Speedo and MPG check - The 1947 - Present Chevrolet & GMC Truck Message Board Network

**Fbks_Cruiser** · 05-30-2018, 05:20 PM

It seemed like my speedometer was off a bit. I decided to check it and the odometer as well as true MPG by running a GPS for a tank of gas.

Turns out the speedometer/odometer is off by about 10% (indicates faster than true speed). When GPS speed was showing 55 mph, speedometer was at about 60-61 mph. Odometer also registered about 10% too many miles.

MPG check with true GPS miles was 11 mpg in mixed town and highway driving.

Here is my setup:

1972 GMC K2500
350/350TH/NP205
4:10 gears (per previous owner, I haven't checked but seems reasonable)
235/85R16 tires
HEI distributor set to 8* base
"RV" cam (per previous owner, I don't know the specs but seems fairly mild)
Weiand 8004 intake
Edelbrock 1406 carb with one step richer than stock metering rods

**Steeveedee** · 05-30-2018, 09:20 PM

That's pretty good. I only get 7 MPG when towing my travel trailer. Life isn't just about mileage.

**Fbks_Cruiser** · 05-30-2018, 10:39 PM

Yep, I didn’t buy the truck for fuel economy. But rather to have something classic and cool to run chores with. However now that my daughter is home from college for the summer and driving my car to work, the 72 has turned into my daily driver for a while.

**Already Gone** · 05-31-2018, 11:47 AM

I checked my speed with a GPS and was out 3MPH at 65. At 65 the GPS said 62. I didn't bother with gas mileage. A factor here for determining speed accuracy is tire size as a lot of times we are not using the same size ( height mainly) that came on the truck from the factory.

jfnar · 05-31-2018, 12:23 PM

My speedo was off by about 5% in the other direction. My truck had short, fat tires on it and no engine or transmission when I bought it. I put in an overdrive MY6 and a 292. I had no intention of keeping the short, fat tires so when I started looking for tires I got a size that made the speedometer correct. In your case a 28 1/2" tire would get it right. That's about the same as mine.

rockyrivermark · 05-31-2018, 12:56 PM

I have a similar issue with the speedometer reading off.
just buy an assortment of driven gears for about $25
Super easy to change and they are colored with different teeth count
Try a few until you get it speed right

**Already Gone** · 05-31-2018, 01:05 PM

Quote:

Originally Posted by jfnar

My speedo was off by about 5% in the other direction. My truck had short, fat tires on it and no engine or transmission when I bought it. I put in an overdrive MY6 and a 292. I had no intention of keeping the short, fat tires so when I started looking for tires I got a size that made the speedometer correct. In your case a 28 1/2" tire would get it right. That's about the same as mine.

Yea, I am running 235/75 15's on the truck and they are 28.85 inches high. I'm not concerned about 3 MPH for runing around. When I go on trips or a few 10 mile daily drive I use the GPS to keep me honest.

**WorkinLonghorn** · 05-31-2018, 01:16 PM

Quote:

Originally Posted by rockyrivermark

I have a similar issue with the speedometer reading off.
just buy an assortment of driven gears for about $25
Super easy to change and they are colored with different teeth count
Try a few until you get it speed right

Yes, this is the obvious answer. In the shops we used to have a big assortment of speedo gears all color coded as rockyrivermark describes.
If you have a mom and pop shop around there they might make a suggestion or even sell you the correct gear for cheap. -BA

**Fbks_Cruiser** · 05-31-2018, 07:20 PM

I'm not too worried about it. As long as I know 60 mph indicated is really 55 mph. A taller tire would fix the problem (less rotations per mile) and that may be in the future but the tires that are on it are brand new. I see lots of other places I would rather spend $1000. You guys are right though, the easiest and cheapest fix is a new speedo gear. I may go down to the local driveline shop and see what they have if it really starts to bug me.

**randy.powell** · 06-04-2018, 09:01 AM

So instead of playing the switch different gears game. go to one of the many online sites that does the calculations for you. you'll need tire diameter (or size), rear end gear ratio, and transmission type. There you will get both the drive gear tooth size as well as the driven gear tooth size.

Looking at the info you have in your setup data you provided, it looks like you need to plus up your driven gear by one tooth. Run the numbers though to double check.

B/R,
Randy

05-30-2018, 05:20 PM	#1
Fbks_Cruiser Senior Member Join Date: Apr 2016 Location: Fairbanks, AK Posts: 329	K2500 Speedo and MPG check with GPS It seemed like my speedometer was off a bit. I decided to check it and the odometer as well as true MPG by running a GPS for a tank of gas. Turns out the speedometer/odometer is off by about 10% (indicates faster than true speed). When GPS speed was showing 55 mph, speedometer was at about 60-61 mph. Odometer also registered about 10% too many miles. MPG check with true GPS miles was 11 mpg in mixed town and highway driving. Here is my setup: 1972 GMC K2500 350/350TH/NP205 4:10 gears (per previous owner, I haven't checked but seems reasonable) 235/85R16 tires HEI distributor set to 8* base "RV" cam (per previous owner, I don't know the specs but seems fairly mild) Weiand 8004 intake Edelbrock 1406 carb with one step richer than stock metering rods __________________ 1972 GMC K2500 Super Custom Matt Last edited by Fbks_Cruiser; 05-30-2018 at 08:34 PM.

05-30-2018, 09:20 PM	#2
Steeveedee Who Changed This? Join Date: Jan 2013 Location: Simi Valley, CA Posts: 10,091	Re: Speedo and MPG check That's pretty good. I only get 7 MPG when towing my travel trailer. Life isn't just about mileage. __________________ ~Steven '70 Chevy 3/4T Longhorn CST 402/400/3.56 Custom Camper Simi Valley, CA

05-30-2018, 10:39 PM	#3
Fbks_Cruiser Senior Member Join Date: Apr 2016 Location: Fairbanks, AK Posts: 329	Re: Speedo and MPG check Yep, I didn’t buy the truck for fuel economy. But rather to have something classic and cool to run chores with. However now that my daughter is home from college for the summer and driving my car to work, the 72 has turned into my daily driver for a while. __________________ 1972 GMC K2500 Super Custom Matt

05-31-2018, 11:47 AM	#4
Already Gone 70+ ( Old Skool Club ) Join Date: Nov 2003 Location: Saskatoon, Saskatchewan ,Canada Posts: 9,073	Re: Speedo and MPG check I checked my speed with a GPS and was out 3MPH at 65. At 65 the GPS said 62. I didn't bother with gas mileage. A factor here for determining speed accuracy is tire size as a lot of times we are not using the same size ( height mainly) that came on the truck from the factory. __________________ 1972 C10 Custom/Deluxe 613 Highlander 406/700R4 1999 White Tahoe LS 4x2 with Z56 Police Package 1992 K1500 GMC Suburban Members I have personally met: MusicMan70 - HeavyD - ChewyChevy67 - StingRay -71SWB4x4 - 67 Burb - DeadheadNM - too much stuff - bc65 - das601 " Circumsatances Do Not Change Responsibility " " The Sky is not the Limit, Your Mind is." Marilyn Monroe .. RIP Charlie Watts 1941 - 2021

05-31-2018, 12:23 PM	#5
jfnar Registered User Join Date: Jul 2009 Location: Jonesboro, AR Posts: 410	Re: Speedo and MPG check My speedo was off by about 5% in the other direction. My truck had short, fat tires on it and no engine or transmission when I bought it. I put in an overdrive MY6 and a 292. I had no intention of keeping the short, fat tires so when I started looking for tires I got a size that made the speedometer correct. In your case a 28 1/2" tire would get it right. That's about the same as mine.

05-31-2018, 12:56 PM	#6
rockyrivermark Registered User Join Date: Apr 2011 Location: Fairview Park, Ohio Posts: 1,030	Re: Speedo and MPG check I have a similar issue with the speedometer reading off. just buy an assortment of driven gears for about $25 Super easy to change and they are colored with different teeth count Try a few until you get it speed right __________________ My build page http://67-72chevytrucks.com/vboard/s...d.php?t=764662 My cabinet building site http://www.cbcabinets.com

05-31-2018, 07:20 PM	#9
Fbks_Cruiser Senior Member Join Date: Apr 2016 Location: Fairbanks, AK Posts: 329	Re: Speedo and MPG check I'm not too worried about it. As long as I know 60 mph indicated is really 55 mph. A taller tire would fix the problem (less rotations per mile) and that may be in the future but the tires that are on it are brand new. I see lots of other places I would rather spend $1000. You guys are right though, the easiest and cheapest fix is a new speedo gear. I may go down to the local driveline shop and see what they have if it really starts to bug me. __________________ 1972 GMC K2500 Super Custom Matt

06-04-2018, 09:01 AM	#10
randy.powell Senior Member Join Date: Jun 2014 Location: Bushwood, MD Posts: 510	Re: Speedo and MPG check So instead of playing the switch different gears game. go to one of the many online sites that does the calculations for you. you'll need tire diameter (or size), rear end gear ratio, and transmission type. There you will get both the drive gear tooth size as well as the driven gear tooth size. Looking at the info you have in your setup data you provided, it looks like you need to plus up your driven gear by one tooth. Run the numbers though to double check. B/R, Randy



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