THX.

01-13-2010, 11:26 AM	#3
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Re: Speedo? Last edited by MobSled; 01-13-2010 at 11:28 AM.

01-13-2010, 03:27 PM	#4
willett Another Day, Another Dollar Join Date: Nov 2008 Location: North Vancouver, BC Posts: 1,316	Re: Speedo? my spedo cable has done some funny things in the past while on a straight stretch of road. it has started to "jump" 0 - 160 and then to 100 and then all the way around and then back down.... etc. i would yank you spedo cable and make sure that there are no crazy bends, as well as lube er' up with a touch of graphite. bigger tires in the rear would suggest that the spedo should be reading lower speeds if anything.... __________________ -78 K20 Cheyenne 454 long box, 4" lift, 35" MT's, '84 cab, '80 box "its hip to be square"

01-13-2010, 03:34 PM	#5
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Re: Speedo? Well that's just the thing I went from a 255/70/15 tire in the back to a 255/60/15...I guess I could be missreading my rpm gauge. On a 350/350 combo with a stock 77 rear diff, while cruising down the highway at 65mph where should my rpm be?

01-13-2010, 03:46 PM	#6
TwoFiftyShifter Registered User Join Date: Nov 2008 Location: Gwinnett GA Posts: 1,803	Re: Speedo? Depends on the axle ratio. If its a 373 it might be 2800 rpm or so. __________________ 1982 K10 SWB 1987 V20 - Sold - Doh!

01-13-2010, 03:52 PM	#7
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Re: Speedo? So 70mph 2300rpm doesn't sound right at all?? Sorry for all the noob questions fellas I got into the muscle scene about 3months ago. Another question is what's a healthy rpm I can take a bone stock 350 up to?

01-13-2010, 04:00 PM	#8
TwoFiftyShifter Registered User Join Date: Nov 2008 Location: Gwinnett GA Posts: 1,803	Re: Speedo? No it really depends on your axle ratio, what I said is close for 3.73, but if you have a 3.07 ratio that would be correct. __________________ 1982 K10 SWB 1987 V20 - Sold - Doh!

01-13-2010, 04:01 PM	#9
TwoFiftyShifter Registered User Join Date: Nov 2008 Location: Gwinnett GA Posts: 1,803	Re: Speedo? 4500 RPM is pretty safe max rpm. __________________ 1982 K10 SWB 1987 V20 - Sold - Doh!

01-13-2010, 04:37 PM	#10
3r!c84 Badass Join Date: Jan 2009 Location: lox. fl Posts: 4,586	Re: Speedo? you went down in tire diameter so it will throw off your speedo.just like changing your rearend gears __________________ Think outside the box? -84c10 (1stcar)→My Build← -00 Saturn 5spd drop a gear and disappear! 1/4:13:90

01-13-2010, 05:03 PM	#11
MobSled Registered User Join Date: Oct 2009 Location: Downey, CA Posts: 110	Re: Speedo? So how do I go about recalibrating my speedometer cable its showing 9mph faster then what I'm actually driving.

01-13-2010, 05:06 PM	#12
3r!c84 Badass Join Date: Jan 2009 Location: lox. fl Posts: 4,586	Re: Speedo? get a diffrent size speedo gear.its on the tranny on the drivers side.at the tail of the tranny.you insreww that and a gear comes out.you can go to gm or a parts store might have them and get a size smaller or bigger(i dont know what way to go)but ask them they will know __________________ Think outside the box? -84c10 (1stcar)→My Build← -00 Saturn 5spd drop a gear and disappear! 1/4:13:90

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