1968 c10 gear ratio for highway

[groundedc10]

I am just looking for everyone's opinion for a good highway gear ratio for my setup. I currently have a 305 with a manual 4 speed. In 4th gear at 60mph the engine is working very hard and in the upper RPM's. I am looking to change my gears so I can cruise on the highway at a higher speed than 60 and not have the engine RPM's so high. I have done some research but don't see any info of anyone with a good setup. Let me know your thoughts. Thanks

[CST10]

3.42 is a good in between ratio. 3.08 is better for long range traveling but might be a little sluggish off the line depending on how much TQ you have.

[HSRACER]

3.08 is a good highway gear

[hugger6933]

I agree with HS, that the 3:08 is a great gear to have out on the road with no overdrive as long as your in a 2 wheel drive with normal sized tires and a 1:1 final drive. 3:42 is getting towards the top end of that range where you would want be with the engine starting to yell pretty loud. Jim

[groundedc10]

Thanks for the advice. I will look into the 3.08

[jocko]

If you're not planning to swap the trans at any point, then a 3.08 or 3.23 is a good choice if your MAIN concern is lowering highway rpm. 3.08 might even almost make granny 1st useful, but not really... So, with the 3.08 you will have pretty sluggish launches in 2nd (the major downside of the 3.08 plan). If you will ever change out the trans for an o/d version, then save some $ now and put in a 3.73. You'll only see a minor difference now, but it'll be the perfect marriage when you put in the o/d trans. Or, better yet, if you're going to do both (eventually), do the o/d trans first, it will make the biggest impact for now, then do the rear later. Also keep in mind that if you go as low (numerically) as the 3.08 now, an o/d trans in the future will be a problem because the rpm will be too low on the highway (final drive ratio too low numerically (i.e. a high gear) with the 3.08 and o/d 4th). You'd have to cruise at 95 just to keep the rpms up..

[Mike C]

The 3.07 gears are easy find used. The other ratios mentioned, not so much. So if junkyard sourcing is your goal...

What size tire do you have? That makes a huge difference. I've got a 29" tire on the back of my short bed with 3.07 gears and TH350 and it cruises well at 65, faster not so much. But with your manual trans RPM should be several hundred less for converter slippage.

[groundedc10]

I have the factory 15" wheels and tires on it and don't plan on changing from that size any time soon. I will for sure hold on the the factory gear set just in case I ever want to go back.

[Mike C]

The easiest way to improve your highway experience is taller tires. What size tire is your "factory" tire? At this point the old G or H bias tires are long gone and whatever is on there now may or may not even be close.

The factory tire is probably equivalent to a 215 75 15 which is about 27" tall. A 235 75 15 is about 28" tall. And a 265 75 15 is about 31" tall.

I had 26" tall tire on the back of my Camaro (4.10 gear 4 speed manual) and went to a 28" tall tire. Made a noticeable difference in rpm for cruising.

If your truck is at stock height, I would look at some taller tires. I had the 265 75 15 on my short bed until I lowered it. And it currently has a 29" rear tire with the 3.07 gear which is tolerable for shorter blasts on the highway.

[HSRACER]

Originally Posted by Mike C

The easiest way to improve your highway experience is taller tires. What size tire is your "factory" tire? At this point the old G or H bias tires are long gone and whatever is on there now may or may not even be close.

The factory tire is probably equivalent to a 215 75 15 which is about 27" tall. A 235 75 15 is about 28" tall. And a 265 75 15 is about 31" tall.

I had 26" tall tire on the back of my Camaro (4.10 gear 4 speed manual) and went to a 28" tall tire. Made a noticeable difference in rpm for cruising.

If your truck is at stock height, I would look at some taller tires. I had the 265 75 15 on my short bed until I lowered it. And it currently has a 29" rear tire with the 3.07 gear which is tolerable for shorter blasts on the highway.

I agree with Mike C, my two trucks with a 3.08 gear had either a 29 inch or 31 inch tire on to make them truely comfortable at 70 plus MPH.

[kieth]

send me a email kieth.goudelock@mhctruck.com I will return to you a spreadsheet that shows usable torque to the ground and speeds with all different tire diameters, all available gears, and a variety of engines.....this will let you find a tire gear and wheel combo that will not only be at the rpms you want but will also keep you from putting together a combo that falls on its face..................Kieth

ps this will come in the form of a excel spreadsheet so you will have to have the excel software to see it.

[Accelo]

The C10's are definitely old school when it comes to gearing. As an example my 2000 Silverado has 3:70's rear end gearing with the overdrive its the equivalent of 2.61 rear end gearing. To put it another way: without the overdrive one would have to need the 2.61 gears to achieve the same highway motor rpm.

In the past I put a 4L60 behind my 1971 396 3/4 ton truck. My mileage went from 10 mpg to 13mgp. A 30% improvement. The biggest difference was the noise in the cab. Much more comfortable to drive on the highway. BTW this truck had 4.11.

If you goal is just mileage I would go higher, on the ratio, than the 308. If burnouts are the rage this would be too high. All the more reason I like the conversions to 4L60 i.e 700R4. One gets the benefit of the lower first gear and the .70 over drive. If you want to figure the equivalent ratio just multiply your differential ratio by .7. If you find a used transmission it can be comparable in cost to changing the rear-end ratio. Look ma: Burn outs and gas mileage!

[Mike C]

Agreed, overdrive is the best solution, especially in a two wheel drive truck. I am not a fan of TH700 R4 though but TH200 4R.

You will not find a used TH200 (most likely)and it will cost you at least $1700 to get a heavy duty one. A professionally built rear with limited slip and new gears will cost nearly $1000 so not that much more.

And the TH200 lets you keep the stock driveshaft while a 700 conversion will need a modification. The reality is a used auto trans is likely to fail within 30,000 miles. I like to joke (not really a joke...) that there are 2 types of auto transmissions: those that have quit working and those that will quit working.

But I plan on the most expensive Bow Tie Overdrives TH200 4R for my short step when I get around to the 427 swap. Reality is that overdrive is what makes new cars more driveable than old cars. Fuel injection is 20%, but the other 80% is overdrive.

[Ohio mike]

What's your guys opinion on my set up, hope I'm not hijacking a post here. I'm building a 67 c10 2wd shortbed, 454 with stock bottom end and mild cam, 1800 stall and built 700r4. Just ordered eaton Posi and debating on keeping 3.08 gears. My goal is to be able to drive it anywhere but I don't want too tall of gear for the od tranny. With a 30" tire rpm @60 should be around 1450 and at 70 should be pushing 1700 rpm. I figure the big block should still be good for plenty of burn outs even with the gears. Too tall or good gear?

[Mike C]

When I had my Impala SS most of the guys went from 3.07 to 3.73. You could get away with the 3.07 but to maximize performance I would go 3.42-3.55-3.73. Obviously with these trucks the 3.73 easiest OEM to come by.

[Ironangel]

Originally Posted by Ohio mike

What's your guys opinion on my set up, hope I'm not hijacking a post here. I'm building a 67 c10 2wd shortbed, 454 with stock bottom end and mild cam, 1800 stall and built 700r4. Just ordered eaton Posi and debating on keeping 3.08 gears. My goal is to be able to drive it anywhere but I don't want too tall of gear for the od tranny. With a 30" tire rpm @60 should be around 1450 and at 70 should be pushing 1700 rpm. I figure the big block should still be good for plenty of burn outs even with the gears. Too tall or good gear?

3:08 is a perfect gear for that big block. Hole shots with sticky tires, then think about going lower. The fact that you have a short bed (lighter weight) lends towards the higher gear. Hard to beat a 3:08 behind a big block, period.

[Accelo]

[QUOTE= I'm building a 67 c10 2wd shortbed, 454 with stock bottom end and mild cam, 1800 stall and built 700r4. Just ordered eaton Posi and debating on keeping 3.08 gears. My goal is to be able to drive it anywhere [/QUOTE]

3.08 Gears
0.7 X overdrive ratio of the 700R4
2.156 Final drive equivalent

I get 2070 motor rpm at 60mph with 3.08 (standard 1;1 ratio transmission)
I get 1449 motor rpm at 60mph with 2.156 (overdrive .7 ratio transmission)
I get 1690 motor rpm at 70mph
Add 3% to the rpm listed if not using a lock up torque converter.

Didn't appear the others figured rpm using the Over drive ratio.

[groundedc10]

My current tire size is 215 75 15. Next year I will switch over to an automatic with an OD. Just looking for a good setup for cruising this summer. I will look into a 3.08. Thanks for the gel and info.

[Ohio mike]

I don't plan on using the lockup converter. I thought for lower speeds I can just keep it in drive and be ok. Posi came today so I'll try the 3.08 and see how it is. Thx for the feedback.

[67ChevyRedneck]

Originally Posted by groundedc10

My current tire size is 215 75 15. Next year I will switch over to an automatic with an OD. Just looking for a good setup for cruising this summer. I will look into a 3.08. Thanks for the gel and info.

*IF* this is true, I would pass on the 3.08 rear unless you absolutely have to have it for just this year. Quite a few members on here have had an OD trans with a 3.08 and with the weight of these trucks it's not a very good combo, some even complained there was only a small fuel mileage improvement as the trans was always dropping to 3rd to go up even small hills.

You also need to find out what gear you have now. If you're new to classics, your definition of the "motor screaming" may be a lot different than someone elses, and you may have a 3.08 now. However, most manual trucks received a 3.73 or 4.11 gearing.

If you do go looking for a 3.08 they were mostly used in automatic trucks. Not a guarantee its a 3.08, but likely. Then you need to decide if you want to stick with 6 lug or go 5 lug...

A 3.42 is the best ratio combo for these trucks with overdrive but hard to find. I have a 3.73 with a 700R. I would not want a taller gear. The truck scoots along well with it and returns decent fuel economy.

[CorDog009]

I like anything from 3.08 to 3.42, maybe 3.55 depending on tire size and trans ratios.

[kolohekane]

my 67 long fleet side with a 350 cid and turbo 350 and 235/75/15 tires is running 3000 RPM at 55 mph. This is unacceptable for my needs. Reading here it seems that a OD transmission is a better option for me rather than a gear change. So if I use a TH700R what would be my expected drop n RPM?

[jocko]

Originally Posted by kolohekane

my 67 long fleet side with a 350 cid and turbo 350 and 235/75/15 tires is running 3000 RPM at 55 mph. This is unacceptable for my needs. Reading here it seems that a OD transmission is a better option for me rather than a gear change. So if I use a TH700R what would be my expected drop n RPM?

Is your truck a C20 and was the original motor a 6 or 307/327? All things being equal, the combo you're describing is approx. a 4.57 rear, which was std for a 68 C20 with a six or sbc (with 29" tire, that is). Any rear gear callouts on the SPID? 4.10 was optional. Keep in mind that if the tire size is non-stock, the speedo will be off, so a GPS is the best way to know your actual mph.

700R4 would drop your rpm as shown below (in the case of 55 mph, I was showing 2912 RPM with a TH350 for comparison)

[clay68c10]

I'm coming up with 2100rpm at 55

Edit, too slow typing, missed the chart. I was close.

[davischevy]

[QUOTE=kolohekane;7867814]my 67 long fleet side with a 350 cid and turbo 350 and 235/75/15 tires is running 3000 RPM at 55 mph. This is unacceptable for my needs. Reading here it seems that a OD transmission is a better option for me rather than a gear change. So if I use a TH700R what would be my expected drop n RPM?[/QUOTE

Between 500 and 600 RPM drop.